I'm a heavy user of two GPS units. I've finally settled on the Sanyo eneloop batteries as being the best option. Sanyo is very honest with their capacity numbers. The eneloop cells also hold a charge for several months with very little self-discharge, which is important if you have extra cells that don't see constant usage. About a 18 months ago, the Sanyo cells were the only ones being built in Japan where the quality control is excellent ... that was the tipping point.
My main charger is a LaCrosse BC-1000 (no longer available as a new product). Recently I needed to charge more than four cells at a time, so bought a LaCrosse BC-700
unit which is every bit as good as the previous model. I've stopped using the higher charge rates and only use the 500mAh rates, so both units function the same for my application. Both units will only work off household voltage ... hasn't been a problem since I have more cells than necessary.
I can recommend the above components without any reservation. There are other good chargers available ... John's MaHa charger is a good one, just a little too expensive for my application. What ever you buy, just make sure they charge each cell separately at a low charge rate ... a charge rate of 20 to 25% of cell capacity works well with negligible heat build up in the cells when charging. As others have already stated, heat is the enemy of longevity. My eneloop cells are approximately 18 months old and still don't show any reduction in capacity.