Keep Having to Replace Battery 1985 300TD

I've had my 300TD for two years now. Between my initial pre-purchase test drive and picking up the new car, the battery went dead and wouldn't take a charge. The original owner put in a new Mercedes Benz battery. A year later, that battery went dead and wouldn't take a charge. I bought an Autozone 49-DL battery (same size and amp/hours as original). A year later, it is dead and won't take a charge. I've been trying to charge it with the battery disconnected from the car and it charges endlessly. Any idea what causes this or where to start troubleshooting?

Bill Brewer - Tehachapi

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