I shoot with a Canon EOS 5D and an f1.4 50mm lens. This combo takes great photos in the dark, but the auto-focus sucks.

One solution it to put a flash on it and set it to "emit, no fire" so that the flash uses a red light for focus assist, but doesn't actually flash. I don't like this solution because the flash doubles the size of the thing I'm lugging around, and I bang it into things all the time.

Another solution is the Canon Speedlite Transmitter ST-E2, which is basically a flash without the flash part: it does focus assist and does it well (it also does other things that I don't care about).

Here's the problem: unlike a real flash, it doesn't run on AA batteries. It runs on these shitty 6v "2CR5" batteries, the disposable version of which are around $5 each. They don't last very long, maybe 1500 or 2000 shots.

I've bought rechargable 2CR5 battery kits from two different manufacturers, and guess what, they don't work. More specifically, they seem to not fit. I can only get the ST-E2 to function with them if I keep the lid open and manually jiggle the battery. They're not making contact somehow. I've tried the obvious things, including taping shims around them, to no avail. Maybe my ST-E2 is defective, but since it always works fine with the disposable batteries, that seems unlikely.

What I'd like is something that plugs into the hot-shoe and does focus-assist, but that runs on AA batteries. Failing that, a rechargable 2CR5 that actually works with the ST-E2. Any suggestions?

Previously.