balancing battery charge

[Bill Wright]

Given two 12V lead acid gell batteries, both new, both the same type, but from different sources, how do I balance the charge before fitting them in a 24V appliance?

Bill

[Adrian]

Chuck 'em both on the same 12v smart charger until they reach "float"?

[newshound]

But not at the same time.

[Dave Liquorice]

How are you going to be charging them, series @ 24 V or parallel/individually @ 12 V? Even if you start with exactly the same charge in both it won't be long before one has more (or less) capacity than the other. Series charging is likely to make things worse.

Making sure that you never discharge below about 1.1 V/cell(*) will help maintain balance but they won't stay in balance.

(*) Memory, have a google for the battery makes specs, you might be able go down to 1 V/cell.

[Brian Gaff]

They will never be matched in any case as obviously they are themselves made of cells in series and each one will have slightly different capacity. When any cell goes down, it is reverse charged by the current flowing through the load and the impedance rises as the voltage drops, so to speak. I used to use these batteries in models and really all one could do was try to catch the voltage dropping below the norm and then recharge them both. Brian

[tony sayer]

Interesting..

Over time I've been to transmitter and comms sites were they sometimes have stack's of 2 volt cells around 500 odd amp hours each, but they are just strung together to make 24 or 48 volts, so are they likely or do they get one cell go "astray" and if so what can be done about it?..

*Astray meaning go wrong capacity wise in some way or 't other;!..

[Dave Plowman (News)]

I don't think it matters. Just start off with them both fully charged in the normal way. After all, each one is cells in series, so you're just adding more. And no two cells will be *absolutely* identical.

[Gazz]

You do a periodical balance charge, which holds the batteries at a higher than normal voltage for a time, my motorhomes charger used to do this every 35 normal charging cycles, it would take the battery bank* up to 15 and a bit volts for an hour or so, causing them to gas vigorousely, part of the battery maintinence was to top up the cells with distilled water every now and then, i didnt go as far as adding the auto pumped watering system tho, as i found a check every 3 months was adequate,

*6 x 232 Ah 6 volt batteries in paralell and series to give me a 12volt 696 Ah battery bank.

BUT, that was for flooded vented cells, not sure what the equalizing voltage for gell cells is, if there even is one, As they are likely to be used in a granny chariot i guess, just charge them both up to full on a 12 volt automatic charger, then connect them together for your 24 volt bank, and charge as one battery from then on.... as that's what the charger will see it as.... and it is just 12 x 2.2 volt cells in series.

[Fredxxx]

Why not? Just parallel them up for 12V.

[Andrew Gabriel]

For float (up to 13.8V), they will be fine together in parallel, as you can leave lead acid batteries on that forever. (Ideal float voltage varies a bit with the precise battery type.)

For cyclic charge (>= 14V), then charging together is not so good as one may be gassing for a while as the other is still charging. As a one-off, it probably won't matter, but you wouldn't want to do it regularly. It is done (in both series and parallel) where they are identical, same age, and replaced together at end-of-life, but then the life you get is that of the shortest lived.

[Fredxxx]

Personally I would have got identical batteries as different makes will likely have different characteristics.

Given the starting state may be different for each battery I would place them in parallel and charge with a charger designed for gel batteries that hopefully has a tight charging spec.

Is this for an alarm or similar, where the batteries will be continuously float charge in situ?

[F Murtz]

Just stick them in series and charge them the same as a 24v battery. My truck had that and did just fine.

[Harry Bloomfield]

Bill Wright brought next idea :

Charge each individually using some sort of controlled charger, to bring each to a full charge after which they should balance.

[Bill Wright]

They are identical, but bought from different places on the same day!

Bill

[Dave Liquorice]

I doubt they are 500 AHr SLA cells, much more likely to be open vented wet cells. I'd expect those to have routine maintenace of checking the electrolyte density (aka the acid concentration) and topping up as required. An open vented cell can be maintained, an SLA can't be by it's sealed nature. This makes open vented cells are also far more "robust" than SLAs.

[Brian Gaff]

Well if they have gone to the trouble of making them individually, one is forced to the conclusion that they change out the duff cells.

Certainly in the model scenario mentioned earlier, the ideal would have been individual cells as the cells in the pack always had at least one that went down first. Brian

[Brian Gaff]

It won't matter the effects everyone has noted p will still occur I'm afraid. Brian

[meow2222]

SLAs arent hard to open and add electrolyte - theyre not sealed despite the name - but the electrolyte pool is very small compared to flooded cells, so electrolyte issues happen much faster.

NT

[Rick Hughes]

a 4 stage charger that includes balancing charge for Gel batteries will take care of it for you .. CTEK make one