Amp usage
Submitted: Thursday, Jan 10, 2013 at 21:21
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Imanoone2u
Another question if I may indulge your brains trust.
To calculate amp usage for an appliance, the formula would be
W/v=amp
Now, I have an 800w 220v coffee machine that I would like to connect to my
batteries via an 1300w inverter.
How would I calculate the power usage here? Do I use the 220v value or the 12v
value?
There is of course the AC V DC difference...
Even if I calculate 800w/12v=66.6a
If the machine operates for 10 min only it would still use
66.6/6=11.1a....a fair bit.
Thought, solutions, options? Suggestions?
Marcel
Reply By: Member - Rosco from way back - Thursday, Jan 10, 2013 at 21:55
Thursday, Jan 10, 2013 at 21:55
You're basically on the money old mate.
Suggest you buy a fire coffee pot. You can't expect 800W out of a 12V system for very long. I doubt it would work satisfactorily even with the engine running and if your alternator could put out 100 amps.
AnswerID:
502186
Reply By: patsproule - Thursday, Jan 10, 2013 at 22:01
Thursday, Jan 10, 2013 at 22:01
Yeah, your maths are right. But factor in the inefficiency of an inverter and the current at 12V will be even higher than that. It's not at all practical.
AnswerID:
502188
Reply By: Imanoone2u - Thursday, Jan 10, 2013 at 22:06
Thursday, Jan 10, 2013 at 22:06
Thanks guys,
I'll save that option for a 'BIG4' caravan park:) plunger here I come
Marcel
AnswerID:
502189
Reply By: Nomadic Navara - Friday, Jan 11, 2013 at 11:11
Friday, Jan 11, 2013 at 11:11
A few corrections here. To cater for the losses in the inverter you divide by 10 and not 12. That gives a current draw of 80 A. If you run it for 10 minutes that would be 80/6=13.3Ah (at this stage you should be talking amp hours and not amps.)
However that is not the full story. You have not taken
Peukert's Law into account. Batteries are not a linear device. If you have a 100 Ah battery you will only get 100 Ah out of it if you discharge over a 20 hour rate (5 A current draw for 20 hours.) If you discharge it at a lower rate you will get more amp hours out of it. If you discharge it at a higher rate you will get less. When you apply Pukert's Equation to your exercise you could be drawing twice the 13 Ah from your battery.
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Follow Up By: Imanoone2u - Friday, Jan 11, 2013 at 11:17
Friday, Jan 11, 2013 at 11:17
Cheers Peter,
I though it was a bit too good to be true. :(
Appreciate everyone's input
Marcel
FollowupID:
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Follow Up By: Robin Miller - Friday, Jan 11, 2013 at 12:52
Friday, Jan 11, 2013 at 12:52
Following on Peters line I notice you use 220 as your volts Marcel - this usually is a European spec and if your inverter is 240v then your coffee maker might be using closer to 900 watts.
100 amps or so isn't really a problem if done right and you don't make two many cups of coffee, it helps to run the car at the same time though.
I run an electric jack hammer from my car at 150 amps for long periods this way and to help I have modified the output voltage of my inverter down to limit max current.
Don't do this unless your in that field though.
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Follow Up By: Imanoone2u - Friday, Jan 11, 2013 at 14:32
Friday, Jan 11, 2013 at 14:32
Thanks Robin, my bad. I did mean 240v. After 35yr in the land of Oz... I'm still not used to it...lol
Don't worry, I'm not tinkering with anything I'm not 100% sure on how to do... Especially if there is high voltage/amps or expensive equipment involved
Marcel
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