Batteries in parallel

I was always under the impression that putting lead-acid batteries in parallel was inadvisable and yet, I've seen a commercial installation with two 12V 100AH sealed lead-acid batteries in parallel. I tried it myself and there doesn't appear to be any problems. Then I put five 12V

7AH batteries in parallel but one of them got a bit warm when charging the five together so perhaps it's not a good idea after all. The "warm" battery appears to charge and discharge normally on it's own so perhaps it just has a slightly different make-up to the others. Anyway, what are the technical objections to batteries in parallel? Alan
Reply to
Alan Peake
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"Alan Peake"

** Only works out OK if they are electrically identical and always in the same state of charge.

Had enough making suree individual cells in one battery comply with that - let alone two independent batteries.

...... Phil

Reply to
Phil Allison

Alan wrote,

I was always under the impression that putting lead-acid batteries in parallel was inadvisable >

Hi Phil, does this apply to nicads?

If I were to wack 6 Nicad AA's (different brands possibly, but the same (cough) specs) into a basic charger, should I make sure they're all competely discharged first? Or is the rule... dont mix 'em.?

Mark kelepouris

Reply to
Mark Kelepouris

"Mark Kelepouris"

** Absobloominglutely !!!!!!!!!!!

** Certainly.

Then charge them slowly - like over 20 hours or more.

If you expect them to be around for long.

........ Phil

Reply to
Phil Allison

Ta.

Mark Kelepouris

Reply to
Mark Kelepouris

Generally not a good idea in anything that is mission critical or where the danger of a fire is an issue. In a remote community where you dont care about explosion or money then sure. But in my setup definitely *no* !

The only exception would be correctly sized PTC between devices to the common bus with some appropriate alarm system and recovery system etc.

Why,

Well because any sink by one LA battery, for whatever reason, is capable of pulling a heck of a lot of charge from all others its connected to and pretty damn quickly. ie. One little insulator punch through makes the cell that has that short able to pull huge currents and end up with a big melted mess or worse a big fire fed by lots of high temperature plastic, sputtering acid and H2 gassing...!

Nicads 'might' be a little more acceptable but I am unsure as to the thermal coefficient runaway issues, some might have inherent mechanisms of safety but the general consensus by the *battery manufacturers* is no, dont do it.

In either case of LA or NiCd - charging might be an issue, in that one cell or battery or however you connect them might be more 'willing' to pull charge than others. Theory might suggest that, hey once its charged its not going to pull more, but due to any SPC issue re manufacture and batching you *will* end up with diversity and this means greater risk and this means, in the presence of a source of energy with low impedance, great potential for really catastrophic knee effects causing disaster. I say no.

If I *had* to put lots of batteries in parallel then I would take great care sizing PTC and any other methods to remove/disconnect a cell that became unreliable.

Having been in power systems and had the honour of putting a power system of 130KW in a remote jungle community in Malaysia and being exposed to other attempts and their repercussions I say a definite no no, dont take the risk...

One would ask, Phils remostrations aside, why you 'need' to do this, cost, cheap, unwillingness to size, delivery, change to site specs what etc ?

--
Regards
Mike
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Reply to
Mike

It is not recommended to put lead acid or variations AGM, SLA etc in parallel. Reduction in battery life is generally 25%, but there is no fixed amount as it depends on each set of batteries how identical they are. I have seen from little reduction to 60% reduction in battery life from 2 batteries from the same production batch. Issue is you have no way of telling how compatible the batteries will be or if any one cell is going to fail. To make a blanket statement 25% appears to be a good medium average figure. If you need capacity and cannot get a single battery of sufficient capacity, to go series with 6.0v or 2.0v batteries.

Because you have seen a commercial installation done that way certainly does not mean anything. There is so much disgusting quality work around in low voltage you can find easily any number of disasters.

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Reply to
Landline

"Baron von der Slahp"

** It may be safer to parallel individual cells, in the case of NiCds.

Then, if a whisker of cadmium ever shorts one cell, its partner will deal with that pronto.

...... Phil

Reply to
Phil Allison

Well, I don't really need to do it. It's just that our local amateur repeater site is so wired by a comms tech who does many similar installations on remote, unpowered sites. with no apparent problems so I thought I'd do the same at home. It's no big problem here to just switch each battery in as required so I'll do that before anything dramatic happens. As for the repeater site, I'll bring it to the attention on the responsible people. Alan

Reply to
Alan Peake

"Baron von der Slahp"

** The *best darn advice* I ever read (by Rod Cooper in WW mag) about using NiCds was NOT to use them to make up a series connected battery !!!!!!!!!!!

Instead - design your device ( with an inverter if needed) to use a single cell - as do many shavers and electric toothbrushes.

Since all the makers specs, when read carefully, only applied to the use of a single cell - this must be what they intended.

...... Phil

Reply to
Phil Allison

How does this relate to the dual battery systems in cars? I used to have my starter battery up in the engine bay and also a deep cycle in the back of the van, the two were seperated by a solid state device to allow the starter to charge first.

From what I'm reading here that could have made a horrible mess if one of my batteries was damaged. Am I missing something or should 4wd'ers be a bit more careful before they go adding a second battery?

-- Michael Heydon

Reply to
Mike

In this case you have a 'something' between them :)

In this case its ok as the batteries are some distance apart so the 'something' and the cabling will help share load and if one cell fails in one of the batteries then its unlikely to make a catastrophic mess, the solid unit between them should be able to sense that and disconnect - well if its smart enough - so you dont end up with two low charged batteries...

Regards Mike

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Reply to
Mike

You can parallel both Lead Acid and Nickel Cadmium batteries that are in a similar state of (dis)charge quite happily. The problems start when a Ni-Cd cell dies and goes short circuit. Fizz, pop etc. It's rare, but it does happen. Make sure you fuse one or maybe even both strings. Lead Acid cells don't seem to lose internal resistance quite as dramatically under normal circumstances. You will find huge banks of paralleled Lead Acid accumulators right throughout the standby power industry.

Don't know about Lithium Polymer, Nickel Metal Hydride or any of the others. Would be very interested to hear from someone about those?

Reply to
Baron von der Slahp

Thanks Phil,

Hmm, that starts to bring up worrisome memories of trying to electrically remove neglected Ni-Cd dendrites. I've actually had a Ni-Cd vent directly into my eye whilst undertaking such a silly act. I was very lucky, so I don't really want to push things by going along that path. Buy new nicads everyone. They're cheap now and always work much better.

I think the parallel pair of individually fused series strings has the advantages maybe, in terms of lesser fusing requirements. Serviceability and safety of recharge cycles in the failed condition becomes a matter of how exactly the configuration fails. Having the fuse blow on the good side leaves you with an under-voltage supply that may be quite seriously overdriven during recharge. If the fuse on the damaged, shortened series branch fails you're left with a happy supply voltage, halved output capacity, and a similarly halved recharge cycle time.

Failure modes for the series string of fused, paired parallel cells are also pretty ugly. One cell shorts, blows the fuse on it's side of the parallel branch, and you end up with a really screwy configuration that I wouldn't like to predict the behaviour of at all. If it blows the fuse on the accompanying good cell's side, then we get the under-voltage supply with serious (though slightly less) potential for being overdriven during recharge again.

Hmm, whicheither of the neither really. They're both a bit of a worry. It's little wonder that battery manufacturer's recommend against all of these.

Reply to
Baron von der Slahp

Go to s.c.e.battery and search for an excellent series of posts within the last three(?) months on this.

Basically, two seems fine practically. Anything more is almost a surity to cause problems.

The better idea is to switch to series connection of two X 6 V 200AmpHr, or even 6 X 2V humungous AmpHr.

It is really a matter of restrictions of getting the batteries into position {:-(

Reply to
Terryc

These batteries are usually designed to support different loads. The standard car battery does the usual starting, lights, etc. The second battery does added devices. So they discharge through different circuits.

The charging system may charge them in parrallel but it should be wired in such a way that one can not discharge the other.

Reply to
Terryc

It astonishes just how much energy is stored in just a single Ni-Cd cell, and what you can achieve with it. I used to make single cell powered strobe lights for model rockets - just an AA or N size cell, chopper, transformer, rectifier, storage cap and Xenon tube trigger circuit. A tiny N cell could easily pump out the 450-500V required, flash for very short duration's at a steady 2Hz, and keep that up for the required 3 minutes or so before the flash rate rolled off. I think the N cells aren't even 1/2 the length of an AA either!

Reply to
Baron von der Slahp

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