I am making my first battery powered circuit. I am confused as to what happens when a battery is getting close to empty. Here is a quick example of what I am confused about:
From my understanding a 12 V battery in a car does not go to 0 Volts when it is empty. A car battery will go between say 12.6 V and 12 V. When it is at 12.6 V it is fully charged and when at 12 V it is considered empty.
Let's say the battery is at 12 V and it is considered empty. Am I still able to reliably use this 12 V in a circuit, but the battery will not be able to supply any current, or am I unable to use this 12V once the battery is empty?
Basically what I am trying to figure out is how to turn off this circuit with a switch. My real application is using a 36 V battery and in normal operating conditions will use 15 A of current. This means I can't really use a normal switch because the voltage and current ratings are too high and it will burn it up.
Instead I was thinking I could use a P channel MOSFET on the positive side of the battery with a switch and a voltage divider shown below:
The circuit will be on when SW1 is not connected to 36 V and it will be off when SW1 is connected to 36 V.
The thing I am unsure of is if I can expect this behavior if the battery is empty? Can I reliably use the voltage of this empty battery, or would it give me unexpected behaviors that turn on the MOSFET when it should be off? Is there a better way to do turn off a circuit rather then using a P-CH MOSFET?