The battery not charging can be attributed to a few things. The battery itself can be faulty and not able to take a charge. The alternator on the engine can fail and the battery will not charge. If the engine has a regulator, it can fail and cause the battery not to charge as well.
On the front of the engine there is a little silver box. On that box there should be some numbers etched into it. If you can reply back to me with those numbers I will be able to tell more about your engine as far as the type of charging system it has on it.
It is also worth mentioning that if the valves are in need to adjustment, it can act like a dead battery and sometimes you can use jumper cables to start it when this happens; further making it seem like a dead battery.
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