Battery behaviour in parallel

[ch2]

If you are charging two batteries set up in parallel, will they always see the same charge current? Say you had a charger capable of delivering 20 Amps, do the two batteries get 10Amps each or will one see a higher current (if it has a different charge for example)? I guess this has to do with the impedence of a battery and how that varies through the charge cycle.

Another question - how often do batteries go closed circuit?

[Nando]

YOU SAID : how often do batteries go closed circuit?

What do you mean : CLOSED CIRCUIT ?.

Batteries in parallel will take current based on their ISR ( Internal Series Resistance ) their charge and their chemistry if different.

Nando

[jimjjnn]

I think he means how often do batteries get shorted out between plates.

[ch2]

Yes Jim, that is what I meant. The batteries that I am using are AGM so I imagine that it must be very improbable.

[ch2]

Nando,

If the batteries are the same (Vision 6FM100), how much would they vary? There will be some differences due to manufacturing tolerances but would they be significant?

[Flux]

If the batteries are the same type, size and age and the connections are of the same length and resistance then they should see equal charge( may take a while to sort out if they start in a different state of charge)

If things are not identical or the same age or the lead resistances are different then the current will be different.

Batteries normally only short out when they are old and abused ( usually after a few years).

I suspect from the question you are worrying about what happens when one does short.

When it happens, it will only be one cell, the good cells of the faulty battery will be dragged up to gassing volts to maintain the terminal voltage. The terminal voltage (and other battery volts) will be dragged down a bit and you will need more charging to keep the volts up.

Nothing spectacular will happen, you will need to be observant to notice it quickly and sharing fuses or breakers will do nothing to protect you from it.

The internal resistance of batteries is very low but the difference in volts between charge and discharge will prevent drastic charging currents.

With wet batteries you should keep an eye on the sg with a hydrometer and you will spot the failing battery.

With SLA the first indication may be venting of the good cells in the bad battery and acid spray about them, or you may spot it as poor performance and constant low volts.

Flux

[ghurd]

SLA - venting - spray?  AGM or gel or something else?

We have used up a lot small ones of 'them' without a physical visual sign. Just unable to hold or maintain a charge.

G-

[Nando]

Flux gave you the answer

Nando

[Ungrounded Lightning Rod]

If the batteries are the same type, size and age and the connections are of the same length and resistance then they should see equal charge( may take a while to sort out if they start in a different state of charge)

Also at (about) the same temperature.  (Which is why you want paralleled batteries at the same height from the floor in your battery rack.)

On my travel trailer (with two batteries) I tie them together with equal lengths of the same guage wire, then hang the ground on one battery's neg post and the hot on the other's plus.  That way each battery sees the same resistance from the tied points:  One jumper's worth.

You can generalize that by daisy-chaining a parallel string of batteries and hooking the plus connection to the battery at one end and the minus to the battery at the other.

(Paralleled streetlights, especially on bridges, are often wired that way (at the cost of an extra conductor the length of the string) so they're all the same brightness.)

[Ungrounded Lightning Rod]

If you are charging two batteries set up in parallel, will they always see the same charge current?

The voltage on a lead acid cell varies by about 10% or so with the state of charge.  So (all other things being equal), the less-charged battery gets more charging current and things tend to even out.  This effect distributes the charging evenly both across the surface of the plates, between the plates within a cell, and among the component batteries of a battery bank.

As others have pointed out, the big caveat is "all other things being equal".  A lot of things besides state of charge affect the voltage of a battery:  Age, temperature, geometry, chemistry (SLA vs. glassmat vs. gell, water level, etc.), amount of sulfation, degree of equalization between cells, whether there are defective cells, and so on.

If one or more of these factors are NOT equal the battery tending to lower voltage comes to full charge while its paralleled partner is at significantly less than full charge.  Then on discharge the less-full one reaches minimum charge while the more-full one is still above it.  The result is that the capacity of the paralleled bank is reduced by the percentage difference between the most-full and most-empty battery.  If the lowest one is at 80% charge when the highest one is full, you lose 20% of the ENTIRE BANK's capacity.

So ideally all the batteries you intend to parallel should be purchased at the same time from the same manufacturing lot, wired and mounted with care to equalize their thermal (ambient temperature and cooling) and charge/discharge environment.  Failing that, at least they should be the same model from the same manufacturer and with about the same history.

But if, say, you get a good deal on a bunch of used forklift batteries that have been rotated out on scheduled replacement rather than failed in service, go for it.  Then charge 'em up, check 'em for approximate balance, hook up the ones that are close, and maybe try to rejuvinate the rest into a better match.  Check 'em from time to time for balance and pull those that are starting to cause problems.

Having five sets of batteries rather than four will compensate for a 20% mismatch in charging states, and having a whole bunch of big, cheap, batteries will give you more power storage than a small number of pricey, minty-fresh, perfectly balanced, small ones.

[The Crazy Noob]

Can you give any more info on how you "daisy-chain" a parallel string of batteries?