safe battery voltage when flashing

hi just a quick question i know you cant charge your car battery while doing any sort of logging or flashing.....but my question is can i use a external battery bank to charge my laptop for Logging ? i cant see how my laptop will set improper voltage to the car ecu while logging ?

i know not to flash the car ecu with the laptop getting charged.... also what is safe recommend battery voltage i should look for checking battery life status if it needs a charge? is 12.45v the lowest you would go and do a re flash...anything past 12.45v i should charge car battery ?