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Old 04-17-2006, 08:35 PM   #1
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My Electrical Math

I need someone to check my work to make sure I have it right and didn't miss something. On our 66 Safari, 22 foot, we need a new converter. I'm trying to verify for myself what size I need. So I'm counting watts. I have:

15 x 7 watt bulbs for lights = 105W
1 Porch Light = 7w
1 Fantastic vent, at 3 amps = 36W
2 more vent fans, say 3 amps = 72 W
1 water pump ~4 amps = 48W
That's 268 watts if all of that stuff is used continually.
I could also add the hitch jack motor and the 12 outlet we use to inflate the airbed, but I wouldn't ever use both at once, so say another 48watts, for a total of:

316 watts = W/12 VDC = 26 Amps, or
316 watts = W/13.6 VDC = 23 amps.

So, if this is correct, then I need no more than a 30 amp converter.

Does anyone see a flaw in my logic? I want to make sure I get what I need, but not overpay for something I'll never need.

John
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Old 04-17-2006, 08:51 PM   #2
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John, OEM Univolts were rated at 40 amps. You would need the extra capacity to use to recharge your battery, but 40 should be more than enough.
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Old 04-17-2006, 09:13 PM   #3
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Quote:
Originally Posted by AYRSTRM2
Does anyone see a flaw in my logic? I want to make sure I get what I need, but not overpay for something I'll never need.
John
John,

Your math is ok, but the logic is a little off.

You use the wattage calculations and amperage estimates to size the battery or batteries that you need. For example, say your lights are used for four hours, and your fans for six hours, and your jack for 2 minutes, etc, you add up all the amp hours, then double it to figure out how much battery you need.

Once you have sized the batteries, then you select a converter that will recharge the batteries in a reasonable time period.

That's the way I would do it.
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Old 04-17-2006, 09:23 PM   #4
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My 19CCD came with a 55 amp converter and 2 group 24 batteries. I would suggest that size as a minimum when replacing or adding a converter.
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Old 04-17-2006, 09:28 PM   #5
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Originally Posted by markdoane
John,

Your math is ok, but the logic is a little off.

You use the wattage calculations and amperage estimates to size the battery or batteries that you need. For example, say your lights are used for four hours, and your fans for six hours, and your jack for 2 minutes, etc, you add up all the amp hours, then double it to figure out how much battery you need.

Once you have sized the batteries, then you select a converter that will recharge the batteries in a reasonable time period.

That's the way I would do it.
Interesting. I hadn't thought about it like that. I was going on the idea, which the converter manufacturers all support, that the converter may someday be used as a straight DC source for the coach.

But converters are rated in amps, not amp-hours, so I'm not sure how you can tell how fast it charges a battery?

John
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Old 04-17-2006, 09:33 PM   #6
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Originally Posted by lewster
My 19CCD came with a 55 amp converter and 2 group 24 batteries. I would suggest that size as a minimum when replacing or adding a converter.
Why? I only need half the amps, and the only 12vdc extras I can see ever adding would be a CO2 monitor and maybe, but probably not, a stereo.

I'm honestly asking, why would I need that much power if I can't count up that much current draw?

John
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Old 04-17-2006, 09:52 PM   #7
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Originally Posted by AYRSTRM2
Interesting. I hadn't thought about it like that. I was going on the idea, which the converter manufacturers all support, that the converter may someday be used as a straight DC source for the coach.

But converters are rated in amps, not amp-hours, so I'm not sure how you can tell how fast it charges a battery?

John
Sure the converter can be used as a straight DC source, and you may decide to use it that way, based on your style of camping. I'm saying that the conventional way to size the converter would be based on battery size and charge rate.

You might go through the calculations and decide instead to buy a converter based on max amp draw.

Converters are based on amps. The maximum amperage of the converter/charger should match the recommended bulk charging rate for your battery bank, plus whatever extra you want to run continuous DC loads.

You would not want to size the converter based on high amp, short duration loads like the tongue jack. The battery can supply those high loads with no problem.
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Old 04-17-2006, 10:53 PM   #8
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Originally Posted by markdoane
Sure the converter can be used as a straight DC source, and you may decide to use it that way, based on your style of camping. I'm saying that the conventional way to size the converter would be based on battery size and charge rate.

You might go through the calculations and decide instead to buy a converter based on max amp draw.

Converters are based on amps. The maximum amperage of the converter/charger should match the recommended bulk charging rate for your battery bank, plus whatever extra you want to run continuous DC loads.

You would not want to size the converter based on high amp, short duration loads like the tongue jack. The battery can supply those high loads with no problem.
Mark, I see your point on the tongue jack. This is good stuff to keep in mind, thanks for the input!

John
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