Evinrude/ Johnson voltage regulator

I have a 1989 25hp Evinrude. These motors do not use a voltage regulator on the charging system. According to the manual, you should only use a non-maintenance free battery, the old kind that you fill yourself. Supposedly, this type of battery acts as a sort of regulator, and a maintenance free battery does not. I am having trouble finding one of these batteries. I'm using a maintenance free battery and during high rpm operation, voltage reaches 16 or higher. Is there any way to convert the charging system to a regulated system? Anyone have a shortened battery life by using the newer type batteries with this system? Thanks for any help.

Your engine uses a rectifier. The rectifier really only "regulates" half of the charging system. This is why you are seeing the voltage build up to 16V. This is normal and the easiest way around it is to turn something on in your boat when running longer distances. Running lights will do the trick.

Thank you for the help.