Batteries in parallel

"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

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

"Mark Kelepouris"




**   Absobloominglutely  !!!!!!!!!!!





** Certainly.

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

 If you expect them to be around for long.




........  Phil

Ta.

Mark Kelepouris

snipped-for-privacy@killspam.internode.on.net says...


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 ?

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


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

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

says...


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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   and now with auto 10-15 min timer for engine illumination option.
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http://niche.iinet.net.au

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.

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.

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?

"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

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.

"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

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!

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
{:-(