[Tom Wesenberg]

How old is the battery? As P.S. said a new battery should read higher, probably closer to 6.4 to 6.7 volts. I'm still using the $2 battery I bought from a junk yard 8 years ago and it also reads 6.2 volts and works fine.

I wonder if you are putting out more than 4 amps? Have you checked the output with a quality amp guage? I find original ammeters fairly accurate if the pivots are oiled, but I find repro ammeters often stick and don't have an accurate reading.