Sounds like your alternator is not charging. If you put a meter on the battery terminals while it's running it should be putting out between 12-15 vdc ( volts dc current). If not then more than likely the alternator is shot. You can take the battery to a parts store and alot of them will check it out for free to make sure you don't have a bed cell. If you're not getting the correct battery volts while running then I would replace the alternator.

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