A car which has 4 cylinders and an engine capacity of  1000cc to 2000 cc needs 250 to 500 amps to start in 2 to 3 seconds to start. This means that approximately 0.25 Ah to 0.5 Ah is consumed every time you start your car. WARNING ! If your car is not starting for any reason, and you continue trying to start it , a 40 Ah battery which consumes 0.4 Ampere to start will theoretically get discharged after you have attempted a 100 starts. In real conditions it does not work like this.A battery will not give you full peak capacity of 40 Ah, your battery may get fully discharged after less than 15 to 30 attempts depending on how well it is maintained and how old the battery is. So avoid excessive attempts to start the battery.