Re: Kicker alternator / charger
The part catalog for your engine shows a "charge coil", it is item 29 on this web page:
Johnson Ignition Parts for 1987 8hp J8RCUD Outboard Motor
That looks like just an AC coil, so you would need a rectifier, and that would give you only unregulated DC voltage, depending on engine RPM. The part catalog shows a "rectifier assembly" as well, it is item 43.
As I am not familiar with that engine, I am not sure if Johnson made anything to control the voltage. The coil looks small enough that its internal resistance would limit the charging current if a battery is connected.
Other usual solution would be just a high powered Zener diode across the rectifier output limiting the DC voltage together with the internal resistance of the coil. The Zener diode could be part of the rectifier assembly.
A "real solution" (stable voltage output independent of engine RPM with good conversion efficiency) would be an electronic DC-to-DC converter after the rectifier and no need for limiting Zener diode. If anything like that is available, it probably would be an after market solution.
Charging from an idling engine is a good question. The output from the charging coil might not reach the desired voltage. In that case you really would need a DC/DC converter. How much power, i.e charging current, such a small coil would provide at trolling speed, that is another question. Those coils are usually sized for the RPM of a cruising engine.
I am sure, that others more familiar with that particular engine and the problem of charging battery from a smallish engine would chime in!
Good luck, Peter
