I remember the good ol' days of Alkaline's, then came NiCad which offered rechargeability but not much longevity and lower voltage. Then Energizer came out with their lithium AA's which I thought were a godsend for on camera flash but expensive, then came NiMh which had promise and a lot more longevity than NiCad but would lose their charge if sitting dormant and their ability to quickly recharge a flash wasn't the best so the Energizer lithium's were still in the camera bag for serious shoots. Over time the formula for NiMh improved and capacities increased and then Eneloop was one of the first to offer low self-discharge and high recharge cycle. I then thought I found a diamond in the rough with NiZn (Nickel Zinc) technology offering 1.6v and extremely fast recharge; however, they don't last as long as a good NiMh but they powered up my flash's blazing fast, but after a few years the batteries and the charger died and it seemed the technology never took off likely because of the onset of our reigning DC champion Lithium Ion. Only issue is Li-Ion doesn't come in AA size, well, that is until recently. Li-Ion promises the same voltage and fast charge (of devices) as the tradition Lithium battery but with the ability to re-charge over and over. None of the top brand manufacturers have put their name on any Li-Ion AA batteries as of yet so the bulk seems to come from Chinese off brands, I bought some Fenix branded versions by they are a bit pricey and I'm not sure if they really give an advantage though I have not full tested them as of yet.
One caveat to all this, for years I was a diehard Powerex fan until one day, within a few months of my warranty expiring on my Powerex charger it died, just wouldn't charge anything anymore, I spoke with Powerex customer service and was polite in asking if they could do anything to help me out given that I did not abuse the product and they politely told me to pound sound, so I did the next best thing in a capitalist society - I threw away their defective product and bought a La Crosse charger in its place which has lasted me over 6 years now. That being said, Powerex does make good products with typically high QC so I still recommend them even though I did not have a stellar experience with their CS.
Today I buy Eneloop batteries for general use, I buy Eneloop Pro for anything needing fast charge (like a flash). I will likely try some Powerex low-self discharge batteries again at some point in the future. I should also note that I have bought a number of "cheap" Chinese branded NiMh because, well, they were cheap; however, most of these bite the dust pretty quick while my Eneloop's keep on going strong. I have had bad batteries with all the brands I buy, it's just something you have to deal with from time to time.
My last point, keep a close eye on specs, batteries like Powerex Pro and others that offer super high mAh are often not low self-discharge which means they start losing their charge pretty quick after pulling them out of the charger. I like to charge up a bunch of batteries and keep them around for when needed as I often forget to charge ones right before a shoot, YMMV.
