My guess is that Battery volts is a load test simulating a battery being recharged by using a known resistance. Its probably a moderate load with the alternator being spun fairly fast, but I couldn't guess what the speed was. Since the accepatable range is in Volts rather than amps, I agree its a really crude test. There may be some inputs the store employee sets to adjust the load or rpm for different units. Need to find someone who spent some time running such testers (and paid attention). My recollection was the only thing they did was look upo (or ask someone who knew) how to hook up the test leads. Probably different setup for self exciting and internally regulated alternators.