Design for a low voltage disconnect?

I'm currently building the final version of my arduino powered car management system.
But before I can continue there's one bug I discovered in my prototype left to solve:

One of the functions of the system is an alarm which is triggered by a 3-axis accelerometer, doors opening or the ignition.
This of course means the arduino has to have power at all times.
Because of this I have already installed a second, larger, battery in my trunk so that my primary battery stays charged to start the car.

But the problem I have encountered is that when I don't use the car for a few days the system discharges the battery too far.
This not only shortens the battery's lifespan, but also causes some kind of fluctuation triggering the alarm.

That's why I'm looking for a low voltage disconnect design that draws as little power as possible.
And preferably cheap too :stuck_out_tongue:

Check this page of Nick Gammon about power saving. - Gammon Forum : Electronics : Microprocessors : Power saving techniques for microprocessors -

But the problem I have encountered is that when I don't use the car for a few days the system discharges the battery too far.

That really shouldnt be happening. Have you got a multimeter to measure the current draw? Maybe you can figure-out what's using so much current and modify the design.

There's almost always a few things drawing current... A clock, the radio's station-memory, the remote door-lock receiver, etc. I built an alarm for my van (with a different microcontroller) several years ago. Sometimes it sits for a few months, and the alarm-LED is still blinking, the door-lock remote works, and the stereo & clock are still working. If it's been several months, I'll charge the battery before I even try to start it, but a few weeks or a couple of months is no problem. (Since the alarm works on 5V, the thing is going to keep running 'till the battery gets down to 6V or so, and voltage-regulator drops-out.)

Yes, the prototype drew too much current, but I've already implemented a lot of power saving measures in my new design.
But at this stage I don't have any figures yet about the power consumption.

Of course I will still have the same problem as before, it will just take longer now to get to this point.
And that's why I would like to disconnect the system once the voltage drops below 10.4V.

That way my battery will live longer and I don't have to worry about the alarm triggering whenever my car isn't used for a while.