My arduino with Bluetooth inside my car draws 50 mA in sleep mode.
Will it drain the car battery?
If 40Ah battery, around 800 hours.
How long you leave it? How many Amp-hours is the battery good for? 50mA is 1/20th of 1 amp, in 20 hours it will have drawn 1 Amp-hour. Average car battery has about 70 Amp-hours full charge.. and if you leave it for 6 months you will need a jump.