Mysterious electrical problem with '91 Sentra

My '91 Sentra needs to keep going to work and back. Recently, the battery went dead quickly (overnight); bought a new battery and installed it. Next morning, it was dead again. Figured it was an alternator problem, got new battery, only connected it to drive car to shop. Car drove fine. Shop says the alternator is OK, battery is OK, can’t see anything that would cause dead battery overnight. They called the problem “intermittent” and are doing tests. What do they need to test? Connection from battery? Short in system somewhere? I need reliable wheels…ok, I’ll settle for wheels that won’t leave me in downtown Tacoma after work with no way home. Thanks.

Walmart has the quick battery disconnects. That could be your work-around until someone finds the cause of the battery drain. The radiator fan may be coming on at odd hours, and draining the battery. This can be caused by a fan relay, or, faulty engine coolant temperature sensor. Replace the radiator fan relay, and see what happens.

The shop needs to test the electrical system for an excessive current drain while the car is parked. They should know this already and how to pin down the troubled circuit by pulling fuses. If the trouble is intermittent, then that makes finding the trouble a lot harder. It sounds like the trouble is pretty consistant though from what you stated.

“Battery drain” is a conclusion. What are the symptoms? Here is the technique for one to use to find the cause of a battery drain: