Hey! I recognize that problem description. Welcome to the forum.
Have you checked the voltage output of your battery pack? You did say it was home-made, so verifying all the wires are correct and the output voltage is ~ 9V wouldn't be a bad plan.
Also, and I'm in full-blown speculation mode, since my camera hasn't shown up yet, I've been reading the repair guide and it talks about a switch that determines if you're using the AC adapter or the battery pack (so I assume it's in the slot the battery plugs into**). It says if that switch is defective (or not being tripped by the battery pack), then you might get a battery warning light.
** There's a slot in the AC adapter about half an inch down from the "top" contacts. I assume that allows the switch to be open. Does your battery pack press that switch down?