Lead acid battery for embedded system
The vehicle's battery can not charge the embedded battery, particularly with a diode in series. The voltage will not be high enough.
If the alternator is working properly it might be able to charge the embedded battery, but only very slowly.
You could look for an ideal diode circuit between the vehicle's alternator and the embedded battery, but then you run the risk of dangerously overcharging the embedded battery.
I think you need to start with a dc-dc converter that boost the vehicles voltage up to 15V or so, then add a charging circuit to that.