How much does it cost to charge a car battery
 

So this guy showed up to my house and asked me if he could charge his trailer batteries- all 7 of them. So I said ok, figured I'd help the guy out.Well hes come back a couple times, Hes using a makeshift 70 amp charger for about 3 hours at a time and connects all the batterys together using very thin wire....I was wondering how much power/ money this charging will cost though the power bill cause I don't want my power bill racked up because of him. Any help would be great.
Thanks

Just buy a battery tender and never call anyone to charge your batteries. Good ones go for around $100. Most of the tenders can maintain around 4-5 batteries year round. They will never over charge the batteries.

Is this guy homeless?

Not quite sure of your situation with this guy.

I am also confused.. Do you own a garage/shop? Why can't he charge his batteries anywhere else? and why is he charging them so often?

I'm also confused! Is there a Super Duty involved in any of this?

that guy is taking advantage of you. You can't tell me that he doesn't have access to an outlet at his residence to charge his batteries. He's probably just looking to get it done with no cost to himself. I guess if you want to keep letting him charge his batteries just keep an eye on your elec. bill and charge him the for the extra.

The whole situation seems very odd.

i'll take a crack at it.

P=IV

70amps at 12volts = 840watts applied to the battery
assume a 50% charger efficiency
so 1680 watts (1.6kW) consumed from your wall
1.6kW for 3 hrs = 5kWhr each 3hr session.

so take the 5kWhr times your kWhr rate on your electric bill and you have your answer.

around here, we are $0.08/kWhr
so it would cost me 40 cents to help the guy out each time.

(a note to the nitpickers ... the battery charger will charge to 14volts, but is probably more than 50% efficience, just to make the math easy i used 12 and 50 ...)

I'm sure there is more to this story.......maybe you had to be there?