for best results:
- avoid 15 minute chargers. They're running the cells hot and so they wont take a full charge, & if there's a fault they can go bang. A 2500mAH will take roughly 10 Amps to charge from flat in 15 minutes, the charger is pumping a lot of juice.
- avoid chargers that do not heave single cell monitoring. Many only work in 2s or 4s. This will not detect or correctly charge a pair of unequally discharged batteries or a pair where one is becoming faulty. They probably bank the cells like this to avoid having too many channels with high current, they just run a higher voltage across 2 batteries in series and they share the same current.
- avoid "timer chargers", you need to be using a negative delta V charger (-dV or -"triangle symbol"V). Voltage of a cell rises as it approaches full charge, as soon as its hit full charge the voltage then starts to drop, this is the time for it to switch to trickle. Timers are sometimes used in addition to -dV to provide a safety backup or stepped charging cycles.
I found the the energizer 1 hour charger does the job for 4 cells. It's got -dV sensing, timer too, 4 channel independant monitoring, with car lead.
Like this:
http://www.amazon.co.uk/Energizer-Hour-Charger-AAA-Batteries/dp/B000BPCW00 (I'm not recommending this seller, I've never used them, its just a quick link to demonstrate the charger I'm talking about), Jessops do them too:
http://www.jessops.com/Products/ProductDetail.aspx?SkuId=30986