Trouble restarting Ferguson to-35

Whenever I turn off my 1955 Ferguson to-35 and then try to restart it, the battery barely turns the engine over and it won't start. After charging the battery for a few hours, it'll start up immediately. The amp meter shows positive amperage (not sure if that's the correct term) while it's running.

Assuming the battery isn't shot, what could be causing it to get drained so much during the operation of the tractor? If it were a bad generator, wouldn't the amp meter show a negative reading during operation? Thanks.