# connecting batteries in parallel or series, myth and theory

I've heard from way back that connecting batteries in parallel is a bad idea.
I've heard a couple reasons. One is that it can be hard to make sure the
batteries are loaded equally. Another reason I heard is more complex. It says that parallel connection should be done only on a per-cell basis, and that a series/parallel combination would thus have parallel connections at every cell ... and therefore multi-cell batteries are bad in this regard while single cell (2V) "batteries" are the way to go.
Some googling revealed a few web pages that seem to not have much reference to any parallel connection issues, aside from making sure the batteries are the same size/capacity.
I would think that self-charging (a stronger battery charging a weaker one connected in parallel) would not be much of an issue for a batteries that are intended to keep a float charge, anyway (lead-acid is all that I am considering here). If any battery has a problem where it can't take a charge and could be a hazard if charged, then it seems one would have a bigger issue by that alone than anything related to wiring them in parallel.
Maybe there are issues when the _number_ of paralleled batteries is high?
Series connection would _seem_ to be more of a problem, but clearly isn't a show stopper at common voltage levels (12 to 48). What I refer to when I say this is the risk that a single weak cell in the series woould be effectively "reverse charged" by the "brute force" of the high voltage series circuit forcing the current in the reverse direction than a charging current. On the face of it, this risk seems like one that, if it could happen, would happen on even a small a voltage as 6 volts (3 cell). But maybe for lead-acid the risk is merely depleting the bad cell to zero and destroying it and nothing more?
What risks ... to the batteries themselves ... exists in a very high number of cells in series wired for very high voltages (120 to 600 volts for example)?
I remember running across, a few years ago, a very large DC to AC inverter that was designed to run from a 576 volt battery configuration (which would be closer to 600 volts). So it might seem that such setups are possible. That wouldn't necessarily say there are no risks, as such a setup would be an industrial one that could be well supervised for the issues that could happen. But I would like to know what potential issues to watch for in a home setup that could be wired to as much as 48 volts.
Would particular sub-technologies of lead-acid batteries matter in this regard (aside from making sure all are the same type)?
Who here thinks connecting batteries (one cell or multi-cell) in parallel is bad ... and why (if you don't know why, I'll assume it's myth).
Is there any advantage to using a big single cell (other than it's really big current capacity, which itself may eliminate the need to even have anything wired in parallel)? Example: Surrette 2KS33PS
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On 7 Aug 2008 18:43:07 GMT, snipped-for-privacy@ipal.net wrote:

Interesting paper on the topic of parallel battery wiring here:
http://www.battcon.com/PapersFinal2002/McDowallPaper2002.pdf
For me, as someone who lives off-grid, has a 48V battery bank, and does his own battery maintenance, the big advantage of having a single string, vs multiple parallel strings, would be the ease of maintenance -- 1 string 24 cells to check; 2 strings = 48 cells to check, etc.
When I purchased my batteries, I could not purchase 2V batteries in the capacity I would have liked. So I have two parallel strings. But the 2V cells are now available in a satisfactory size, so when my present bank needs to be replaced, I will be using a single string.
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| Interesting paper on the topic of parallel battery wiring here: | | http://www.battcon.com/PapersFinal2002/McDowallPaper2002.pdf
It seems from that, parallel string operation is mostly OK, with one notable exception being parallelizing to increase deliverable current capacity (as opposed to parallelizing to increase time capacity and reduce current per string). If one string fails under a high current load, it could be a big problem for the other string.
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On 8 Aug 2008 02:43:39 GMT, snipped-for-privacy@ipal.net wrote:

However, I believe they are mostly discussing systems that spend most of their time on float charge, with occasional
But the authors also described a situation where one string would discharge more than another. Those circumstances -- "... a system that is designed for long discharges, but is subjected to frequent shallow discharges" could describe many off-grid systems. The result is that one string, (in the paper the "high-rate" battery), receives the brunt of the cycling duty, and may age prematurely as a result.
This is probably not a significant issue with two matched strings that are placed in service together. But this phenomenon is, I believe, the source of the recommendation to not mix batteries of different ages in off-grid systems. --ron
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| On 8 Aug 2008 02:43:39 GMT, snipped-for-privacy@ipal.net wrote: |
|> |>| Interesting paper on the topic of parallel battery wiring here: |>| |>| http://www.battcon.com/PapersFinal2002/McDowallPaper2002.pdf |> |>It seems from that, parallel string operation is mostly OK, with one notable |>exception being parallelizing to increase deliverable current capacity (as |>opposed to parallelizing to increase time capacity and reduce current per |>string). If one string fails under a high current load, it could be a big |>problem for the other string. | | However, I believe they are mostly discussing systems that spend most of | their time on float charge, with occasional | | But the authors also described a situation where one string would discharge | more than another. Those circumstances -- "... a system that is designed | for long discharges, but is subjected to frequent shallow discharges" could | describe many off-grid systems. The result is that one string, (in the | paper the "high-rate" battery), receives the brunt of the cycling duty, and | may age prematurely as a result.
If merely aging prematurely is the result, that may not necessarily be bad if the other string(s) age less at the same time. This spreads out your need to replace batteries over time. If all the strings aged identically, then your whole battery bank will need to be replaced all at once when the time comes.
| This is probably not a significant issue with two matched strings that are | placed in service together. But this phenomenon is, I believe, the source | of the recommendation to not mix batteries of different ages in off-grid | systems.
If the effect is that the _older_ strings gets _older_ faster, and the others do not, it just means _one_ string to replace sooner and _fewer_ strings to replace later on.
I'm still concerned about a string with a shorted cell. But I can see this would still be a problem in general, as the effect to the stunted string is the float voltage would be a couple volts higher (more significant the shorter the string).
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On 8 Aug 2008 18:32:00 GMT, snipped-for-privacy@ipal.net wrote:

And if it is the newer string that gets older faster? ... Well, maybe you won't get the longevity you thought you'd paid for.
In any event, the "best" setup does depend to some extent on the application and goals. --ron
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| On 8 Aug 2008 18:32:00 GMT, snipped-for-privacy@ipal.net wrote: | |>If the effect is that the _older_ strings gets _older_ faster, | | And if it is the newer string that gets older faster? ... Well, maybe you | won't get the longevity you thought you'd paid for.
Right. So we need to know which it is ... which string gets older faster.
| In any event, the "best" setup does depend to some extent on the | application and goals.
How about a power backup system (charged by the grid) that gradually shifts to a renewable power system (charged by solar, wind, etc) and then eventually to a completely off-grid system (or at least a sell-only grid system).
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On Aug 9, 6:09 am, snipped-for-privacy@ipal.net wrote:

Phil
People have explained several reasons for "NOT" paralleling batteries. It should be avoided wherever possible.
You response tells us that you have/want parallel batteries and you are looking for someone to tell you it's ok.
So, Yes it is ok. As long as you also accept that it is the second best option and are prepared to take all responsibility for your choice of battery bank.
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In alt.engineering.electrical snipped-for-privacy@gmail.com wrote: | On Aug 9, 6:09 am, snipped-for-privacy@ipal.net wrote:
|> | |> |>If the effect is that the _older_ strings gets _older_ faster, |> | |> | And if it is the newer string that gets older faster? ... Well, maybe you |> | won't get the longevity you thought you'd paid for. |> |> Right. So we need to know which it is ... which string gets older faster. |> |> | In any event, the "best" setup does depend to some extent on the |> | application and goals. |> |> How about a power backup system (charged by the grid) that gradually shifts |> to a renewable power system (charged by solar, wind, etc) and then eventually |> to a completely off-grid system (or at least a sell-only grid system). |> |> -- |> |WARNING: Due to extreme spam, googlegroups.com is blocked. Due to ignorance | |> | by the abuse department, bellsouth.net is blocked. If you post to | |> | Usenet from these places, find another Usenet provider ASAP. | |> | Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) | | | Phil | | People have explained several reasons for "NOT" paralleling batteries. | It should be avoided wherever possible.
If you'd like, feel free to summarize the thread. I plan to look it all over again once it seems all the responses are done.
| You response tells us that you have/want parallel batteries and you | are looking for someone to tell you it's ok.
No. If there is an alternative, I'm willing to consider that. So what would you suggest as an alternative? More in series and increasing the voltage?
| So, Yes it is ok. As long as you also accept that it is the second | best option and are prepared to take all responsibility for your | choice of battery bank.
It's 2nd best compared to what is 1st best?
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On Aug 10, 5:09 am, snipped-for-privacy@ipal.net wrote:

A single string of 2V cells of the correct Ah rating. If there is no other option then limit the parallel strings to 2.

A single string of 2V cells of the correct Ah rating.

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In alt.engineering.electrical snipped-for-privacy@gmail.com wrote: | On Aug 10, 5:09 am, snipped-for-privacy@ipal.net wrote:
|>
wrote:
|> |> | |> |> |>If the effect is that the _older_ strings gets _older_ faster, |> |> | |> |> | And if it is the newer string that gets older faster? ... Well, maybe you |> |> | won't get the longevity you thought you'd paid for. |> |> |> |> Right. So we need to know which it is ... which string gets older faster. |> |> |> |> | In any event, the "best" setup does depend to some extent on the |> |> | application and goals. |> |> |> |> How about a power backup system (charged by the grid) that gradually shifts |> |> to a renewable power system (charged by solar, wind, etc) and then eventually |> |> to a completely off-grid system (or at least a sell-only grid system). |> |> |> |> -- |> |> |WARNING: Due to extreme spam, googlegroups.com is blocked. Due to ignorance | |> |> | by the abuse department, bellsouth.net is blocked. If you post to | |> |> | Usenet from these places, find another Usenet provider ASAP. | |> |> | Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) | |> | |> | Phil |> | |> | People have explained several reasons for "NOT" paralleling batteries. |> | It should be avoided wherever possible. |> |> If you'd like, feel free to summarize the thread. I plan to look it all over |> again once it seems all the responses are done. |> |> | You response tells us that you have/want parallel batteries and you |> | are looking for someone to tell you it's ok. |> |> No. If there is an alternative, I'm willing to consider that. So what would |> you suggest as an alternative? More in series and increasing the voltage? | | A single string of 2V cells of the correct Ah rating. | If there is no other option then limit the parallel strings to 2.
Let me summarize/rephrase, since my intentions didn't pass through the layers of conversation. What is the alternative for paralleling cells/batteries/strings when the desired capacity is NOT available in single cells or small batteries?
If a 24V system needs to be doubled in capacity, and larger cells is not an option, maybe you'd suggest a 48V system, especially if no investment has yet been made in the 24V ssytem. But what if the system is already at 48V? Then what? Even higher in voltage?
|> | So, Yes it is ok. As long as you also accept that it is the second |> | best option and are prepared to take all responsibility for your |> | choice of battery bank. |> |> It's 2nd best compared to what is 1st best? | | A single string of 2V cells of the correct Ah rating.
As you go up in single cell capacity, other issues come up. These can be things like cost (double capacity single cell might be 3x the cost) and handling (you can't lift the double capacity cell).
What I want to know is not so what _the_ best system design is, but information about the advantages and disadvantages so that I could weigh one design over another. ONE way to explain this might be in terms of the cost of everything. For example, just how much of a savings on smaller cells/batteries makes it worth going that route, in your opinion/experience. If I can build a system of one string of single cells for \$30000, and would like to reduce the price, at what price level would _you_ decide it's worth going with parallel strings? \$10000?
For me, once I can get a good handle on the _actual_ issues of parallel strings vs. parallel cells vs. parallel batteries, then I could answer the above for myself. Right now I'm getting answers like "batteries obey the laws of physics" without any explanation of what laws apply (there might be more laws that apply beyond the obvious ... and that is crucial to know).
Right now I'm not getting much better answers here than I got from Googling.
But I did get one useful answer that the measurement of a single cell can be masked when another is in parallel to it ... but that just tells me it is better to parallel the strings rather than the cells (which also happens to be a lower cost option).
And maybe some big rectifiers to isolate the strings from cross-charging might be called for. It would then seem to me the only way to keep the strings charged with the rectifiers in place is to separately charge each string. But that might be an economic benefit from smaller chargers.
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On Aug 11, 4:54 am, snipped-for-privacy@ipal.net wrote:

If by "desired capacity" you mean that there is no cell/battery at all that will allow for a single series string, even at a larger capacity than desired, then you will have to use parallel strings. 2 parallel strings max. Keep in mind that cells with a capacity of 1500Ah are available.
If by "desired capacity" you mean the ones at a price you want to pay... Get a grid connection

See above.

As pointed out above if cost is the defining point of "desired capacity" then you should get a grid connection.
Lifting batteries has never been a problem for tool using humans.

Design by price is often used by those that want to buy a Rollex for a hundred dollars and complain because it keeps lousy time. At no time should you choose parallel strings on cost, unless you have not got the money at all to do better.

Parallel strings do not charge and discharge in a uniform manner. Two parallel strings are second best choice, but with care and diligence with maintenance are acceptable although if you lose a single cell you still have to replace the lot.

Cost again. If the cost of a PV system is more important than a PV system that is properly designed and built then you would be better off with a grid connection.

Been there, done that, didn't work and the tee shirt dissolved in the wash.

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In alt.energy.homepower snipped-for-privacy@gmail.com wrote:
| If by "desired capacity" you mean that there is no cell/battery at all | that will allow for a single series string, even at a larger capacity | than desired, then you will have to use parallel strings. 2 parallel | strings max. Keep in mind that cells with a capacity of 1500Ah are | available.
What I am doing is finding out what I need to know in order to balance the need for a good system and the need to make the system affordable. If batteries at twice the cost for capacity also last longer because the system works better, that favors the more costly batteries. This is the kind of thing I'm trying to find out. Knowing all the issues with parallel strings vs. parallel cells vs. monster cells, as well as issues with selected voltages (for example 48VDC vs. 240VDC) all figure into the design choice. Other factors that have to be considered include whether I can lift the batteries by hand (only small ones) vs. needing a forklift (more cost, more energy concerns, and also spacing concerns).
I have not made the decision, yet because I do not believe I have all the information I need. What I have gotten so far does suggest the best way to go would be the big Surrette 2V cells, 24 in series for 48VDC. Then for larger capacity, split the loads into separate battery/inverter systems. Some of these decisions will depend on what money is available when the time tim build comes around.
| If by "desired capacity" you mean the ones at a price you want to | pay... Get a grid connection
The idea is to eventually get off the grid _and_ do it without carbon based fuel usage (e.g. no gas/diesel generator).
| As pointed out above if cost is the defining point of "desired | capacity" then you should get a grid connection.
It's cost WITHIN the plan of going off-grid.
| Lifting batteries has never been a problem for tool using humans.
The selection of tool could affect things like battery layout, etc. Smaller batteries have an advantage. Larger batteries are not ruled out.
|> What I want to know is not so what _the_ best system design is, but information |> about the advantages and disadvantages so that I could weigh one design over |> another. ONE way to explain this might be in terms of the cost of everything. |> For example, just how much of a savings on smaller cells/batteries makes it |> worth going that route, in your opinion/experience. If I can build a system of |> one string of single cells for \$30000, and would like to reduce the price, at |> what price level would _you_ decide it's worth going with parallel strings? |> \$10000? | | Design by price is often used by those that want to buy a Rollex for a | hundred dollars and complain because it keeps lousy time. At no time | should you choose parallel strings on cost, unless you have not got | the money at all to do better.
There seem to be some with opinions that say parallel is, while not the best choice, not all that bad, either. More may say that when considering rectifiers to isolate them from charging each other. But rectifiers have a cost, too (including a voltage drop).
I'm exploring all options. I'm not interested in specific advice on what I should do (at least not without well explained why) ... I'm interested in the information to make the best decision in the circumstances that will be present at the time the decision is to be made.
|> For me, once I can get a good handle on the _actual_ issues of parallel strings |> vs. parallel cells vs. parallel batteries, then I could answer the above for |> myself. Right now I'm getting answers like "batteries obey the laws of physics" |> without any explanation of what laws apply (there might be more laws that apply |> beyond the obvious ... and that is crucial to know). | | Parallel strings do not charge and discharge in a uniform manner. Two | parallel strings are second best choice, but with care and diligence | with maintenance are acceptable although if you lose a single cell you | still have to replace the lot.
Information I want is to know just how close that second best is to first best. Once I get all the issues, I may be able to come up with some idea of this.
|> Right now I'm not getting much better answers here than I got from Googling. |> |> But I did get one useful answer that the measurement of a single cell can be |> masked when another is in parallel to it ... but that just tells me it is |> better to parallel the strings rather than the cells (which also happens to |> be a lower cost option). | | Cost again. If the cost of a PV system is more important than a PV | system that is properly designed and built then you would be better | off with a grid connection.
It's about what cost to get off the grid.
|> And maybe some big rectifiers to isolate the strings from cross-charging might |> be called for. It would then seem to me the only way to keep the strings |> charged with the rectifiers in place is to separately charge each string. |> But that might be an economic benefit from smaller chargers. | | Been there, done that, didn't work and the tee shirt dissolved in the | wash.
Do you know why it didn't work? Or are you just assuming that because it had rectifiers, that must be why?
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On Aug 13, 3:59 pm, snipped-for-privacy@ipal.net wrote:

Batteries are the very heart of your system. That should tell you something about the choice of battery/ cell used. You sound like a guy looking for an excuse to use golf cart batteries.
In the last twenty five years I have seen just about every combination of battery supply. I have seen things like a twenty four volt system made up of batteries of 3 different capacities, 4 different ages and 6 different brands. Not a pretty sight.
Best choice is a single series string of the correct capacity. Failing that being possible the second best option is no more than 2 parallel strings to make up the capacity.
Your choice, you spend the money, you wear the consequences. Let's say you are willing to spend half a million to build your house, you're going to quibble over 50k to power it.

Easy, not particularly cheap.

It costs what it costs to do it so it serves your energy needs. Cheap batteries are cheap for a reason.

So it take two people to move a battery into place. Once it it there it ain't going anywhere. I made a harness for larger batteries so they could be handled with ease by two.

Batteries are DC, what are the rectifiers for?

You still sound like a guy looking for an excuse to use golf cart batteries.

For a maximum of two parallel strings with lots of maintenance and care you will not have too many problems.

Ah, in that case you need to know what your daily load will be, how many days of autonomy is required.

Sorry, no rectifiers. Did try diodes. Oh, it worked alright. It was just not worth the effort. The strings always ended up at different voltages and needing charging to bring them back into line. Then there was the game of musical cells trying to get a balance in each string.
But if you have the time on your hands, go for it.

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In alt.engineering.electrical snipped-for-privacy@gmail.com wrote:
| Batteries are the very heart of your system. That should tell you | something about the choice of battery/ cell used. You sound like a guy | looking for an excuse to use golf cart batteries.
You sound like a guy wanting to tell everyone to do things exactly the same way you do things, without being willing to tell them why, whether you actually know why or not.
| In the last twenty five years I have seen just about every combination | of battery supply. I have seen things like a twenty four volt system | made up of batteries of 3 different capacities, 4 different ages and 6 | different brands. Not a pretty sight.
And were you there when it blew up?
| Best choice is a single series string of the correct capacity. Failing | that being possible the second best option is no more than 2 parallel | strings to make up the capacity.
Why?
Specifically I want to know why for the "no more than 2" part. But I also want to know how to balance the decision between first best and second best against other factors that may push for that second best. Merely ordering the option into 1st, 2nd, 3rd, etc., is not it.
| Your choice, you spend the money, you wear the consequences. Let's say | you are willing to spend half a million to build your house, you're | going to quibble over 50k to power it.
If you have nothing more to add, then what you have said so far will not play much, if any, role in my evaluation of the designs to use. This is because you aren't providing information I consider useful. Again, I am interested in the technical information to go into a design decision that balances multiple needs, not the "what I did" or "what I would do" (unless I get a sufficiently significant number of those responses from verified engineers).
|> | If by "desired capacity" you mean the ones at a price you want to |> | pay... Get a grid connection |> |> The idea is to eventually get off the grid _and_ do it without carbon based |> fuel usage (e.g. no gas/diesel generator). | | Easy, not particularly cheap.
Fine. I didn't expect it to lower my costs relative to being always on the grid.
|> | As pointed out above if cost is the defining point of "desired |> | capacity" then you should get a grid connection. |> |> It's cost WITHIN the plan of going off-grid. | | It costs what it costs to do it so it serves your energy needs. Cheap | batteries are cheap for a reason.
I expect to pay more than the cost of being on-grid. But this is NOT an infinite finance to work in.
|> | Lifting batteries has never been a problem for tool using humans. |> |> The selection of tool could affect things like battery layout, etc. |> Smaller batteries have an advantage. Larger batteries are not ruled out. | | So it take two people to move a battery into place. Once it it there | it ain't going anywhere. I made a harness for larger batteries so they | could be handled with ease by two.
I want to do it with one person. Maybe that means a fork lift machine and the space to move the fork lift around. Or maybe it means a block and tackle assembly that can slide on an overhead rail. Or maybe it means all the cells/batteries placed on individual roller carts with locking wheels. Or maybe it means having smaller batteries and doing the lifting more often. All these options are best balanced out knowing all information (not just the cost, but also the technical implications, the latter of which I was hoping I could get pointers to from someone here).
|> |> What I want to know is not so what _the_ best system design is, but information |> |> about the advantages and disadvantages so that I could weigh one design over |> |> another. ONE way to explain this might be in terms of the cost of everything. |> |> For example, just how much of a savings on smaller cells/batteries makes it |> |> worth going that route, in your opinion/experience. If I can build a system of |> |> one string of single cells for \$30000, and would like to reduce the price, at |> |> what price level would _you_ decide it's worth going with parallel strings? |> |> \$10000? |> | |> | Design by price is often used by those that want to buy a Rollex for a |> | hundred dollars and complain because it keeps lousy time. At no time |> | should you choose parallel strings on cost, unless you have not got |> | the money at all to do better. |> |> There seem to be some with opinions that say parallel is, while not the |> best choice, not all that bad, either. More may say that when considering |> rectifiers to isolate them from charging each other. But rectifiers have |> a cost, too (including a voltage drop). | | Batteries are DC, what are the rectifiers for?
They can do things like ensuring that one bank does not cross change another. They can allow separate chargers for each bank.
|> I'm exploring all options. I'm not interested in specific advice on what |> I should do (at least not without well explained why) ... I'm interested in |> the information to make the best decision in the circumstances that will be |> present at the time the decision is to be made. | | You still sound like a guy looking for an excuse to use golf cart | batteries.
I am someone looking for the technical information that would be a valid basis for deciding what circumstances that golf cart batteries can be used in, and what circumstances they cannot be used in, where "circumstances" involves a lot of things that I don't even know, yet.
|> Information I want is to know just how close that second best is to first |> best. Once I get all the issues, I may be able to come up with some idea |> of this. | | For a maximum of two parallel strings with lots of maintenance and | care you will not have too many problems.
What issues will happen with 2 parallel strings?
What issues will happen with 3 parallel strings that would not happen with 2?
|> |> Right now I'm not getting much better answers here than I got from Googling. |> |> |> |> But I did get one useful answer that the measurement of a single cell can be |> |> masked when another is in parallel to it ... but that just tells me it is |> |> better to parallel the strings rather than the cells (which also happens to |> |> be a lower cost option). |> | |> | Cost again. If the cost of a PV system is more important than a PV |> | system that is properly designed and built then you would be better |> | off with a grid connection. |> |> It's about what cost to get off the grid. | | Ah, in that case you need to know what your daily load will be, how | many days of autonomy is required.
Days of autonomy will gradually shift to being whatever number of days God decides not to offer me the opportunity to charge up (e.g. no wind, no sun).
I can't say some particular exact number of days I want to be sure the system will continue to operate in. If I say some number, I have to face the issue of what to do when it happens to turn out longer than that. I expect to start with a low number and build up to a higher number.
|> |> And maybe some big rectifiers to isolate the strings from cross-charging might |> |> be called for. It would then seem to me the only way to keep the strings |> |> charged with the rectifiers in place is to separately charge each string. |> |> But that might be an economic benefit from smaller chargers. |> | |> | Been there, done that, didn't work and the tee shirt dissolved in the |> | wash. |> |> Do you know why it didn't work? Or are you just assuming that because it had |> rectifiers, that must be why? | | Sorry, no rectifiers. Did try diodes. Oh, it worked alright. It was | just not worth the effort. The strings always ended up at different | voltages and needing charging to bring them back into line. Then there | was the game of musical cells trying to get a balance in each string.
Did each string have its own charger and charge controller?
| But if you have the time on your hands, go for it.
I don't know whether I will or won't. I'm looking for the specific technical info to learn that allows me to figure it out. I do not know if you know what it is I want to know, or not. But I do know you are not saying it, or pointing to it, for whatever reason that might be.
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On Aug 14, 2:52 am, snipped-for-privacy@ipal.net wrote:

I have told you why, parallel strings do not charge/discharge in a uniform manner. That is the truth.

No, I was called to fix it after it developed terminal behavior. They seldom "Blow Up".

I have told you why, parallel strings do not charge/discharge in a uniform manner. That is the truth.

Two strings is manageable without to much pissing about. The decision is to buy a single string of the correct Ah capacity. If this is not possible then use two parallel strings. Money before good design is false economy.

Not true. You want someone to give you a good reason to choose the lowest price option over good design. No matter what you decide, be it a single string or six parallel strings, if one cell dies and the batteries are older than a yera or two then you will be replacing th whole bank. That the way it works.

Good, because it will cost you far more than keeping your grid connection.

You have a grid connection, you will find that PV can not replicate that level of energy for what you feel is a reasonable cost. What have you done to reduce your energy needs.

You have been given a great deal of information in this thread.

A diode is not a rectifier.

You can use golf cart batteries, they can be wired in series, parallel and series/parallel in as many strings as you feel are required. Most people only ever do this once. Their second battery bank is usually a single string of cells with the correct Ah rating.

Parallel strings do not charge/discharge evenly. Two strings and you must monitor the state of charge and equalize the strings more often. Basically twice the work of a single string. Three parallel strings and you might expect to do four times the maintenance. In the end you will have repetitive cell failure. Because of the cost you will replace cells/batteries rather than replacing the whole bank, then you will remove a string to use the cells/batteries to replace dead cells/ batteries in the remaining strings.
I know, you would never do that.

No. you have to decide when designing your system. This is one of the factors that sizes the battery bank. God doesn't give a rats ass whether you sit in the dark or not,

"DAYS of AUTONOMY" is not a sliding scale. It is what your batteries will provide at the load designed for. What are you going to do, add and subtract batteries as required?

Hang on. You want the cheapest option. Now you want to build multiple systems.

Oh, I know what it is you want to know alright. But to understand you need to listen. You want some one to tell you what you want to hear.

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In alt.engineering.electrical snipped-for-privacy@gmail.com wrote: | On Aug 14, 2:52 am, snipped-for-privacy@ipal.net wrote:
|> |> | Batteries are the very heart of your system. That should tell you |> | something about the choice of battery/ cell used. You sound like a guy |> | looking for an excuse to use golf cart batteries. |> |> You sound like a guy wanting to tell everyone to do things exactly the |> same way you do things, without being willing to tell them why, whether |> you actually know why or not. | | I have told you why, parallel strings do not charge/discharge in a | uniform manner. That is the truth.
I'm not doubting the truth of this. I'm wanting to understand the means by which this happens, and the degree to which it happens. I want to understand this enough to know how well it may be mitigated. For example the non-uniform charging may be dealt with by smaller chargers isolated on each string (not one big charger trying charge the strings in parallel).
|> | Best choice is a single series string of the correct capacity. Failing |> | that being possible the second best option is no more than 2 parallel |> | strings to make up the capacity. |> |> Why? | | I have told you why, parallel strings do not charge/discharge in a | uniform manner. That is the truth.
I'm not doubting the truth of this. I'm wanting to understand the means by which this happens, and the degree to which it happens. I want to understand this enough to know how well it may be mitigated. For example the non-uniform charging may be dealt with by smaller chargers isolated on each string (not one big charger trying charge the strings in parallel).
|> Specifically I want to know why for the "no more than 2" part. But I |> also want to know how to balance the decision between first best and |> second best against other factors that may push for that second best. |> Merely ordering the option into 1st, 2nd, 3rd, etc., is not it. | | Two strings is manageable without to much pissing about. The decision | is to buy a single string of the correct Ah capacity. | If this is not possible then use two parallel strings. Money before | good design is false economy.
I would not say it is not possible. But there may be (when it comes time to do this) reasons to prefer the smaller batteries. Reasons can include the desire for one man handling (can depend on available space). Or it could be the desire to start small and migrate to larger.
I want to explore means to manage the load balancing before deciding. Maybe it will be the case that those methods are not worthwhile. I cannot say today. Today is the learning time. The decision is later. The conditions will be known then.
|> | Your choice, you spend the money, you wear the consequences. Let's say |> | you are willing to spend half a million to build your house, you're |> | going to quibble over 50k to power it. |> |> If you have nothing more to add, then what you have said so far will not |> play much, if any, role in my evaluation of the designs to use. This is |> because you aren't providing information I consider useful. Again, I am |> interested in the technical information to go into a design decision that |> balances multiple needs, not the "what I did" or "what I would do" (unless |> I get a sufficiently significant number of those responses from verified |> engineers). | | Not true. You want someone to give you a good reason to choose the | lowest price option over good design.
No. I want to know all the reasons for all the choices.
| No matter what you decide, be it a single string or six parallel | strings, if one cell dies and the batteries are older than a yera or | two then you will be replacing th whole bank. That the way it works.
What happens if you have ONE string of several very large cells/batteries and one dies? Are you saying you don't have to replace the whole bank in this case?
|> I expect to pay more than the cost of being on-grid. But this is NOT an |> infinite finance to work in. | | You have a grid connection, you will find that PV can not replicate | that level of energy for what you feel is a reasonable cost. What have | you done to reduce your energy needs.
That is a work in progress. Much of it will be done when I start going with some battery power. More will be done later. This is not a one big step all the way project.
|> I want to do it with one person. Maybe that means a fork lift machine and |> the space to move the fork lift around. Or maybe it means a block and |> tackle assembly that can slide on an overhead rail. Or maybe it means all |> the cells/batteries placed on individual roller carts with locking wheels. |> Or maybe it means having smaller batteries and doing the lifting more often. |> All these options are best balanced out knowing all information (not just |> the cost, but also the technical implications, the latter of which I was |> hoping I could get pointers to from someone here). | | You have been given a great deal of information in this thread.
Quite a lot of it is unsupported "what to do" advice.
|> They can do things like ensuring that one bank does not cross change another. |> They can allow separate chargers for each bank. | | A diode is not a rectifier.
Rectifiers are made of diodes (or other things).
|> |> I'm exploring all options. I'm not interested in specific advice on what |> |> I should do (at least not without well explained why) ... I'm interested in |> |> the information to make the best decision in the circumstances that will be |> |> present at the time the decision is to be made. |> | |> | You still sound like a guy looking for an excuse to use golf cart |> | batteries. |> |> I am someone looking for the technical information that would be a valid basis |> for deciding what circumstances that golf cart batteries can be used in, and |> what circumstances they cannot be used in, where "circumstances" involves a lot |> of things that I don't even know, yet. | | You can use golf cart batteries, they can be wired in series, parallel | and series/parallel in as many strings as you feel are required. Most | people only ever do this once. Their second battery bank is usually a | single string of cells with the correct Ah rating.
And maybe I will migrate to that. One possible path is that I would start with a small bank and migrate to a larger bank. I may parallel things in the interim. I will also be looking at possible circuits to manage the load balancing between parallel strings. Maybe that won't be practical. I'm sure you'll say it won't but I want to know why, if that's the case.
| Parallel strings do not charge/discharge evenly. Two strings and you | must monitor the state of charge and equalize the strings more often. | Basically twice the work of a single string. Three parallel strings | and you might expect to do four times the maintenance. In the end you | will have repetitive cell failure. Because of the cost you will | replace cells/batteries rather than replacing the whole bank, then you | will remove a string to use the cells/batteries to replace dead cells/ | batteries in the remaining strings.
If it is the case that cost is why I would replace one cell/battery at a time, then cost would clearly prohibit the monster cells.
| I know, you would never do that.
I believe I can make the right decision ... the one that is right for my situation, once I know all the issues, how they behave, their workarounds, methods of managing them, etc.
|> Days of autonomy will gradually shift to being whatever number of days God |> decides not to offer me the opportunity to charge up (e.g. no wind, no sun). | | No. you have to decide when designing your system. This is one of the | factors that sizes the battery bank. | God doesn't give a rats ass whether you sit in the dark or not,
|> I can't say some particular exact number of days I want to be sure the system |> will continue to operate in. If I say some number, I have to face the issue |> of what to do when it happens to turn out longer than that. I expect to start |> with a low number and build up to a higher number. | | "DAYS of AUTONOMY" is not a sliding scale. It is what your batteries | will provide at the load designed for. What are you going to do, add | and subtract batteries as required?
Apparently we have a different understand of this term. I don't know what yours is, then. So it's pointless to proceed on this.
|> |> |> And maybe some big rectifiers to isolate the strings from cross-charging might |> |> |> be called for. It would then seem to me the only way to keep the strings |> |> |> charged with the rectifiers in place is to separately charge each string. |> |> |> But that might be an economic benefit from smaller chargers. |> |> | |> |> | Been there, done that, didn't work and the tee shirt dissolved in the |> |> | wash. |> |> |> |> Do you know why it didn't work? Or are you just assuming that because it had |> |> rectifiers, that must be why? |> | |> | Sorry, no rectifiers. Did try diodes. Oh, it worked alright. It was |> | just not worth the effort. The strings always ended up at different |> | voltages and needing charging to bring them back into line. Then there |> | was the game of musical cells trying to get a balance in each string. |> |> Did each string have its own charger and charge controller? | | Hang on. You want the cheapest option. Now you want to build multiple | systems.
Your understanding of "cheapest option" is likely not the same as mine.
|> | But if you have the time on your hands, go for it. |> |> I don't know whether I will or won't. I'm looking for the specific technical |> info to learn that allows me to figure it out. I do not know if you know what |> it is I want to know, or not. But I do know you are not saying it, or pointing |> to it, for whatever reason that might be. | | Oh, I know what it is you want to know alright. But to understand you | need to listen. | You want some one to tell you what you want to hear.
You want some one to do what you tell them to.
I can see I'm not going to get anything useful from you.
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On Aug 14, 5:21 pm, snipped-for-privacy@ipal.net wrote:

It happens because every cell has a different internal resistance. You may see nothing wrong with spending the rest of your life trying to get several strings of batteries to a balanced state of charge but most peopls have lives beyong the battery room

Starting small and adding to a system is another false economy.

Start with your energy audit. Size a system to suit and play what if on paper for a while.

Until you know what goes into system design you will fail to understand the relationship of the parts.

That is absolutely right, one dead cell in a bank of batteries more than a year old, you replace the lot.
Parallel strings are more prone to cell failure due to unequal charging.

You will find that is incorrect. You size a system to your need and build it.

Yep. that's often the case. You might try Swinbourne University and see if they will sell you the notes and work book for the "Introduction to Renewable Energy" course.

Yes that's right. They are used to change AC to DC.

The reason why is that all cells have different internal resistance, so they all charge/discharge at different rates. As a battery a single series string has the aggregate of all the cells resistances and is basically balanced. Parallel strings must be constantly monitored to keep them in a balanced state of charge.
And when it comes to using a hydrometer, a single string is a lot nicer to do than a number parallel strings.

A single series string of the correct Ah rating will, with proper care, out last parallel strings. Good quality batteries will have a longer warranty that cheaper batteries.

Time will tell.

Could be.

No, I have given you information. What you decide is your choice.

You got an awful lot if you listened. You are fishing for an answer that fulfills your idea of solar power. Reality may be different.
Learn to size a system, play what if with the numbers, cost your sizings. Parallel strings are second best choice for home power systems and should be avoided wherever possible.

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In alt.engineering.electrical snipped-for-privacy@gmail.com wrote: | On Aug 14, 5:21 pm, snipped-for-privacy@ipal.net wrote:
|>
|> |> |> |> | Batteries are the very heart of your system. That should tell you |> |> | something about the choice of battery/ cell used. You sound like a guy |> |> | looking for an excuse to use golf cart batteries. |> |> |> |> You sound like a guy wanting to tell everyone to do things exactly the |> |> same way you do things, without being willing to tell them why, whether |> |> you actually know why or not. |> | |> | I have told you why, parallel strings do not charge/discharge in a |> | uniform manner. That is the truth. |> |> I'm not doubting the truth of this. I'm wanting to understand the means |> by which this happens, and the degree to which it happens. I want to |> understand this enough to know how well it may be mitigated. For example |> the non-uniform charging may be dealt with by smaller chargers isolated |> on each string (not one big charger trying charge the strings in parallel). | | It happens because every cell has a different internal resistance. You | may see nothing wrong with spending the rest of your life trying to | get several strings of batteries to a balanced state of charge but | most peopls have lives beyong the battery room
This argument can also be applied to limit the number of cells in series, too. Are you limiting your thinking about charging to one which only involves one single charger system and charging all batteries together?
| Starting small and adding to a system is another false economy.
And you suggestion is to start out as big as you will ever need all at once at the start, even though you don't need anywhere near that capacity now, and may not for years?
|> I want to explore means to manage the load balancing before deciding. |> Maybe it will be the case that those methods are not worthwhile. I |> cannot say today. Today is the learning time. The decision is later. |> The conditions will be known then. | | Start with your energy audit. Size a system to suit and play what if | on paper for a while.
This does little good until _after_ one has all the scientific information about batteries.
|> No. I want to know all the reasons for all the choices. | | Until you know what goes into system design you will fail to | understand the relationship of the parts.
Until you are offering this level of information, what good is your advise?
|> What happens if you have ONE string of several very large cells/batteries |> and one dies? Are you saying you don't have to replace the whole bank in |> this case? | | That is absolutely right, one dead cell in a bank of batteries more | than a year old, you replace the lot. | | Parallel strings are more prone to cell failure due to unequal | charging.
But _you_ don't really know why that happens, and as a result of that, _you_ are not able to envision ways around it. But the important thing is, _you_ can't offer real scientific information because you simply don't have it. All you have is specific "do what I (would) do" advice, and nothing more.
|> That is a work in progress. Much of it will be done when I start going |> with some battery power. More will be done later. This is not a one big |> step all the way project. | | You will find that is incorrect. You size a system to your need and | build it.
Such an idealist. Maybe you should work in the _commercial_ UPS market or something.
|> Quite a lot of it is unsupported "what to do" advice. | | Yep. that's often the case. You might try Swinbourne University and | see if they will sell you the notes and work book for the | "Introduction to Renewable Energy" course.
Is that a course on the science of how batteries work, or just a technician class on how to install batteries using common "still thinking in the box" approaches?
|> Rectifiers are made of diodes (or other things). | | Yes that's right. They are used to change AC to DC.
That's not the only thing they can do. Do not assume that because there is more need for one thing (converting AC to DC) that no other need (isolating DC directions) is valid to consider.
|> |> |> I'm exploring all options. I'm not interested in specific advice on what |> |> |> I should do (at least not without well explained why) ... I'm interested in |> |> |> the information to make the best decision in the circumstances that will be |> |> |> present at the time the decision is to be made. |> |> | |> |> | You still sound like a guy looking for an excuse to use golf cart |> |> | batteries. |> |> |> |> I am someone looking for the technical information that would be a valid basis |> |> for deciding what circumstances that golf cart batteries can be used in, and |> |> what circumstances they cannot be used in, where "circumstances" involves a lot |> |> of things that I don't even know, yet. |> | |> | You can use golf cart batteries, they can be wired in series, parallel |> | and series/parallel in as many strings as you feel are required. Most |> | people only ever do this once. Their second battery bank is usually a |> | single string of cells with the correct Ah rating. |> |> And maybe I will migrate to that. One possible path is that I would start with |> a small bank and migrate to a larger bank. I may parallel things in the interim. |> I will also be looking at possible circuits to manage the load balancing between |> parallel strings. Maybe that won't be practical. I'm sure you'll say it won't |> but I want to know why, if that's the case. | | The reason why is that all cells have different internal resistance, | so they all charge/discharge at different rates. As a battery a single | series string has the aggregate of all the cells resistances and is | basically balanced. Parallel strings must be constantly monitored to | keep them in a balanced state of charge.
If the resistances in _series_ are different, that can result in an imbalance as well. It's just more easily hidden becuase the single string still has a behaviour like a single string. It charges up at whatever rate it can and it discharges at whatever rate is demanded up to what it can. All this while one cell is forced to go along until it dies. But you won't see it coming unless you are monitoring each cell separately by some other means. And then you are into the same amount of work to manage the system, anyway.
| And when it comes to using a hydrometer, a single string is a lot | nicer to do than a number parallel strings.
For a moment there, I thought you might say that BIG MONSTER CELLS are easier to measure than dozens of little batteries because the latter means taking a measurement dozens of more times.
|> If it is the case that cost is why I would replace one cell/battery at a |> time, then cost would clearly prohibit the monster cells. | | A single series string of the correct Ah rating will, with proper | care, out last parallel strings. Good quality batteries will have a | longer warranty that cheaper batteries.
But your advice is to not have any strings of batteries at all until such time as one can have the biggest baadest string bought all at once. No economic planning involved.
|> Apparently we have a different understand of this term. I don't know |> what yours is, then. So it's pointless to proceed on this. | | Days of autonomy is how long your batteries will supply your daily | load with no input.
I can't tell you what the future might hold in terms of the longest period that might be when dealing with solar and wind sources. But I guess you are the one with the crystal ball.
|> | Oh, I know what it is you want to know alright. But to understand you |> | need to listen. |> | You want some one to tell you what you want to hear. |> |> You want some one to do what you tell them to. | | No, I have given you information. What you decide is your choice.
You have not giving me the information I seek. All you have given is advice of what you think I should do, and simplistic reasoning (like "parallel will charge unevenly" without considering mitigating methods).
|> I can see I'm not going to get anything useful from you. | | You got an awful lot if you listened. You are fishing for an answer | that fulfills your idea of solar power. Reality may be different.
I don't have the idea, yet. I'm seeking scientific information (NOT The end results like "batteries will charge unevenly") that explains what is going on and what methods might exist to deal with it and manage it.
| Learn to size a system, play what if with the numbers, cost your | sizings. Parallel strings are second best choice for home power | systems and should be avoided wherever possible.
Learn the reality that size needs change, finances are not all at once, etc. Lots of that might work in commercial UPS systems for companies that need a short ride through an outage to maintain "five nines" uptime. My goal does not include any "five nines" uptime for my home.
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On 14 Aug 2008 07:21:57 GMT, snipped-for-privacy@ipal.net wrote:

Start small. If it works go big. Don't even consider trying to start small and expand the system. You can always use the "small" system for a dedicated application (like running a relatively low-drain system in the house - perhaps the computers and entertainment systems, or lighting circuits etc. Always good NOT to have "all your eggs in one basket" with an alternate energy system.

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