It sounds like your winch may be drawing too much power, which is causing your bike to stall and drain the battery. Even though the battery seems fine and holds charge when the bike is off, the winch might be overloading the electrical system when in use. This could be due to a faulty winch motor drawing excessive current, a poor connection in the wiring, or an issue with the charging system not keeping up with the power demand while the bike is running. Check the winch motor for excessive amperage draw, ensure all connections are tight and clean, and test the battery under load. Also, verify that the charging system is functioning properly to keep the battery charged while using the winch.