Hey John T

I have a sparky question. Can you tell me how much amperage a battery charger will draw charging a 12 volt battery at 40 Amps. I have looked all over the net and on battery chargers but that must be top secret info. Thanks.

I'm gonna see how my answer compares to John's...

12 Volts X 40 Amps = 480 Watts / 120 Volts = 4 Amps.

The efficiency of your charger is likely in the 75% to 95% range so the actual current will likely be 5% to 25% MORE than 4 Amps making it between 4.2 and 5 Amps. (And, then, of course, there's power factor!)

How'd I do, John?

Hey Bob,

I'm NOT John T but I am a sparky. Think you have it covered.

I would guess you are right on.

I concur. If it was a inverter driven charger with no transformer, it might be the low end of that range due to efficiency. Jim

Yo Bob, you done good in my FWIW opinion:

Yo Fudd, as typical the good Bob plus Teddy and I usually agree and he's as much or more a sparkie then myslef in my opinion as my knowledge is pretty limited to only old tractor electricals and AC Power Distribution, Im not much on electronics. ALSO when it comes to AC Power Distribution there are several more current practicing electricians and engineers sharper then myself as Im a bit rusty on the topic, but hey I do my best and am "usually" correct I think or I wouldn't throw it out there.

THAT BEING SAID (I'm an attorney remember, I cant help using disclaimers lol) I take the same approach, only may modify it slightly to account for a few minor what I consider realistic closer to actual voltages.

When the battery is charging I would guess (since its a 12.6 volt creature if full charged) the chargers output is more like say 13 to 14+ volts subject to battery state and chargers design and capacity. If its supplying 40 amps at say 13.5 volts, that's 540 watts output. Like Bob I don't know the chargers efficiency, if an older noisy buzzing heat producing unit it may be as low as 75% but if more modern maybe 90% (but I just cant say for sure that's only a guess)

Then like Bob in order to get 540 watts out you may (due to heat losses) be putting "guessing" "around" 700 watts in and 700/120 = 5.8 amps

So, (like Bob) the answer depends on the charger and its efficiency and capacity and the battery condition, but Id guess its a safe bet to figure the charger is drawing at least 4 and up to 6 amps on its 120 VAC Input ?????????

Happy Veterans day and thanks to all the Vets. All gave some, some gave all

John T

Thanks guys, I appreciate the help. Happy Veterans Day to all you Veterans!