A stator is basically a generator on earlier model outboards, or an alternator on later models. Its job is to supply the electrical energy needed to operate the ignition system on an outboard boat motor. It also contains charge coils that provide charging current for the battery.
A stator works by a series of magnets in the flywheel passing over the coils on the stator. This creates alternating current. On older outboards, some stators were used mainly for battery charging through the regulator, while the ignition system itself was powered from the battery.
Stators come in many output ratings, commonly from 6 amp up to 40 amp. A lot of people assume that even a small-amp stator will fully charge the cranking battery all the time, but that is not true. An outboard stator only reaches its rated output at wide open throttle.
That means with lower-output stators, the charge output through the regulator / rectifier at idle or low rpm is very minimal. If you idle for long periods, troll excessively, or run accessories that draw a lot of current, the battery may actually discharge instead of charge and could leave you stranded.
On motors with less than about 16 amps of stator output, it is a good idea to use a battery charger to bring the battery to full charge before heading out on the water.
You might ask why manufacturers did not simply use higher-amp stators on smaller motors. The answer is that the power required to generate that much electrical output would significantly reduce the motor’s ability to power the boat. For example, putting a 40 amp stator on a 30 or 40 horsepower outboard could reduce power at the prop dramatically. The engine would lug down, overheat, and could even seize if run hard long enough.
