Q: Batery drains sitting in driveway with engine off.

asked by on December 30, 2016

My battery will fully discharge within a few days when it is driven. I have taken my car and had the electrical system checked several times over the past few months and nothing is wrong. I have even had battery changed twice to be sure with no improvement. I was told my problem may be in the dashes electrical system, If true do you know what the problem might be?

My car has 120000 miles.
My car has an automatic transmission.

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If you request a parasitic power draw diagnostic the responding certified mechanic will track this down for you and let you know of the required repair. First, the battery needs to be load tested to exclude the possibility that you have a faulty battery. If the battery is 4 years old or older, and particularly if it only marginally passes a load test, getting a new battery will definitely be amongst the possible resolutions. If the battery is "good" though, there are really only two basic causes of the issue you are having: 1) there is a parasitic power draw on the battery, when the car is off, due to what we call a "phantom load". Such can be an electrical device with switch contacts (or relay contacts) that just won’t "open" but phantom loads can also occur from shorts to ground. This is very easy to measure. You just simply wire an ammeter in series with the battery and measure the electrical load that is flowing when the car is off. Typically a car without "phantom loads" will still draw around 20 to 50 milliamps or so to run the clock, security system, computer and so forth. That is normal and all cars will draw some power when off. It is when you get above that modest level of 20 to 50 milliamps, that a car battery can be drained, if the car sits without running, over a period of days or weeks; or 2) The charging system is not properly re-charging the battery as the car is running, causing it to slowly discharge, depending on exactly what accessories you are running, engine speed, run time and so forth. Of course, it is possible that both causes exist but such would all be checked out in the context of the parasitic power draw diagnostic. Hope this helps and I hope we can assist you in resolving this.

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