battery - What precautions should I take for carrying batteries/shooting in the heat?

So I'll be shooting out in the sun for the next few weeks. My camera was overheating in the sun a few days ago and switched bodies for safety reasons. The flash at the end of the shoot was warm and the AA Ni-CD batteries were burning but the temperature warning never showed up on the flash.

I was carrying extra batteries on my backpack (AA and camera ones) and they were warm, as with everything else.

I'll be out carrying my gear in the heat (during the hottest parts of the day). Is there any advice on carrying extra batteries in the sun or in use during the day? I don't have a backup flash but I have a backup body so that will help the shoot there.

Am I being paranoid about my Eneloops exploding under use in a flash and my camera batteries going bad?

Answer

NiMH (which is what eneloop are, not NiCd) won't explode in heat you can survive in, but may suffer. The most likely effect is a significant reduction in capacity, most of which will recover on cooling and recharging. Reducing the drain on the flash batteries will help (e.g. by swapping between sets frequently, or only using the flash as much as strictly necessary.

The Li-ion camera batteries shouldn't get too hot and (unless they're very cheap no-brand replacements) will have a thermal cutout. They are much more dangerous if they do fail.

Keeping your spare batteries in an insulated container (or just wrapped in a fleece jacket) will limit their temperature rise before you use them. I wouldn't cool them below room temperature before doing this, but they could be wrapped in with something fridge cold. If you do this, be sure to wrap the batteries tightly in plastic (small resealable bags work well) to make sure that no condensation can reach the contacts.

Try to hold the camera and flash out of the sun as much as possible. If you're doing a walk-shoot-walk type of activity, I suggest taking the batteries out of the camera before packing it away and caring them separately. The padding of your bag is a decent insulator and will keep the heat in, allowing it to spread to the camera electronics.