Car Battery Drain – What is It?

how to find a battery drainI have a Infiniti G35 and recently the battery has been dying time and time again, I took it to the shop and tested the battery and the alternator. They said both were fine, but while hooked up to the tester it showed the
battery draining while the car was off.

Do you think it is possible to have a weak cell in the battery that still shows as good but could cause it to continue draining dead?

Please help!


Hello there,

You most likely have a “voltage drain” and will need a mechanic to help you track it down.

Some common car battery drains are from

1 Dome lights, glove box, trunk and hood lights that are ON

2. Electric seat motors and power antennas that are stuck in the on position and still running (listen for a humming noise)

3. Radios, cell phone chargers, GPS devices that are faulty

A drain is any electric sensor, relay or device that is ON when it should be OFF and it sucks the electricity out of the battery while the engine is off.

Finding the drain can sometimes be time consuming and expensive, so make sure you have a good/qualified mechanic doing this for you.

This is a really good video showing how to find a battery drain

What we would do if you were in my shop is perform a basic electrical system test on the alternator and the battery then hook up some test equipment that will show us how much of the battery power is being used with the engine and ignition key OFF.

If we determine the drain is too large for the battery to handle, we then start unplugging items and tracing down components until we find the culprit of the drain.

Now, there is ALWAYS a drain on the battery with the key off, items like radio and clock memory, and the computer system are always ON and consuming small amounts of battery power…that is normal. What is not normal is large amount of power being used.

A strong battery can withstand a interior dome light being left on over night, a weak battery can not. A strong battery can NOT withstand the voltage draw of an electric seat motor that is stuck in the ON position.

In some cases we will need to keep the vehicle over night to see how much of the drain is being produced and what is causing it. This can be tricky stuff and requires a wiring diagram of the vehicle and a vast knowledge of the electrical system as well as how to properly use the test equipment.

I highly recommend you seek a qualified mechanic if you are unable to locate the source of the drain yourself. Paying the wrong mechanic $75 a hour (and it takes him 4 hours to find the problem) will cost you more than paying the right guy $100 an hour and he finds the problem in 2 hours.

Please share this with your friends,
Austin Davis

6 Comments on "Car Battery Drain – What is It?"

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  1. patrick walsh says:

    I had a new sensor put in my 2006 battery drains overnight with brand new battery.

  2. Lavonne says:


    I have a 2001Maxima – problem started a week ago. something is draining battery, its not the altnator or starter. Each morning need to get a jump. Need idea what problem could be


    • Austin Davis says:

      You need to start ruling things out, check for accessories left on….glove box, passenger sunvisor light, trunk and hood lights. electric antennas, radios, power seats and window motors etc. etc. if nothing is obvious then you will need to have a “battery drain test” done by your mechanic and he will have to slowly disconnect items until he can track down the problem.

      I prefer to install a NEW battery when testing, if an internal cell inside your battery is bad it will not stay fully charged. Don’t rule out the battery just because it looks good or is fairly new, it could still have problems internally.

  3. Damian says:

    I had a drain in the alternator! I thought the job of the alternator was to CHARGE the battery not drain it? huhh shows what I know.

    • By Austin Davis says:

      Yes, that is very possible. The alternator can short out internally and cause a drain on the battery. The alternator can basically stick ON and suck power from the battery.

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