Like most new cars these days, our Mustangs will cut current to lights that are left on too long. Like the trunk light, overhead console lights, and headlights. What component is doing that? Is there a module in the fusebox? If you turn on the ignition, that seems to reset the device.

 

How does the circuit work? When the ignition is ON, a transistor acts as a diode and keeps a cap charged up. When the ignition is turned OFF, that capacitor is discharged down in a couple minutes, by the base current of the transistor—perhaps 5 or 10 nA. When the voltage on the cap gets low enough, all the transistor turns off, and the lights turn off.
You asked for it.

 

Karman.

I'm not an electrical engineer, and that made sense to me.

 

On a serious note, I was wondering the same thing, and then wondering why Ford didn't put the headlights on a system such as this. If you leave your headlights on, there goes the battery.

 

So the headlights are not on the battery saver feature? I know the interior lights all will turn off in about 10 minutes. Headlights stay on until the battery dies?

 

OK, one more time, a legitimate question without the remarks from the peanut gallery. What is the "battery saving device", where is it and how does it work? Maybe there's something in the electrical part of the shop manual if somebody can take a look.

 

You do get to hear that dandy warning chime too until the battery dies.

 

I think that it is part of the SJB (Smart Junction Box) now.
I am just guessing, but that darn thing seems to monitor all light functions.

 

The battery won't completly die. It will have like 4-5% of power left

 

It's controlled by the SJB (Smart Junction Box). When water shorts out the SJB, your lights will malfunction. Previously Ford used a LCM (Lighting Control Module) to handle all lighting controls but they changed it to a SJB which handles more than just lights. There's also a Bussed Electronic Control center where the grounds attach so the SJB can monitor lighting controls as well (which one is on, etc...)