# How many Watt Hours are consumed to jumpstart car?

I bought one of these portable jump starters. This one says in specification that it can start a big 6L gasoline engine up to 12 times on a single charge. Simple math implies that each jump-start attempt should consume 100%/12=8.5%. At least for big engines.

Now I am using it to jumpstart Chevrolet Aveo with a smaller 1.6L engine. I have tried to jump start my car with it like 10 times for 3-4 seconds each time and the portable jumpstarter's charge status has barely dropped from 74% to 72%.

This has left me wondering on how many WHr typically are needed to jump start a small 1.6L engine?

While I see that starter spins the belt in my car, is there a chance that actual load (e.g. camshaft) is somehow not connected to the belt and hence the charge on the portable jump starter drops by insignificant amount? By the way I am not able to start the car and I am also getting code "P0340: Camshaft position sensor error". See this question for more context.

• For your car assume 12v and 400A, that gives power , multipy by time 3 to 4 seconds , convert seconds to hours and you have your answer in Wh... – Solar Mike Sep 23 '17 at 22:48
• 12V*400A/3600seconds*3seconds=4Whr/attempt. 10 attempts would be 40whr. However, my portable jumpstarter lost 2% charge during these 10 jumpstart attempts. So the total capacity of that portable jump starter would have to be 40*(100/2)=2000Whr which is way above the claimed jumpstarter capacity of ~14whr. I guess it could be that the lead-acid battery is still doing the job then. Or 400A is peak power and does not remain constant during these 3 seconds. – user389238 Sep 23 '17 at 22:57
• No the current draw is not constant - electric motors create back emf ... And your battery will most probably be helping as you mention unless it is completely dead or broken – Solar Mike Sep 24 '17 at 9:05
• another thing going on here is the meter claiming 2% discharge on that unit is likely not at all accurate. – agentp Sep 24 '17 at 12:27