My Tohatsu 8hp outboard has an 80w charging system, which I discovered did not have a voltage regulator after the installation of my solar charging system. After some research I found this was standard on most small 2 stroke outboards. It seems that the outboard manufacturers assumed that anyone using a small outboard for charging would probably never have fully charged batteries, so the batteries would always accept the charge, and voltage regulation wasn’t important. Probably a fairly reasonable assumption unless you have some other chargeing system onboard like I now do.
I discovered the lack of voltage regulation when trying to top off the charge of my secondary battery with the engine before combineing the batteries as the primary was already topped off from the solar panel, and I didn’t want to charge one by discharging the other. After about 10 minutes of running the engine at crusing RPMs I checked the battery voltage at the terminals with a multimeter, and found it was over 16v!
The charging system works off of a dedicated charging coil under the flywheel, that based on engine speed produces a AC current up to 40v at full throttle, and a rectifier converts the AC current into DC current at (roughly) half the AC voltage. That means at full throttle the engine will be putting out roughly 20 volts, way too much voltage if you don’t have a half charged battery hungry to absorb the power.
With the help of George, aka ThatBoatGuy, a moderator at seafaring.com, I found a combination rectifier and regulator that was meant for a Briggs & Stratton engine, model 797375, and confirmed the wireing of my engine, and how it install it.
Since the regulator only has three 16ga wires, two yellow AC Input wires, and one red DC output wire, installation was pretty simple.
The first thing I did was remove the old rectifier to determine how it was connected. Unfortunantly I had to cut the output wire from the old rectifier as it was hard wired to a Y adapter that was used to feed the wireing harness for the engine, so my goal of having a direct part for part replacment wasn’t quite possible, but with the installation of snap plugs on the end of the cut wires would allow easy re-installation of the old rectifier if I needed.
To begin the actual installation I cut off the red and yellow connectors, leaving the bare copper wire. Since this is a marine installation not the lawn tractor this was intended for, I tinned the end of the wires with solder to help prevent corosion. Then I used Anchor adhesive lined shrink wrap butt connectors to add about 8 inches of 16ga tinned marine wire to the end of the wires and sealed the connectors with a heat gun. At the end of the extensions I installed snap plugs to plug into the existing wireing harness without having to modify anything.
I hooked everything up, routed the wires, cleaned the paint off the mounting point so it would make a good electricial connection (it grounds via the case), and bolted the regulator in place.
I started the engine to test the regulator, and no matter what RPM the engine was at, or for how long, the voltage at the batteries never got above 14.2v, Success!
UPDATE 10/2/11: After a summer with pretty poor winds that caused us to motor a lot, this is still going great. I also found through my monitoring of the charging system that the batery voltage reading displayed by my chart plotter is not exactly accurate. It seems Garmin tried to factor in a certain degree of expected voltage drop, and thus it actually reads anywhere from 0.2v to 0.6v higher than the batteries are actually at.