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Q: Brand new battery keeps being drained. I found the interior light switch does not work. Could this be the culprit?

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Bought the car used. Has brand new battery. Keeps being drained and car won't start if not driven for greater than 16 hrs. No problem was disclosed. Already checked all the fuses and removed the fuse for the cigeratte lighter that looked bad. So frustrating as I really can't afford a fishing expedition at a dealership.

My car has 137000 miles.
My car has an automatic transmission.

A: Even though you have a "new" batt...

Even though you have a "new" battery, it does have to be load tested just to be sure. Such is not a problem though as many large battery vendors will do it for free. Once you have confirmed the battery, you want to check the output of the alternator and compare against the Factory Service Manual (typically output should be around 13.8 to 14.5 volts but the FSM spec is the one to rely on). If all that checks out, you have a phantom load on one of the car's circuits that is drawing a significant amount of current when the car is off. Normally, some battery power is required to keep the car "alive" when it is sitting in your garage (alarm, computer, clock, etc.) but this current draw is so small, at around 50 milliamps, that it won't discharge the battery "quickly". Basically, to find the phantom load(s), when the car is off, you measure the amp draw from the battery to each and every circuit fed by the battery. If you do this methodically, eventually the circuit that is discharging the battery will show up. That individual circuit(s) is then traced to identify the faulty device and or wiring issue that is letting current flow when it should not flow.

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