how do I know which to get?
- low-self-discharge NiMH rechargeable batteries are beginning to replace alkalines because they can retain 70%-85% charge at 2-3 years. battery types are tenergy centura and sanyo eneloop. typically can deliver less current (mAH - milli-amp-hours) than normal rechargeable cells. compare mAH and price, and storage time and see what you like. tenergy's D cells are NOT like some mfr's C or AA cells in a D shell. this is a full-on D. tenergy's D cells are NOT like some mfr's C or AA cells in a D shell. this is a full-on D. they are heavy too.
- tenergy premium NiMH cells at handle large draw current for most stuff (but there are special cells which handle even higher current than that I think at all-battery.com). tenergy's D cells are NOT like some mfr's C or AA cells in a D shell. this is a full-on D. they are heavy too.
- 18-minute or 1-hour-fast-charge cells, for when you need speed, and not necessarily long charge life, and don't mind not having the full charge capacity (seems to be falling into disuse?) all-battery.com no longer sells these, but you can find them on ebay.com.
- if you still want to go alkaline, there are industrial alkalines from newark.com and digikey.com. and there is also duracell procell at b&h photo and newark and digikey, especially if you like stuff that lasts long and comes in bulk.
- don't use differenly-charged cells together, the more-charged cell will reverse-charge (bad) the less charged cell. this applies to plain-old alkalines too.
- don't mix/use cells with different make, model, or mAH together in the same device/toy/unit. what you are going to find is that discharge curve characteristics between manufacturers and models are different. if there are differences at all, it could at some time cause the revse-charging (bad) problem mentioned earlier.
- when charging, don't put 2 cells in the same channel. it will do the reverse-charging thing (bad), except that now you have also shorted 2 power cells in parallel, which isn't good, since there are usually slight differences in charge characteristics between cells.
- smart chargers with separate lanes it is ok to charge disparate/different kinds of cells in each channel.
- avoid the 14-24-hour brainless "trickle" chargers for standard cells! toss those at the recycling companies or don't buy they are usually very cheap, or may come with a camera kit. use a smart charger or a smart volume charger always.
- make sure that when you charge batteries, that they are topped off.
If you leave the batteries alone for a month or more, put them back in a smart charger like BC1HU or the fancier TN190. These charge the cells individually at a fast clip. by that time they have discharged some, and some cells have discharged more than others.
Don't mix different manufacturers of rechargeables, different mAH of rechargeables, or with alkalines in a toy or product, or you could cause damage to the cells, cause leakage, or explosion due to the difference in potential. if one or more cells are discharged partially, the cell that is charged the most will reverse-charge the partially discharged cells, and then cause the aforementioned ill effects - they weren't meant to be charged backwards. so when you use them, make sure they are topped off.
beware of cheap chargers such as trickle-chargers that take 14-24 hours to charge cells - they can over-charge or under-charge, or leave one cell charged more than another. throw it away and get a smart charger. you will save your batteries.
NiCd batteries have a memory, and are not as good as NiMH and won't charge much below the lowest charge. however, they can handle high surge currents. they also don't like staying on the charger. when using, they should be discharged fully and charged fully. see wikipedia article. this is the reason for the discharge button on chargers.
Maha Energy makes 2700mAH AA NiMH cells, the next highest (highest is 2900mAH) capacity, but the 2700mAH actually has more real power output (real mAH) in tests. lasts a long time in a most digital SLR cameras. all-battery.com sells 2600mAH NiMH cells by the boatloads (or in boxes of 4), for $1.60/AA each and they're pretty good from my experience.
If you can't wait an hour or more for the charge time, how about 30 minutes? the T8000 is a special fast charger that requires special AA 2300mAH batteries.