Ask the Expert: The vehicle won’t start after sitting for more than a day, but the battery tests good. What could cause this?

Q: The vehicle won’t start after sitting for more than a day, but the battery tests good. What could cause this?

A: If you have a vehicle where the battery goes dead overnight, a parasitic draw could be the culprit. A parasitic draw is caused by a load draining current from the battery while the ignition is turned off. In newer vehicles, a small amount of draw is normal due to computers and modules needing to retain settings such as radio presets, clock, and memory seat settings. This draw is low enough that a vehicle can sit for long periods of time and not have any issues. A parasitic draw test is used to measure how much current the vehicle is using when it is shut off and to determine whether the draw is normal or excessive.

To perform a parasitic draw test, you will need a digital multimeter, and basic tools to remove a battery cable.

First, make sure you have a fully charged battery and that all accessories have been removed and turned off (cigarette lighter chargers, dome lamps, sun visors, etc.). Remove the key from the ignition.

Next, disconnect the negative battery cable (be sure to retain all customer settings before disconnecting the battery). Set your digital multimeter to the highest amperage setting and connect the meter in series between the negative battery cable and negative battery post.

If you have done everything correctly, you should see a reading on your meter. Let the vehicle sit for at least 30 minutes to an hour to make sure all modules in the vehicle have gone to sleep. Typically, the rule for parasitic drain is that it should be less than 50 milliamps after the vehicle has been shut off. Any more than 50 milliamps, you should consider it excessive and will need to locate the source of the drain.

Information provided by: Power Probe