why a lipo then? if you have room just add a second battery like you do for stereo systems/rv's
your entire project always runs off the second battery parrellel to the cars 1st battery. alternator directly
and a ssr connects them when the car is running.
u can't really do this with a lipo cause you might destroy it.
car alternator charges >battery>goes through diode or relay to prevent start batt from discargeing
into second battery(or lipo with safeguards (charger))>> into your project box/disconnect switch.
that would be best i think but i have room in my car and the 1st battery is in the trunk. you dont need any code or arduino control and u can power anything 12v for a while off a second battery, and everthing charges up when u start the car.
make sure its a deep cycle battery
Give me more info about your restrictions, and preferences. i love to help