Skip the auto shop - Our mechanics make house calls

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

asked by on

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Looking at used Mitsubishi Eclipse 2007, 100,000 miles. What is the average wear in miles on this vehicle?

Hi there. This is a very difficult question to accurately answer; as there are several variables that impact engine wear and tear such as routine service and maintenance schedule and type of driving completed. However, on the average, typically Mitsubishi...

Q: My 2008 Mini Cooper S has a check engine light on and the code is P0977.

The code P0977 will set if the transmission control module (TCM) records an abnormal voltage reading from a shift solenoid in the transmission. A vehicle with the P0977 trouble code will generally be driveable but due to the potential for...

Q: Why is my car idling high

This may be related to a faulty idle air control valve or possibly a throttle stop that may need to be adjusted. The idle air control valve monitors the air intake as it is mixed with fuel prior to being...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...