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Old 29-01-2012, 19:00   #1
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Check My Math - DC Charger AC Draw - Generator Sizing

I'd appreciate it if some of the more electrically gifted would check my logic here and I hope the general formula will be of interest to others.

The basic logic I'm trying to nail down, is if my battery charger draws X Amps AC while charging, how big a generator do I need to provide X. Problem is X is not provided in same units so we need to do some math.

My charger says its max draw is 12A (rms) at 90 VAC.
My charger says its typical draw is 8.5A (rms) at 120 VAC.
I believe I can ignore the max draw number since I will be using a 120VAC generator in my solution.

Volts x Amps = Watts
120V x 8.5A = 1020 W

So, I need a generator which can provide a SUSTAINED 1020W minimum just to met the AC needs of the charger.

Do I have that correct?

Thanks a lot!
-patrick
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Old 29-01-2012, 19:16   #2
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Re: Check my Math - DC Charger AC Draw - Generator Sizing

Hi Patrick,

Not sure why there would be a spec for draw at 90 VAC since that would be a serious under voltage in the supply and would not expect to see that low a voltage in any normal situation.

But if the charger is drawing 8.5 amps at 120 VAC that would equate to 1020 Watts. However, the spec you quote says typical but what is the max draw? You should make sure that the generator has the capacity to supply the maximum load from the charger. Do you know the maximum output DC of the charger? That would give an estimate of the max AC draw. To get the exact AC draw from the DC output you would also need to know the power factor but you can get reasonably close without that.

Skip
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Old 29-01-2012, 19:25   #3
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Re: Check my Math - DC Charger AC Draw - Generator Sizing

Another math check. What is the max output of the charger? (e.g. 12v amps x 12). This should be less than the 8.5 amps x 120v since there is some loss in the charger.

Also, genets don't last if run above 80% of maximum for any length of time. Really happier at 70%.

Carl
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Old 29-01-2012, 19:56   #4
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Re: Check my Math - DC Charger AC Draw - Generator Sizing

Another thing to consider is the quality of power from the genset. You didn't mention the size or type you will be using, but the smaller 2-pole high rpm diesel gensets have very dirty power. You can expect current draws to increase 25% if using this type of generator.

Also, I don't understand the "typical" draw spec of the charger. Chargers do not have "typical" draws - rather they draw the maximum possible at full output and decrease accordingly as the battery charges. So an "average" or "typical" number is meaningless. You will need to spec a generator at the maximum charger draw and expect it to hold there as a continuous rating.

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Old 29-01-2012, 20:37   #5
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Thanks for the replies.

The max and typical numbers are right out of my Xantrex TruCharge 40 manual.

I thought that 90VAC number was weird too, but there it was. There were no additional "draw" numbers for the AC side.

-p
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Old 29-01-2012, 20:44   #6
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Re: Check my Math - DC Charger AC Draw - Generator Sizing

Sounds like you may be considering a small portable genset. For a boat your size, the EU2000i by Honda is just about the smallest which will be really useful in recharging your batteries. It can power a battery charger of about 75-80 amps or even a bit larger if they have power factor correction. With the smaller Honda (1000 watts), you're limited to a much smaller battery charger and, of course, a much longer recharge time.

Bill
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Old 29-01-2012, 20:48   #7
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Re: Check my Math - DC Charger AC Draw - Generator Sizing

Guess the model 40 means 40 amps output at a nominal 12V. That would mean the power in Watts at the max of 40 amps would be 480 W.

With a 100% efficient charger (of course does not exist) AC Watts in would equal DC Watts out. Drawing 1020 W to produce 480 W is already pretty inefficient so I would think the 8.5 A would be a max AC draw.
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Old 30-01-2012, 05:40   #8
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Re: Check my Math - DC Charger AC Draw - Generator Sizing

Quote:
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Drawing 1020 W to produce 480 W is already pretty inefficient so I would think the 8.5 A would be a max AC draw.
There are two factors at work in this poor "efficiency". One factor is energy efficiency- the power lost to heat in the charger's internal circuits. But the other is power factor.

Battery chargers can have very bad power factors- some down to about .7. That plus an 85% power efficiency is what gives the very poor observed "efficiency".

That generator that you calculate that has to put out 1020 watts is really putting out 0.7 x 1020 = 700 when you consider the .7 power factor. But the amps it needs to put out is 1020/120 or about 8.5 as noted. A Honda EU2000i should be comfortable with this as it is rated at 13 amps continuous.

But you might think a Honda EU1000i would come close to powering 1020 watts and even more comfortable at 700 watts. Not so. The 8.5 amps exceeds its continuous amperage spec.

David
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Old 30-01-2012, 06:24   #9
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Re: Check my Math - DC Charger AC Draw - Generator Sizing

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There are two factors at work in this poor "efficiency". One factor is energy efficiency- the power lost to heat in the charger's internal circuits. But the other is power factor.

Battery chargers can have very bad power factors- some down to about .7. That plus an 85% power efficiency is what gives the very poor observed "efficiency".

That generator that you calculate that has to put out 1020 watts is really putting out 0.7 x 1020 = 700 when you consider the .7 power factor. But the amps it needs to put out is 1020/120 or about 8.5 as noted. A Honda EU2000i should be comfortable with this as it is rated at 13 amps continuous.

But you might think a Honda EU1000i would come close to powering 1020 watts and even more comfortable at 700 watts. Not so. The 8.5 amps exceeds its continuous amperage spec.

David
Hi David,

Of course all correct. I understand both reasons for the inefficiency (BSEE before turning into a boat bum). I would put it a bit differently. The generator would still be putting out or producing 1020 Watts but due the power factor, if you assume 0.7, only 714 Watts will be transfered to the charger. But I think that is what you are saying anyway?
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Old 30-01-2012, 06:25   #10
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Re: Check My Math - DC Charger AC Draw - Generator Sizing

Quote:
Originally Posted by Patrick_DeepPlaya View Post
Thanks for the replies.

The max and typical numbers are right out of my Xantrex TruCharge 40 manual.

I thought that 90VAC number was weird too, but there it was. There were no additional "draw" numbers for the AC side.

-p
Usually such spec is interpreted as the lowest AC voltage the charger will accept to function and what max. amperage it will require in this case.

This also gives the max input power: 90A * 12V = 1080VA

To estimate the required generator power a PFC of this charger is required. Should be specified on the charger. Look for a number between 0.6 - 1.0 next to PFC or Cos phi.

Since we are given the max input power, there is no need to know the conversion efficiency factor.
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Old 30-01-2012, 07:53   #11
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Re: Check My Math - DC Charger AC Draw - Generator Sizing

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Since we are given the max input power, there is no need to know the conversion efficiency factor.
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Old 30-01-2012, 08:57   #12
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Re: Check My Math - DC Charger AC Draw - Generator Sizing

I would think unless you have a really big battery bank a 1000w generator will power it just fine. If you want extra for other loads, get a 2000w.

Or you could just buy a couple of solar panels for less trouble than a portable generator, Plus a battery charger, just to charge some batteries.
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