2009mercury Milan v6 won't hold a charge

We replaced the battery. The dealer cannot find the problem. They asked us to leave it with them for a few days. If the car sits 36-48 hrs it won’t start. We confirmed there were no lights on or anything was left on or plugged in. We checked it by opening the door to see if the overhead light came on every 12hrs. After 48hours the car again would not start. What could be the intermittent problem?

There are many computers/modules in todays vehicles. And some of these computers/modules can stay awake for up to an hour before they go to sleep or turn off. If any of these fail to go to sleep or turn off they can draw enough current from the battery to pull the battery down to where it isn’t able to start the engine.

In days past one would disconnect the negative battery cable and connect a test light between the battery cable and the negative post on the battery. And if the test light illuminated it meant there was a parastic draw on the battery. On todays vehicles that can’t be done. Because if there is a computer/module that is failing to go to sleep, as soon as the battery cable is disconnected it forces these components to go to sleep. So it can’t be determined if this is what’s causing the draw on the battery.

Instead, to test for a current draw on the battery from a component failing to go to sleep, you measure for a voltage drop across the fuses. If a circuit is drawing current from the battery it will cause the element in the fuse to heat up. This heat causes the resistance to increase at the fuse element. So a voltage drop should be detected across the fuse on a circuit drawing current from the battery. Or you can use what fuse isn’t like others method when measuring for a voltage drop across the fuses for a current draw.


We returned to the dealer who determined after three days that it was the radio which they replaced under warranty

Thanks for having the courtesy to post back with the fix. It may help other drivers with similar problems.

Ed B.