OK- - - -couple of comments- - - -no, the 12V trickle charger won't work well as a battery maintainer for the 6V. You could find a heavy duty wire-wound potentiometer like a really big radio volume control and use it as a voltage dropping resistor to bring the voltage down to 7 volts or so for a battery maintainer for the 6V. Drop me a PM or an email and I'll draw out a circuit you can use for that. You'd need a 0-200 ohm potentiometer with about a 500 watt rating for the circuit I have in mind.
Even with the sulfated battery that would result from low electrolyte level, the 15+ volt reading is a little high from the regulator. There's really no need to replace the regulator- - - -just try adjusting the one you've got first. The set of points behind the Field wire is the voltage control. You need to very slightly decrease the spring tension on those points by either bending a tab or turning a screw. A very small adjustment makes a BIG change, so proceed carefully. Adjust the voltage to 14.5-14.7 volts with the engine at a fast idle (1500 RPM) against a new, fully charged battery. Be aware that the setting will lower slightly when the regulator cover is installed because the cover is a better conductor of magnetism than air, so adjust it .1 to .2 volts higher than the final reading you're looking for.
The final check on voltage is the electrolyte level in the battery. If you're needing to add water frequently, the voltage adjustment is a bit too high. Good luck!
Jerry
