connecting batteries in parallel or series, myth and theory

Yes, I've done some anlalytical work in this area. And you are right, if one cell in a series-parallel setup fails for some reason. The string in parallel will discharge itself into the defecitive cell and in fact reverse charge the other good cells in the series string with the defective cell in it. Depending upon the energy content of your cells, you could really have a nice fire. Yes, a diode in each string that is then connected in parallel with another string would prevent this. The added burden is that of added cost for the heavy duty diode you might need and of the voltage drop you would experience from that diode while inoperation. And, of course, you wouldn't be able to charge the string.

My work was with a battery pack that consisted of mutliple series- parallel D cells set up as a back-up for a medical device. Yikes! I hope they took my advice and added the diode to each series string.

In alt.engineering.electrical Ron Rosenfeld wrote: | On 8 Aug 2008 02:43:39 GMT, snipped-for-privacy@ipal.net wrote: | |>In alt.engineering.electrical Ron Rosenfeld wrote: |>

|>| Interesting paper on the topic of parallel battery wiring here: |>| |>|

|>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).

In alt.engineering.electrical m II wrote: | snipped-for-privacy@ipal.net wrote: | |> Right. But there is distinction of paralleling single cells individually |> versus paralleling a single battery of N cells, or a string of N cells. | | | I have never seen single cells paralleled. They are always connected in | series to get the desired voltage. Then, another group of identical | series cells may be paralleled with it.

So the "reason" is "everyone else does it".

Aren't all the plates within one cell already parallel? By extension of that, paralleling individual cells would make sense because it retains the very same concept.

| Yes, I've done some anlalytical work in this area. And you are right, | if one cell | in a series-parallel setup fails for some reason. The string in | parallel will discharge | itself into the defecitive cell and in fact reverse charge the other | good cells in the | series string with the defective cell in it. Depending upon the energy | content | of your cells, you could really have a nice fire. Yes, a diode in each | string that | is then connected in parallel with another string would prevent this. | The added | burden is that of added cost for the heavy duty diode you might need | and of | the voltage drop you would experience from that diode while | inoperation. And, | of course, you wouldn't be able to charge the string.

Paralleled strings mean "-" connected to "-" and "+" connected to "+". That is the correct charging polarity. If one string is longer by one cell than another, the shorter one won't be reverse charged. But it can be _over_ charged. But you face that risk of overcharging even if your charger applies the voltage for N cells when the string only has N-1 cells.

The current flow during charging is reverse of that during use. So it is heavy usage that has the possibility of reverse charging a weak cell.

| My work was with a battery pack that consisted of mutliple series- | parallel D | cells set up as a back-up for a medical device. Yikes! I hope they | took my | advice and added the diode to each series string.

For alkaline cells, which might be what gets put in such a pack, charging them at all could be a major hazard. One string in parallel with another, especially if one is weaker, could lead to that. So with this battery technology, I'd say the diodes are essential.

But my question is focusing on lead-acid cells/batteries intended for power reserve, as typically seen in off-grid systems and in backup power systems.

| I've been running paralleled 12 volt batteries in my motorhome for a decade.

Maybe it is the case that any issues that might exist for paralleled batteries (or strings) increases with the length of the string. One of the things I am considering is what voltage. From that, then, what kind of system. I am mostly centered on 48 volts as there are inverters available for that, and it is also the DC rated voltage for common circuit breakers. This would be for a backup system that may in time become an off-grid system.

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

In alt.engineering.electrical Ron Rosenfeld wrote: | 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).

There are plenty of non-members on that list.

from

Yup, that's why I first looked at

Don't you need to be a certified designer/installer in order that your clients might benefit from the government rebates?

--ron

Mine has 4. No isolation diodes, just short jumper cables between like poles.

John

-- John De Armond See my website for my current email address

Not compression start? I'm disappointed.

Yep, and a whole world of people who aren't. What a surprise. It is possible to be accredited and not be a member.

from

Yes, that's the only reason to be accredited. There is no other benefit to having accreditation. But as I no longer install full systems, why should I keep giving them money.

I suppose that it is the same for the US. Which means that you did not design or install your system. But once installed you became an expert. That is a common occurrence. A person moves to a community, has a house built for them, has a PV system designed and installed for them, and all of a sudden they are master builders and PV experts in the eyes of the uninformed.

I have never received a rebate for my system. I built my house and designed and installed my system.

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.

Read that paper again and work out the physics. Assume two strings of the same make/model/size but one is older. Note that the older string will have a lower capacity. Think about the relative resistances. Which string will supply most of the initial load? That will be the string that cycles most for shallow discharges, and will most likely age the fastest.

I would only use parallel strings if I had no other choice. In my case, I have two parallel strings, but that's because I could not obtain batteries of the size and capacity I wanted to set up a single string. When these need to be replaced, it will be with a single string of 2V cells (which are now available in a suitable capacity for me). Maintenance is much easier with fewer cells to care for.

--ron

from

There you go again George. It seems it is not possible to have a civil conversation with you. You quickly start making false assumptions by which you try to put down myself (and others).

I can play that game also. As I've previously written, I did design my own system, after spending a few years educating myself. I relied on both published experts like Gipe and Strong; and conversations with researchers at NREL who devised simulation programs, as well as engineers at Bergey and Surrette to help me understand things, and provide some assurance that my design would accomplish the goals I set.

I don't pretend to be an expert, as you pretend to be.

And dealing with real experts has made it relatively easy for me to recognize the BS artists and con men such as yourself who frequent both the commercial landscape and the Internet world, and whose skills are limited in so many areas.

It doesn't surprise me that you are NOT accredited. Maybe you had a provisional accreditation at one time based on completing the required training (although I seriously doubt that). But clearly you would have failed the BCSE requirements for Full Accreditation, which require that your skills be verified with real case studies.

--ron

from

Me thinks you have that ass about. I never pretend.

Actually, I am one of the handful of people that has read the BCSE competency standards. Most in the industry don't even know such a document exists, and that includes many of the executives.

This is one of the reasons I no longer have accreditation. The process for re-accreditation is largely dependent on the applicant attending the annual piss up, subscribing to a couple of magazines and installing "ONE" system that may or may not be inspected some time in the next ten years. Oh, and don't forget to hand over the $420 dollars.

Your slur is not unexpected.

Not at all. If you want 48 volts and have two volt cells to play with, that is 48 jumpers for the paralleling the 48 cells. Add another 24 for the series portion. That is a total of 72 jumpers. It's easier to make two batteries first, then parallel them with two jumpers.

No. They are each in series with the one next to it. The power comes in on one terminal, through the electrolyte and out the other plate. There is no chance of any circulating currents within one cell. They all share the same electrolyte bath. If there were only two large plates in a battery, we wouldn't call them paralleled. Take those two plates and roll them up in a cylinder shape. It may look a lot different, but they're still not paralleled.

Now, there *are* multiple plates on each polarity because of space constraints. There's no room for two huge plates with the same surface are. Changing the shape of each polarity plate won't make it parallel.

mike

--=20

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.

Larger oil patch diesels often have hydraulic starters. Huge electric ones are too hard on the ring gear and assorted fittings. A hydraulic motor can be eased on slowly to full power.

There is a smaller auxiliary engine that first starts up to provide the hydraulic pressure. That engine usually drives the alternator also. In nicer set ups, the coolant for the large engine is also warmed up a bit before start up. They have to, as in cold weather, the lubricating oil is like molasses and 12 or 16 pistons don't like moving in it.

mike

--=20

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.

You mean compressed-air start? During my year of OTR truck driving I saw one ragged-out old yard dog with air start. Sucker fired the thing off right beside my sleeper while I was napping. I almost hit the roof.

Batteries and starters are so good now that starting a big diesel is just like starting a car. Mine has glow plugs but I've never used 'em. The company tractor that I drove didn't have any start-assist. Just turn the key and it cranked. Even in -20 deg weather. It was the big 13 liter, 525 hp Detroit too. Amazing.

-- John De Armond See my website for my current email address

Not at all. If you want 48 volts and have two volt cells to play with, that is 48 jumpers for the paralleling the 48 cells. Add another 24 for the series portion. That is a total of 72 jumpers. It's easier to make two batteries first, then parallel them with two jumpers.

Yes your correct plates within a single cell are paralleled and always 1 more neg plate per cell. The power available from a given cell is given by the area of active material in contact with the electrolyte as it's a chemical reaction that provides the power the more power required the more plates are paralleled inside the cell, which is why you end up with a 2 volt nom cell and AHrs dependent on number of plates inside a cell.But it is hard to keep the parameters of each cell identical, in series this is not much of a problem in discharge there will be slight diferences but will be corrected but cells or batteries in parrallel have general recharge problem as the charging device only see's the best voltage of the strings and inevitably sulphation will occurr in some cells this will degrade the available capacity of the cell and will lead to early failure unless a manual charging system is employed to ensure all cells are fully recharged at regular intervals.

No. They are each in series with the one next to it. The power comes in on one terminal, through the electrolyte and out the other plate. There is no chance of any circulating currents within one cell. They all share the same electrolyte bath. If there were only two large plates in a battery, we wouldn't call them paralleled. Take those two plates and roll them up in a cylinder shape. It may look a lot different, but they're still not paralleled.

Now, there *are* multiple plates on each polarity because of space constraints. There's no room for two huge plates with the same surface are. Changing the shape of each polarity plate won't make it parallel.

mike