More Batteries

Rechargable lithiums from cell phones are available through surplus electronics catalogs. Be sure to purchase a recharger with them, since lithiums need their own charger to maintain their lifespan. You'll need to solder connectors to the batteries since there aren't battery holders made for them (the phone has its own set of tabs that connect to the contacts in the battery case). I've found three electrical contacts int eh batteries I've nmodified. Two are needed for charging, the third is a control circuit to prevent over charging or over heating.

Near Space Batteries

It can get cold inisde a BalloonSat. Alkaline batteries will have a problem providing current in high drain cases. Normally I use lithium batteries because they can handle -60 degree F temperatures. Good photo-lithium batteries are avilable at Walmart and Target in the camera department.