Run motor and measure voltage across the battery terminals. As Brian said, you should get ~ 13.5 to 14.5 vdc. If not, voltage regulator/rectifier is the most likely failure. To be sure, measure AC voltage from stator to the voltage regulator/rectifier. For most Merc's, this should be ~> 16 VAC at idle and is the yellow leads to the regulator/rectifier. If the AC voltage is > 16 VAC, then the regulator/rectifier is bad. There are other tests as well. Get a service manual (the OEM one) for details. One of the most typical causes of regulator/rectifier failure is high resistance battery connection(s) or removing a battery lead with the engine running. Good luck.