Similar to the drill idea, I've got 2 test batteries made outta old laptop batteries. I use them I just alligator clip the positive to the power to lights wire, and check if the lights work without having to start the bike. I got one wire coming out at 6v and one at 12. They're cheap. works great, easy to charge. Same cells as most drill stuff, but easily bought online.
I've also used the little on/off NiMH packs for bikes w/o lighting coil. they're super duper easy. like 20-30 bucks on ebay/amazon:
last a long time, easy wiring, complete sealed unit with a on/off switch and even a USB to charge other junk if you wanna.