I have two questions 1- how can I calculate car battery life with Raspberry Pi 3 model b ? 2- the voltage stable for the car is 12.6 V, is their negative impact for the start-up of the vehicle when I use his battery when the car is powered off ?
The answer to both questions varies a lot.
The factors affecting battery life are:
Common car battery capacities are in the 40-80Ah range. The bigger the car and engine, the larger the battery usually is. 1Ah of capacity means that you can draw a current of 1A for 1 hour before the battery is empty.
The higher the cpu load, the higher the power draw. Also, if you use perhiperals such as wifi or the camera, power usage will increase.
The maximum current usage is 2.5A. An idle pi 3b will use about 0.23A according to these measurements:
https://raspi.tv/2016/how-much-power-does-raspberry-pi3b-use-how-fast-is-it-compared-to-pi2b
So there is an up to 10x difference in battery life depending on what you are doing with the pi.
The car battery will have roughly 12-14 volts depending on its state of charge. The pi wants 5 volts. A linear regulator will consume 1 Ampere of 12 volts from the battery to produce 1campere of 5 volts for the pi. So it will have something like 40% efficiency. A good switched regulator will have something like 90% efficiency, so it would require only 0.1A from the 12V battery to feed an idle pi 0.23A at 5V.
So in the worst case you can be drawing 2.5A from a 40Ah battery. It will last 16 hours.
In the best case you would be drawing 0.1A from a 80Ah battery, so the battery life will be 800 hours.
In any case you should plan to leave maybe 50% of the battery capacity unused to guarantee that the car can still start.