I accidentally flew my ignition battery down last week. The engine started cutting out in the air at over half throttle but ran fine below that. I landed and checked my battery. 1.6 volts under load, 3.5 unloaded.
A battery that reads 4.5 volts under a 1 amp load will fly the plane for a long, long time.

Many people don't realize that once you foul a spark plug, it's good only for the trash can. If you pull it, blow it out, and stick it back in like you do a glow plug you'll be rewarded with a sore starting arm.