There are 5 possible causes for poor battery charging:
Bad battery, bad/loose/corroded wiring connection, bad fuse, bad rectifier/regulator, bad stator (usually in that order)
How to test:
1. Take battery out of ski to auto parts store and let them load test it (I've seen brand new batteries bad from the start)
2. Carefully trace then disconnect, clean, and reconnect all wire leads to and from the battery.
3. Check/change the fuse (On my ski it would be the 20 amp one)
4. Check the output voltage of the rectifier/regulator (should be 15 volts). You really need a 6 pin test harness to do this properly, but you can get some idea, by checking the voltage at the battery while the engine is at idle (should be near or above 13 volts) Obviously, if you get a bad reading here, you need to check the stator next, to be sure it is not the problem.
5. Check the stator/pickup coil output voltage. This step could only be done easily by a dealer unless you have the proper test harness (and I think it is a different harness than the one used for the rectifier/regulator.