How much do you charge?
A bunch of rechargeable AA batteries. (Not shown here are the ones I use with my PocketWizards!)
Working with Speedlites (small, portable flash units), I use quite a few AA batteries. But I don't work with these units quite as frequently as I used to since I have gradually been embracing studio monolights (when weight/size is not an issue).
If you use your Speedlites constantly, run-of-the-mill NiMH rechargeable batteries will do just fine. But if they tend to spend weeks on shelves between uses, you run into the problem of batteries that are no longer fresh when you need them because of their high rate of self-discharge.
For this reason, I have switched to using LSD NiMH batteries that barely lose their charge when not in use for extended periods of time. (Sanyo Eneloop and Ansmann Max-E are examples of such batteries.) LSD batteries are somewhat more expensive (and actually have about a 20% smaller capacity), but they are actually worth it, in my situation, because they are always ready to go.
Storage and Transport
To carry the batteries around, I use plastic containers that have room for four batteries (the same amount used in a Speedlite unit). Each kit is numbered and I place a rubber band on the group that was recharged first. Whenever I switch the batteries of a unit, I go to the kit with the rubber band and place the rubber band on the following (next number) kit, so I always use the batteries in the optimal order.
To identify which kit needs to be recharged, I alternate the direction of the batteries, like so:
Bottom kit is neat (i.e.: ready to use.) Top kit is disordered (i.e.: needs to be recharged.)
With so many batteries, I need a charger than can handle a bunch of batteries. Although this is less of an issue with LSD batteries, not all the batteries in a kit discharge at the same speed (especially in the case of self-discharge). Therefore, a charger that would charge group of batteries together would be fooled and charge some batteries too much and others not enough.
My 8-bay charger
The charger that I use charges each battery individually, so I can use any number of batteries, alternate sizes (AA or AAA) and even different brands and capacities, which means that each battery is optimally charged. Moreover, the charger has a "Reset" option that will drain what's left of the charge down on each battery before charging again, which gives you even "fresher" batteries.
The LCD displays allow you to estimate how far you are in the process, but the charger stops charging when the work is done, so you don't have to worry about forgetting the batteries in the charger for too long. (If that wasn't enough, the charger can deal with any power input (110-240V) and also comes with a car adapter.)