What are the best batteries for this? Did you just get the NiMH recharge batteries? Thanks.
There are a lot of batteries out there to choose from but I have come to trust the Sanyo Eneloop "hybrid" batteries because they can hold a charge for a long long time. I can't tell you how many times I have put a speedlite on my hot shoe and the Duracell or Energizer rechargeable batteries were completely dead. Whatever you choose go for reliability over raw power.
Now, I have quite a collection of Sanyo Eneloop (2000 mAh) AA batteries but I am slowly and steadily replacing those with the newer Sanyo XX Eneloop (2500 mAH) batteries. You'll get a faster recycle time and around 25% extra pops with the XX series (rough estimate). Four XX series AA batteries were enough to last me through several hundred photos in my remote 600EX-RT units during a wedding reception at night. These seem to be a good mix of power and reliability.
In the near future, I plan to update the review with some measured times between a few different batteries and the difference between using the 4 AA batteries in the speedlite body and adding an additional 8 AA batteries with the Canon CP-E4 battery pack. Using the battery pack obviously makes the recharge faster and will last longer since you now have 12 batteries but at the moment I only have enough XX series batteries to fill the 3-600EX-RT speedlites and not the battery pack (still using the 2000 mAh for that) but I need to compare for myself to see if it would be worth paying all that extra money and ditching my older batteries. By the time I finish updating all of my batteries to the XX series, Sanyo is bound to release the even more powerful XXX series with 3000 mAh and that would be something.http://www.bhphotovideo.com/c/product/766725-REG/Sanyo_XX_Powered_by_Eneloop.html
Also, you should avoid quick chargers and especially avoid rapid chargers. They will destroy your batteries and they will never reach their full potential after a charge. In order to get the most out of your batteries you need to use a slow charge and that's when I found the La Crosse Technology Alpha battery charger through a forum somewhere. I had to purchase mine from Amazon. If you have a lot of batteries you might want to get two of these since it takes so long to charge (usually at least 8 hours). I wish they had a charger that held more than 4 AA's.