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Most cheap AA battery chargers charge two batteries at a time. Many devices use 1 or 3 aa batteries, meaning it's difficult to make sure you've always got two compatible batteries in the charger.

My fairly limited understanding of chargers is that they trickle feed energy into the batteries until the voltage in the circuit increases to a theshold, indicating that the batteries are charged. So it seems to me that I could short one of the two battery holders, and put the battery in the other one, such that the battery charges twice as fast.

Is this possible? If so, how can I figure out if it will change too fast and damage the battery? If not, is there any reasonably reliable way of charging a handful of single AAs, given only a cheap double AA charger? (I would prefer not to damage the charger, but don't mind modifying it, if necessary). Also, most of these batteries are NiMH, but there are a couple of lithium batteries, and there might be a NiCad or two as well.

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You could figure out which method your charger uses by comparing the battery capacity and the charging time: As I understand a trickle charge will be done with about 1/30C, which would mean that charging would take about 30 hours. I also recommend to look for a better charger if you can afford it, so you can discharge your batteries properly before recharging. I invested some money there, because the eneloop batteries I use aren't cheap too, so I want treat them properly. – 0x6d64 Jul 28 '12 at 10:52
@0x6d64 Well, most of the chargers I have have the current output printed on them, so I can just figure it out from that, no? I know, if I were to buy a new charger, I'd buy a better one. But as it is, I prefer not to create more land-fill. and I'm broke :) – naught101 Jul 28 '12 at 11:25

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If the charger is a genuine slow trickle charger then one battery in place of two batteries in series may well be OK.

Otherwise, shorting out one battery position will have variable results.

This may scramble the few neurons available to simple battery charger controllers and mean that the battery never charges or over charges and may not stop charging. Clamping a two battery power supply to a one battery voltage will probably cause more current than intended unless they saw you coming and took due precautions.

Two batteries in series is not a good match to some of the best charge termination methods. Often used is negative delta V where terminal voltage at constant current drops slightly at fully-charged. With two batteries in series this "signal" is more likely to be missed.

One possibility is to use an oldish deadish or low cost battery in the second position. This should not be so dead as to read "high" under charge but need not have the capacity of the good battery.

A diode string that drops about 1.4V when used in place of the second battery may work OK. Use of 2 x 1N400x diodes MAY be OK. Maybe a 1N400x + a Schottky + ...? A zener or clamp regulator that drops 1.4V may work.

Charging a NiCd battery in a NimH charger may work and may not. If you are able to measure battery voltage under charge and stop charging at about >= 1.4V it will work OK (probably).

Lithium primary batteries must not be recharged.

Lithium secondary batteries MUST NOT be charged in any charger not designed for them, that their days (and yours)(and the charger's) may be long on the face of the land.

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Thanks for the detailed answer. The note about delta V is good to know. I think I'll have to do a bit more reading before I understand the last paragraph :) – naught101 Jul 28 '12 at 14:40

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