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Old 03-19-2015, 03:17 PM   #1
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Battery to Inverter to Fridge equation problem

One of our major sources of apprehension occurs on moving/provisioning day. If we are off shore power for more than 2 hours our residential fridge reaches 45 degrees and beyond.

I have 2 zombie deep cycle batteries (brought back to "life" with Epson Salts), a 750watt inverter and a fridge that draws 6.5 Amps AC.

Throw out the 2 batteries and replace with 1 DuracellŪ AGM Deep Cycle Marine and RV Battery - Group Size 31:
1 amp hour rate:68.2
100 amp hour rate:110
20 amp hour rate:105
3 amp hour rate:85
5 amp hour rate:86
6 amp hour rate:87.4
8 amp hour rate:90
BCI Group Size:31
CCA at 0 degrees F:800
MCA at 32 degrees F:1000
Minutes at 15 amps:348
Minutes at 25 amps:210
Minutes at 5 amps:1265
Minutes at 50 amps:87.4
Minutes at 75 amps:53
Minutes at 8 amps:706
Reserve Capacity:200
Volts:12

I'm looking for a cheap temporary solution for sitting at Walmart or somewhere where I could plug the fridge into the inverter and lower the temperature. I know the 750 watt inverter is borderline, 1000 or 1500 would probably be required. But what I don't know the answer to is how long the battery could power the fridge.

Thanks
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Old 03-19-2015, 03:24 PM   #2
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Your current setup doesn't work?


Seems like you're only using 11Ah minus whatever loss you have in your invertor.

750W is just barely big enough (6.5A x 110V = 715W).
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Old 03-19-2015, 05:53 PM   #3
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MT4Rinner, you left out the conversion of 6.5 amps @120 volt, equals 65 amps @12 volts.

To the OP, You are using more the half of your battery capacity, the amount you should not exceed, in a hour .

That is the case, only if the refrigerator is running all of the time. If it runs a third of the time, you should be able to run it just around 2 hours before the battery needs charging.

This does not take into account any inefficiencies of the inverter.

If you are only using the batteries for travel, get 1 more of the same battery.

Good luck
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Old 03-19-2015, 07:02 PM   #4
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I'm OK on understanding the AC side. I figure 6.5 * 120 = 780 watts +++ since it's a compressor add 1.25 to 1.5 on start up. So I know I'm not doing too good with a 750 watt iinvertor.

It's the DC side I'm fuzzy on. How do we get to 65 amps DC? AND do I always compare the draw to the 1 amp hour rate?
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Old 03-19-2015, 07:38 PM   #5
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780 watts X 12 volts = 65 amps.

Most deep cycle batteries use the 20 hour rate.

A 100 ah battery should only be drawn down to 50%. Leaving 50 AH usable.
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Old 03-19-2015, 07:43 PM   #6
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Quote:
Originally Posted by twinboat View Post
780 watts X 12 volts = 65 amps.

Most deep cycle batteries use the 20 hour rate.

A 100 ah battery should only be drawn down to 50%. Leaving 50 AH usable.
I'm sure you meant 780W / 12V = 65A.

W/V = A
V x A = W
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Old 03-19-2015, 07:47 PM   #7
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Thanks to everyone. Now I've got it!!!
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Old 03-19-2015, 07:53 PM   #8
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Whoops, yup
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Old 03-19-2015, 09:11 PM   #9
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Suggest to use a Kill-a-watt meter to see what the real average power consumption of the frig is over time.
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Old 03-19-2015, 09:39 PM   #10
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Suggest to use a Kill-a-watt meter to see what the real average power consumption of the frig is over time.
I have done exactly that for my Samsung RF-197 fridge. My first test with the Kill-O-Watt worked out to 1.81 KwH per day. That is 1800 Watts per day on 120 VAC.

I just finished my second test because my fridge has changed the way it is running now due to a control board problem. The latest calculation is 2.03 KwH per day. That is 2000 Watts per day now. The reason it is higher now is because of the defrost cycle frequency that it has gone into which is not normal.

I will have it fixed one day but unless it has a catastrophic failure and quits altogether that fix is lower on the priority list, especially if I have to remove the fridge to install a new board.

The Kill-O-Watt devices are great tools.

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Old 03-20-2015, 08:39 AM   #11
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MT4Rinner, you left out the conversion of 6.5 amps @120 volt, equals 65 amps @12 volts.
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Old 03-20-2015, 02:51 PM   #12
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What I don't I don't understand is, when on shore power drop the temp as low as you can go. Once off of shore power - DO NOT OPEN REFRIGERATOR - until you are back on shore power OR you start your engine and let that power your inverter.
And less rule of thumb is 10 * AC current = DC current. Then you have to allow for the Inverter power required, which can be as high as 6 amps.

Whirlpool refrigerator, 660 amp capacity, 960 Watts solar, 2800 PSW inverter/charger. DIY'er
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Old 03-20-2015, 05:34 PM   #13
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We drop as low as possible on shore power. We do not open fridge. But is still goes up about 4-5 degrees per hour. Especially when it is hot. 454 big block eats gas idling. Just looking for the formulas, some easy fixes and if I do spend $$$, it will be on something that can be built on down the road.
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Old 03-20-2015, 06:54 PM   #14
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If it's struggling to maintain temperature , I would look at getting a Sine Wave inverter. The refrigerator compressor may be running slow on the MSW inverter.

Think about a 1000 watt one.

Good luck
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