When the heater runs why does battery gage go way down?

The heater draws a lot of power from the car's battery, which can cause the battery gauge to go down. This is because the heater uses a lot of electricity to generate heat, and the battery is the source of that electricity. When the heater is turned on, the battery has to work harder to generate more electricity, which can cause the battery gauge to drop.

In addition, the heater can also draw power from the alternator, which is another source of electricity for the car. When the alternator is working hard to generate electricity for the heater, it can also cause the battery gauge to drop.

If you notice that your battery gauge goes down when the heater is turned on, it's important to have your car's electrical system checked to make sure that everything is working properly.