GPZ600 Ice Racer Charging Problems

I ahve been given an ice racing sidecar based on a 86 GPZ600. The motor is strong but it has a propensity to boil batteries!!.
Does anyone have either a wiring diagram so I can troubleshoot why I am getting between 16 and 18 volts across the battery at fast idle, or better still, if anyone has a design for a skeleton wiring harness which still retains a charging circuit?
I considered running constant loss but discounted that as the only way to start the beast is on the starter and I would like to keep the battery fully charged just in case I stall it partway thru a race.
All help appreciated.
Picture of the beast to follow.

Thx
Bob Manifold
203 213 2949

sounds like a regulator problem to me