Hi All -
I'm trying to decide on a battery technology to use for a project - it needs 2.7V min, though it can take up to 5.5V. The case it's going in will fit two AA batteries (about 2 1/4" x 1 1/4"), though I'm not stuck on that form-factor - rectangular or whatever will work, so long as it will fit in there.
I also want to include a charging circuit for it if possible. So one can run it off the batteries, or plug in a wall-wart to run/charge. Technology-wise, it looks like the choice is between :
4) Lithium - non-rechargable
5) Standard alkaline - non-rechargable
Max current draw will be around 80mA, usually somewhat lower. (Lots of LEDs). Preference would be for easy availability - as in in-stock at RadioShack :)
One non-rechargeable possibility that looks pretty good is using two 3.6V 2.1Ah AA batteries in parallel for 4.2Ah. That would be good for around 52 hours of use I believe at 80mA draw. Did I do that right ?
There's absolutely no way to recharge lithium cells is there ?
NiMH AA cells have large capacities, but are only 1.2V per cell. I would need either four 1/2 AA cells for 4.8V, or a 3-cell pack for 3.6V that fits in the space of two AA ...
NiCD doesn't seem to have particularly good capacity, plus there's the memory effect to have to deal with. I suppose the charging circuit could be "smart" and either not actually charge it until the battery is almost completely depleted. Or it could cycle the battery itself, first discharging it all the way, then charging it back up again.
That requires a load though - hrm, could turn on all the LEDs full bore, but still, not the best of solutions.
So, what are people using these days ? How would you go about this ?
I've had mention in another forum to use something like a MAX756 step-up converter, which will allow pulling the last dregs/ergs from the batteries. Sounds good. I wonder if there's a combination chip that does also does charging ? Maxim has a lot of charging chips - have to look through them.