Charge 2 x 6V on 12V Charger

Yep. Thats exactly what a 12v SLA is.

NT

If they are at the same state-of-charge when you start, yes. Otherwise, the battery that has the lower open circuit voltage might be damaged.

You'd be much better off using a 6 V SLA charger and putting the batteries in parallel, because each battery would be less likely damaged through cell reversal.

--Winston

"Winston = Wanker " Olin Frank

** Err - the other way around f*****ad.

** Advised against by all battery makers.

** Completely nuts.

Cell ( voltage) reversal can only happen during discharge.

... Phil

Aie-Yup! Jess more 2V cells in series, son..

Not quite correct; the statement is _close_ when mentioning a cell in that series connection. Excess charging current (the stupidity of a fast charge) candamage any battery (one or moer cells). Nominal charging current will not do any damage (stated is the two batteries are in same/similar condition).

damaged, no.

Charging cells in series does not cause any cell to see reverse voltage. Ever. Connecting lead acids in parallel can cause serious problems, both to the battery and to the nearby user, and is never recommended.

NT

Two 6 V SLAs.

One is charged to nearly 100% or say 6.3 V. We place it in series with one that is charged to about 35% or say 6.05 V

At 10 A discharge we can think of the top battery as a resistor measuring 0.63 ohm and the bottom battery equivalent to a 0.60 ohm resistor.

The top battery will have 51% of the charge voltage across it and the bottom battery will have only 49%.

If we begin charging them at say 10 A, the battery charged to nearly 100% will get about 3 W more charge power than the relatively discharged battery.

Successive charge and discharge cycles will cause the difference in state - of - charge to widen until eventually, the bottom battery sulfates and fails.

That's why we never add acid to individual cells of a conventional wet cell battery. It causes an analogous imbalance which can result in the failure of the battery, if it cannot be equalized.

--Winston

Reverse voltage can happen on discharge, not charge.

One can equalize the charge between them and connect them in parallel safely. Many diesel - powered cars had two 12 V batteries in parallel, for example.

Here, we see three batteries in parallel for trailer use:

--Winston

?

We've no idea what the OP's discharge conditions are

Pdiss is usually a non-issue

no, when the fuller cells are fully charged, further current only charges the less charged ones. Charging equalises.

Sure, but its not appropriate advice for someone that is discharging them separately, and doesnt even know whether they can be connected in series or not.

NT

No it won't. The battery that is discharged the most gets the most charge power.The lower the charge on a battery, the higher its internal resistance. The .63 ohm equivalent top battery will receive higher power than the .60 ohm equivalent bottom battery. P = I^2 * R, so the top battery gets more.

Seems moot anyway, as the op specified batteries of the same brand, capacity and usage. If one wants to discuss batteries of different usage/discharge level/specs/whatever, then it is best to independently charge, where the charge can be tailored to the individual battery.

Ed

(...)

Would it be good practice to assume that the OP's discharge conditions are optimal?

I don't think so.

A large fraction of that power is not dissipated but continues to charge the battery beyond 2.12 V per cell. This is a largely useless 'surface charge' but my point is that the battery does not know when to stop charging (until the electrolyte boils off that is). :)

For sustained, large amounts of charging current, yes.

We 'get away' with a long duration, high current 'equalizing charge' with wet cell batteries (and the accelerated conversion of electrolyte to vapor 'gassing') because we can carefully top up the cells with distilled water to keep them from drying out.

We have no way to replenish water in sealed lead acid batteries, so we don't use an aggressive equalizing charge on them.

Beyond a threshold of difference of charge, it's a race between the weak tendency of the charges to equalize and the strong tendency of the higher voltage battery to 'hog' the charging power (in series).

(...)

'Blanket' statements can be misleading, with costly consequences.

SLA batteries in good condition, of the same description and age, that have been operated under identical circumstances can *generally* be placed in series to form a battery of higher voltage or in parallel to form a battery of higher current.

Anyone doing so will want to perform 'due diligence' to assure themselves that they are being safe and will get the desired electrical characteristics because no two batteries age in exactly the same way.

--Winston

(...)

In a parallel circuit, yes. These batteries are in series, though.

I don't believe that is true. I don't think ESR changes much with state of charge.

I think that my 'equivalent resistance' of a relatively discharged battery looks lower than the 'equivalent resistance' of a fully charged battery. 'Equivalent resistance' is just ESR plus (the battery voltage divided by the charge current).

Watch the ammeter next time you charge a battery and see if charge current starts out low and then increases as the battery charges, or starts high and then decreases with increasing battery charge.

(Then you say:)

I agree with your third statement but not your first or second.

:)

Note that the OP did not mention state of charge. One would not place them in series (or parallel) without being sure they were at the same charge level.

I have two 5 Ah SLA batteries of the same lot number from the same manufacturer. They were both used in burglar alarms. One of the batteries measures

6.36 V and the other 6.05 V because only the second alarm was turned on. Can I expect these batteries to perform to spec if I use them in series? No.

I agree. That is the most prudent approach.

It is also moral and correct to match the battery's state of charge, then charge them in series or parallel using the proper sequence for the chemistry in question.

--Winston

"Winston"

** Should be " charge " - right ?

... Phil

Generally it is not a big deal. After all, those 6 SLA's are actually 3 2V SLA's in series!!!!! So your just adding to it.

In any case the issues is that one or more cells maybe "dead" which can cause problems. If the cells are functional and you have some way to monitor the temperature or have a safe location for charging and can deal with possible destruction of the whole battery then it will be ok.

For example, suppose one cell in a two cell battery has a short. What happens? The other cell is being charged at twice the voltage(and hence current) it should be. This will cause the battery to heat up and a possible explosion, meltdown, fire, etc. With a proper charge this shouldn't happen as the full discharge voltage is generally higher than if a dead cell occurs, e.g.,

Full charge: 4.2V Full discharge no load: 4V Full discharge loaded: 3.5V One cell shorted: < 2.1V

So a decent charger should be able to easily detect a dead cell this case and refuse to charge.

Understand that most batteries are composed of multiple cells so what you are doing is not any different than what the manufacturer does except they has a single container. In some cases such as LI it maybe a different story.

If you take any due precaution you should be just fine.

agreed

Pdiss is still a nonissue

agreed

charging power is a non-issue, charge is what counts, i x t. Both get the same, inevitably

.

Recommendations to connect lead acids in parallel, without the necessary knoweldge or checks, is liable to lead to a bad result.

I doubt the OP will. People don't usually.

NT

(...)

Not 'absolute charge power', I agree. 'Relative charge power' is the core issue, though.

Which is unfortunate because, in our thought experiment, the top battery requires no more I or T and the bottom battery requires nearly a full dose of both I and T if it is to perform to spec and indeed, survive over the long haul.

It would be much safer to just remove the batteries from the series string long enough to charge the 'lower' battery independently until it's state of charge matched that of the upper battery, as Ed alluded to earlier.

(...)

As is true for series - connected batteries. :)

(...)

Thus the value of 'Plain Applied Technology'. Our discussion will show up in subsequent searches and will highlight this important consideration so folks who want to do it right can continue to inform themselves.

This is good.

--Winston

its irrelevant. charge i x t is what counts, and thats invariably the same for both bateries.

/If/ the batteries are discharged asymmetrically, one will eventually need water adding. That's all. Normal maintenance.

If the op had a 6v charger, I dont think the question would have been posed.

worst case outcome: normal maintenance required after a while.

NT

(...)

I don't believe the 'bottom' battery would survive. Stored in a consistently discharged state, it would sulfate and fail. If it did survive, then the 'top' battery would have to be over charged pretty consistently in order to fully charge the bottom battery. I don't think it would be wise to abuse either battery in these ways.

I've never added water to an SLA. I see that it is possible though.

That will be what I learned today.

Lots of us have lab supplies that can be used to top up the charge on a battery until it can be paralleled with it's mate for equalization.

Lots of us are virtually enmeshed in abandoned wall-wart supplies that can be pressed into service, for that purpose, with a series resistor and careful monitoring.

These aren't called 'battery chargers' though. :)

Worst case outcome, failure of the lower battery due to sulfation and / or failure of the upper battery due to the effects of over charging.

--Winston

Connect the two 6V batteries in parallel until equalized, then charge in series. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
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** The OP does not have a 6 volt SLA charger - he has a 12 volt one.

Most SLA chargers work as a current limited PSU with a fixed voltage limit of 13.7 volts for a nominal 12 volt battery. 13.7 volts is carefully chosen so 6 fully charged cells will draw only a small current - the terminal voltage must be raised to 15 or 16 volts to overcharge the battery.

When a discharged battery is connected, charging commences at the current limit and as voltage rises it gradually tapers off until at full at charge ( ie 13.7 volts ) it is only a trickle. The limit current value depends on the amp hour rating of the cells so charging is completed in about 2 to 4 hours.

So, if you connect the wrong voltage battery ( 6V instead of 12V ) high current will flow until the battery explodes - the better designs recognise when the voltage across the battery is dangerously low and refuse to apply charge.

The interesting question is what happens if one or more cells in a battery are in a much lower state of charge when connected to the charger.

If it is only one, charging will drop to a trickle with that cell not fully charged. This is because 5 fully charged cells require 13 volts to be applied to pass significant charging current and the remaining cell requires about 2 volts. The total needed of 15 volts is simply not available.

A similar scenario applies for 2 out of 6.

However, if it is 3 out of six charging will still continue at a slow rate until all 6 cells are topped up.

... Phil