connecting batteries in parallel or series, myth and theory

And, I haven't seen where anyone has posted anything that would counterdict what you are saying, (especially with all the conditions you added) nor did I notice where anyone was suggesting that parallel batteries were a first best option, (always or not always).

Don't you think there may be a few less budget constraints, for those building "a commercial UPS system (or submarine battery)" system? (Boy!- the "homepower" system I could build if I had an unlimited budget, it would not look anything like what I can realistically expect to build on my own. It would be neat to design a system that included dedicated "operators" and maintenance men. That could really change a few things.)

Would you be surprised to find that "homepower" battery banks were maintained at something less than military precision or that maintenance schedules might be somewhat different without having dedicated personnel standing watch?

Do modern subs use lead acid batteries anymore? (I doubt they are building any subs with an eye to saving the taxpayer some money. )

Are there a lot of "Homepower" battery systems running at

You could have added the traditional DC electric vehicle conversion, they normally have serial battery connections to provide a high DC voltage to a drive motor (sorta like your submarines).

While all of us can, and do, appreciate the experience an ex-submariner can bring to the discussion, is it always going to be the answer? Will it ever become OT?

If we all had the options and resources of those who build and maintain our submarine fleet, to draw on, when considering or building "Homepower" systems, then we might want to pay closer attention to how it was done in W.W.II subs. As it is, we shouldn't be constrained to, how it was done on a sub. Nor would the design that the sub builders decided on, be likely to match "Homepower" needs. (Unless, we include Al Gore's Houseboat.)

Luck; Ken (Bozo)

Hmm.. lets see what I can drag up.

APC also likes paralleling batteries. Here is a wiring diagram for extending the uptime on one of their UPS's.

Here is another one.

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.

The thing about computer back battery systems is that the batteries are replaced every 5 or 10 years Whether they need it or not. They are also not cycled often.

Cobblers.

It's the ONLY way I'd hook up syncro inverters for parallel operation.

The difference between the "charging voltage" and the "dischange voltage" on the lead cells is on the order of 0.2 volts. If there are more than 10 or so cells in series the "normal" stack would not excesively charge a "one shorted cell" stack. But a nominal 12 volt system might be a problem which may explain why that fire truck mentioned in another post almost set itself on fire.

The "one short" stack would tend to be over charged and monitoring the the chargin voltages would show that something is wrong.

| Are there a lot of "Homepower" battery systems running at | 430vDC or 270vDC? Neon John is having trouble finding | simple, cheap 48vDC inverters.

Yikes! Where does one find fuses/breakers for 430V DC?

| The difference between the "charging voltage" and the "dischange voltage" on | the lead cells is on the order of 0.2 volts. If there are more than 10 or | so cells in series the "normal" stack would not excesively charge a "one | shorted cell" stack. But a nominal 12 volt system might be a problem | which may explain why that fire truck mentioned in another post almost set | itself on fire.

I'm thinking that I would go with a 48VDC system, so that would be 24 cells. But my concern also includes the issue of _reverse_ charging a bad cell in the string. It would seem to me that the higher system voltage raises the voltage point where a bad cell could fail to keep up and end up reversing.

| The "one short" stack would tend to be over charged and monitoring the the | chargin voltages would show that something is wrong.

Would it be good to have separate charging controllers per string?

| 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?

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.

| 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.

I take that as a personal insult. There is nothing wrong with golf cart batteries if his storage needs are modest. If the discharge is kept at some respectable level, they will last decades. Most batteries don't die, they're murdered.

Not everyone needing a car needs a limousine.

mike

--=20 __ __ __ __ __ __ __ __ / /\ / /\ / /\ / /\ / /\ / /\ / /\ / / / /\ \/ /\ \/This space for rent/\ \/ /\ \/ / /_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/

Densa International=C2=A9 'Think tanks cleaned cheap'

Due to the insane amount of spam and garbage, I block all postings with a Gmail, Google Mail, Google Groups or HOTMAIL address. I also filter everything from a .cn server.

Yes. It is true that most batteries are murdered. OTOH golf cart batteries commit suicide.

The point is that batteries are the heart of a system. Bad heart, bad system.

You're right, there is nothing wrong with golf cart batteries, they make great door stops.

I'll tell you something about golf carts.

This guy goes to his doctor and the doc says that there is nothing a bit of exercise won't make better and suggests that the guy take up playing golf. The idea is of course that the guy will be walking around the golf course at least once a week.

The guy says, "Hey that sounds great." He goes out and buys a golf cart to drive himself around the course. A year later he drops dead from a heart attack climbing the four front steps to his house after a round of golf.

This is the same mentality that buys golf cart batteries for a house system and wires them up in six parallel strings.

Buying your heart from the cheap shop is suicide.

Yep, but they still buy the limo.

Now you can take that as an insult if you like. But, Golf cart batteries only last for decades if they are used as door stops. Most people who are capable of learning only ever buy one set of GC batteries for their house system. Those that have bought two sets have a learning disability.

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.

Do you think that commercial UPS systems build them in series because it costs more than a bunch of 12V batteries in parallel? Wouldn't the manufacturer that wanted to corner the market do a 'forehead slap' and say "Doh! We can use the cheap stuff like homepower systems and beat everyone else in the market on price!" Ever wonder why none of them have opted for the lower price "homepower" system methodology??

Properly sized cells in series cost *less* in the long run.

Yep. The reactor plant needs a back up. Diesel engine and battery.

I should think since there are a lot of grid-tie inverters with voltage inputs in that range, there should be inverters available.

The topic is the best way to connect cells together for large capacity. Just about every commercial battery bank is strickly a single string in series. Even those manufacturers that are trying to make money selling their product (where cost is obviously very important).

Maybe they are drawing on knowledge and experience beyond what is available in a "homepower" news group??

daestrom

Funny, you go to GE's links for these UPS's

So it would seem they didn't follow the UPS manufacturer's installation guidelines?

And what APC *doesn't* tell you is how long a multi-parallel bank like that will last. Why would they care if you buy the batteries from them (I don't have a kind opinion of APC based on experience with their batteries lasting less than two years).

I don't see anything in those that suggests two parallel strings. Guess I'll have to take your word for it.

daestrom

I think that they design and build their systems to meet their or their corporate customer's needs, in line with what they may be willing to pay. In the case of submarines it would be more whatever is needed, for national defense, regardless of the cost. In any case, they can pretty much afford any battery they can dream up, the average DIY homepower, doesn't have that luxury.

My point was that because of that last factor they are able to use materials and equipment that would never be available to anyone designing and constructing their own Homepower solutions. Also that they can design for a prescribed maintenance environment, that again includes resources beyond that likely for the Homepower enthusiast. (I have trouble getting the kid to mow the lawn, once every blue moon; wouldn't want to consider the results of expecting him to play Plant Engineer.)

For the 430v commercial UPS you described "36 12v batteries connected in series", are we to assume that they had a 430vDC inverter, that provided a voltage and current capacity to run the commercial installation? Do you think that if the commercial firm could not afford that 430v DC inverter, they would have gone with that design? The DIY homepower guy is much more limited to what is more generally available or what he can modify to serve his needs. No contracting with those firms that won't talk to you without a proper letterhead.

Then there is the question of what is appropriate for the scale of the project. Any significant commercial operation will consume a great deal more energy than the average wasteful American home; the successful Homepower setup uses less power than the average wasteful American home.

The design requirements aren't at all the same.

Still lead acid? Not some of the newer much more expensive exotic batteries? They should want to gain the space savings if nothing else. In anycase, I haven't seen any sub battery banks available at my local government surplus auctions, and short of that I will have to use what is available, and affordable.

So you see the homepower guy, being able to afford the products of Co-Generation contractors?

I think there are already postings and links posted, in this thread, that bring your "Just about every commercial battery bank is strickly a single string in series." statement into question. There is also the fact that "battery banks" come in all sizes, to serve many different power storage requirements. (While it seems to be my fate to run into those who think this way:) You can't just pick the largest or most extreme example you know anything about, and say all must conform to that design.

Even if it is the best design for that large scale implementation, its requirements and design features, even some of its safety features, may have little or no relevance to the design of a smaller system operated in a different environment.

Maybe they are drawing on knowledge and experience Not "beyond" what is available in a "homepower" news group?? But not relevant to what is discussed in a "homepower" news group??? It is great that they know how to work with huge, warehouse sized battery banks, of hundreds of six foot tall 2v cells, good for them. What they may need to do to design, manage and maintain such systems, can make for interesting reading, and who knows there may be some valuable lessons to be learned, but the average homepower effort isn't funded or equipped to conform to any major commercial battery bank design, in most aspects.

Luck; Ken