I have a 91, tohatsu 70. my battery wont charge, I'm getting 8.5v at idle and around 15v at 3/4 throttle. what should I be getting? checked all wires/connections, look good.
8.5 volts at idle says the battery is not charged. It should read 12.6 or thereabouts with the engine off. 15+ volts at 3/4 throttle is pretty normal for an engine with an unregulated charging system. Don't know if your Tohatsu has a regulator or not. What may be going on is that you actually have a battery that this on the way to battery heaven. Have it load tested at any auto parts store. It may also be that your voltmeter is not reading correctly. A battery that is fully charged should read 12.6 volts whether the charging system is working or not. It should not immediately drop to 8.5 volts.
I agree 8.5 volts normally says the battery has 2 bad cells. So has 4 good cells and 2 bad cells. I would fully charge the battery with a battery charger then let it sit for a couple of hours then check the battery voltage before starting. It should be 12.6 volts. 6 cells at 2.1 volts per cell or 12.6 volts.
Turn on the navigation lights for a light load and check voltage before starting.
Did you keep your battery charged over winter and spring?
I would never check with the battery not connected.
With an alternator most of the time it will blow tha alternator.
Outboards many times do not use alternators but instead stators.
Better and safer to check at the battery with the motor connected.