Just wanted to pass along a few recommendations, based upon many years of professional experience, with regard to AA rechargeables. If you are just starting out, maybe this will be helpful and save you some money and frustration.
1) I wouldn’t purchase the ‘grocery store’ variety of AA rechargeables….they’ll do in a pinch, but there are better options albeit at a higher price. I do recommend a set of lithium batteries for each speedlight to keep in your equipment bag for backup if all else fails.
2) The batteries I use for both personal and professional work are: Powerex 2700 mAh, Delkin Devices Extended Power 2900 mAh (from Amazon) and AccuPower 2900 mAh. There are a lot of choices out there and you should do your own research; however, these are the AA units I know I can count on for longevity and rapid recycling when photographing events like weddings.
3) As important as the batteries themselves are the battery chargers; here again, forget the run-of-the-mill units. Make an investment in a ‘smart’ charger which will not only charge, but condition them as well thus extending your batteries’ life. Look into the Powerex Smart Charger, Maha Powerex Wizard One and related Maha chargers, the AccuManager 10 and chargers from Tenergy and La Crosse Technology. Yes, they are more expensive but you will save money in the long run.
Take Care and Keep Shooting!
(The brands mentioned here are registered trademarks of their respective manufacturers / I am not a compensated endorser of any equipment or accessory provider)