12V lead acid battery - what mA to charge at?

[alo]

Hello...

I have a 12V 2.8Ah sealed lead acid battery... and a battery charger that can be set from 50 mA to 300mA.

What should I set the charger at?

Not too sure if the battery is any good, but I thought I might try and recharge it to see if I could get some use out of it. It had been a backup battery in an alarm system.

Thanks

[Andrew Gabriel]

A guide is to divide the capacity by 7, and use that as the max current. That would be 400mA max in your case. Lower is OK.

However, SLA chargers don't normally have current settings. Are you sure the charger is intended for SLA batteries?

They can be quite useful. I have a 7Ah one that was changed out

12 years ago, and still has its full capacity. Depends heavily on how it has been treated. Running them flat is not good, and neither is overcharging them.

[Peter Parry]

Lead acid batteries are charged by setting the endpoint voltage (13.8V for a 12V battery). NiCd/NiMH batteries are charged by setting the charging current.

Sealed Lead Acid (SLA) are slightly different in that they are charged by setting endpoint voltage but with an additional current limiter and deliberate small overcharge. This is usually done automatically by the charger.

It isn't usual for a SLA charger to be adjustable in the way you describe - are you sure it is a SLA charger and not a NiCd/NiMH one?

[alo]

Well this is the item I bought:

there is no 6/12V switch on the one I received.

[Andrew Gabriel]

I suspect it's just a power supply with a series resistor; I have something similar (although the resistor is just the transformer windings and rectifier resistance). Off-load, it's about 17V, and left connected, that's what it tries to get a battery up to, which will wreck it. If you use it, monitor the battery voltage with a meter and switch off at 14V (or 13.8 as Peter says). When the battery is charged, the voltage starts climbing quite quickly if you continue trying to charge it with inappropriate controls. I have an old 3.2Ah one waiting to go to recycling which someone gave me after overcharging it with this sort of charger, and the sides are ballooned out along the cell boundaries.

[Peter Parry]

That appears to be a Skytronic 690.003. Albeit a rather expensive one, they are mostly about GBP5. It is NOT a sealed lead acid battery charger but a simple unregulated charger for vented batteries.

you got a part number or any other identification on the device you have?

[The Natural Philosopher]

10 hour rate is generally reckoned to be safe 'forever' so 280mAh.. or go for a 1 hour charge at 2.8A.

[alo]

On Mon, 20 Jul 2009 17:15:26 +0100, Peter Parry wrote: .

All I can see on it it is:

MW2168

Maybe I should buy something else.

Thanks

[NT]

It may be ill suited to repeat use, but if the OP just wants to charge it once to see if it still works, just charge at c/10 for 10 hours, after which it'll be more or less fully charged. C/10 for 2.8Ah is

280mA.

NT

[Peter Parry]

Google is wonderful thing :-)

Is this it :-

so you have a constant current NiCd/NiMH charger which won't do your SLA battery much good at all.

a simple version of the sort of thing you need.

[Fredxx]

I can only recommend he uses the charger in conjunction with a DVM, where he stops the charging process as soon as the terminal voltage rises above 14 volts.

You're assuming the battery hasn't got sufficient residual capacity, for the battery not to become overcharged.

I think the OP's best solution is to obtain the right SLA charger.

[alo]

Well I ditched the dodgy charger bought off ebay, and bought the one above.

The battery I was testing is defunct, but a new replacement one (for my shed alarm) will now be kept charged by the above unit.

Thanks