Car battery voltage sensitive device

I am looking for a device or small circuit to detect my car battay voltage level and overload protecting device.
This is what I want to make or find:

When I run few devices from my car battaery, I want to cut off switch before my battery drains . If my battary is drain to the low I cannt start my car, I want some device to detect battaery thresh hold level.

Much appreciate your help for this.

Any web side, products or design idea welcome.

Read my postings to this thread.

Battery Monitor

It's difficult to measure state of charge voltage on a battery under load, but here is an attempt. Example: If there is a 10 Amp load on a 100 AH battery the circuit would trip at 73% capacity. With a 1 amp load it could discharge down to 32% before it would trip.


Load C/100 C/20 C/10 C/5
Trip 32% 50% 73% On

100 AH 1 Amp 5 Amps 10 Amps 20 Amps

Thank you all, I got the idea.