3.8l v6 Mustang 2004, randomly won't start but disconnecting and reconnecting the battery works

Question

This problem has occurred 3 times now, so it is a running trend.

Every time it occurs, it goes like this:

I get in the car, insert the key, push in the clutch, and turn the key all the way to start. I'll hear the starter motor crank once and cut off immediately in the middle of the crank. After which, turning the key from accessory to start does nothing. There is no power in the rest of the vehicle.

Now if I remove the key, pop the hood, disconnect the battery terminal, then immediately reconnect it (I've done nothing else). I can now start the car.

I'm not sure if any of these cause the issue, but I've generally noticed this problem has happened:

1. On hot days
2. Right after the car was stopped or a few hours after I've last driven it (such as a short trip to the apartment mailbox and back)

Does anyone have an idea why disconnecting and reconnecting the battery lets me start the car again? Why this is happening?

I do have a couple of mods on the car, but for the large majority of the time I have no issues starting the car.

• Aftermarket LED headlights and tail lights
• Aftermarket radio
• Aftermarket footwell lights, fuse tapped (in the correct direction) to the accessory fuse.

Answer 1

One thing it could be, and I have had this happen a few times, is that the battery has an internal link failure that goes open circuit based on vibration / temperature or the stress level of the driver (much like the stress detector in photocopiers to produce a paper jam).

Sometimes a solid thump on the top or side of the battery can get it to reconnect. Often just the action of loosening and then retightening the battery terminal can be sufficient.

Best test is to swap the battery and see if the vehicle still fails to start - if it does fail then it is something else.

If the other vehicle gets the same problem then buy a new battery.