[SBorovkov]

В описании имхо мутно написано о контроле за выключением.

описание режимов:

P0: In this mode, the M2-ATX behaves like a regular ATX power
supply. If J6 is connected to the motherboard, M2-ATX will also send a gratuitous “ON pulse” to the motherboard right after power
is first applied. If IGNITION is connected to the battery, the unit will shut down if battery is < 11.2V. This is to protect the
battery from over-discharging. If Ignition is not hooked up, M2-ATX will operate from as low as 6V. For more information, please
consult the “UPS mode” documentation on the product page.

1.2 Power challenges in a Vehicle PC
The M2-ATX is addressing these issues by cutting off the 5VSB rail after a pre-defined amount of time (see jumper chart,
HARDOFF). When 5VSB is always active (HARDOFF=Never), M2-ATX constantly monitors the battery levels. When battery level
drops below 11V for more than one minute, M2-ATX will shut down and re-activate only when the input voltage is > 12V.

2.0 Mode of operation
...
Ignition=OFF. M2-ATX waits for “OFFDELAY” in seconds (see jumper chart) and then it turns the motherboard OFF by sending a signal to the motherboard’s
ON/OFF switch. Your computer should turn off gracefully (shutdown procedure). After shutdown, 5VSB will still be provided for another “HARDOFF” seconds.
In the event where the shutdown process is longer than “HARDOFF” (Operating System gets frozen, etc), power will be shut down hard, turning off all power
rails. During the HARDOFF procedure, the battery levels will be constantly monitored to prevent deep discharge situations.

Из всего этого по-моему следует, что аккумулятор контроллируется жестко только в случае hard-off=never.
А в режиме включенного зажигания m2-atx выключит комп только при падении ниже 6-8 вольт, в зависимости от нагрузки на блок питания.