Can I Charge 2 AA batteries with my battery charger (with a car adapter) connected to a 12v 2AH battery?

Question

If I connect my AA battery charger (with car adapter) holding two drained 2600mah AA batteries to a fully charged 12v 2AH Sealed Lead Acid (SLA) battery, will it completely charge them? Do I need a bigger SLA? 7AH perhaps? Is there some formula? Where can I find these formulas?

Answer 1

>> 2AH

That is kind of small for 12V.  But given you are going from 12V to 3 volts (1.5 x 2), you should be able to charge them fully.

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2AH means that it can draw 2 amps for 1 hour to drain the battery.  Given you have 12v, you will NOT be draining the full 2 amp.

Volt x Current = Volt x current (with 100% efficiency).

Good Luck..

Answer 2

2 Amp-hr means you can discharge at a rate of 2 amps for a maximum of 1 hour. Or 1 amp for 2 hours, or 1/2 amp for 4 hours.

Now your 2600mah are 2.6 amp-hrs each, so you need 5.2 amp-hours to charge them. Which seems like more than the 2 amp-hrs your lead-acid battery has.

But amp-hrs are a strange unit, used only for batteries, for advertising reasons. The actual unit should be watt hours, which is a unit of energy. You can get watt-hours by multiplying by the battery voltage.

So the lead acid battery holds 2 amp-hr x 12 = 24 watt-hrs.

The AA batteries of 2.6 amp-hrs are actually 2.6 x 1.2 = 3.1 watt-hrs

So now it looks better, 24 watt-hr battery to charge 2 - 3.1 watt-hr batteries (total 6.2 watt-hr).

And you would be fine, except for inefficiencies in the system. Specially in the charger, which may only be 50% efficient. The charge-discharge cycle of batteries is 80% at best.

So we actually have 24 x .5 x .8 = 9.6 watt-hrs. But still higher than 6.2, but not by a lot. Given the uncertainties like efficiencies, I'd try it first.