If you're doing very heavy shooting then get the Canon CP-E4 external battery pack.
ALSO... when using this pack you still need the 4 internal batteries in the flash. The external pack holds 8. But you get a chocie of puttting it in a mode where all 12 batteries (4 internal + 8 external) work together OR a mode where the internal batteries run the electroncis, communication, etc. and the externals only power the capacitors to do the firing. This is the better mode to use because it means the internal batteries will never get hot. But it also means your distributing the load across 8 batteries instead of 4.... so you're halving the load on each battery.
When Lithium batteries are being rapidly drained, they will heat up. This is "normal" for that type of battery. I typically use the Panasonic Eneloop rechargeables which are "low self discharge" batteries. A normal rechargeable will lose power just sitting unused (self discharge) and can lose as much as 20% of it's capacity just in a day or two. They really need to be charged up just before they are used. The low-self-discharge are much slower... they "claim" that after sitting unused for a year they only lose about 10% of their stored power (I'm not sure how much of that is marketing vs. reality but I do know they hold a charge much better.)
I *think* eneloop batteries are actually NiMH (Nickel Metal Hydride).
If you use ordinary single-use alkaline batteries, then I've learned to be obsessive about removing the batteries when not using them. Alkaline batteries can leak if they are allowed to be run completely dead ... and they can slow-drain to completely dead just sitting in an used device.
Tim Campbell
5D III, 5D IV, 60Da
