Shuting down ("softly") the computer to save the battery
I have just received all the components for my CarPC. The power supply is a DC-DC,250W,ATX (SPS-DX250H) and I am planning to use a circuit that switches to an auxiliary sealed lead acid battery (which is charging only while engine is running) in case the car's battery voltage drops under 9V (the minimum input voltage the PSU needs)or when I remove the key from ignition.
Now I am wondering how can I make the computer to SOFT shut down after a predetermined amount of time in order to avoid discharging of the aux battery more than 30% (maximizing the life of the battery). I was thinking of using the Window's Power Options in order to put the system in stand by after 10 mins, but this is not shuting down and I'll have to mess with the computer at least every 10 mins even when I am in the car and hear music.
Do you have a better idea, a piece of hardware, circuit (schematic),software?
I would appretiate any help!