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Old 11-20-2015, 04:19 PM   #1
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Battery Math,,,,,

Greetings all!

I have a 2011 Phaeton. It has 6 - 6 volt batteries. My question is,,,,, I have a residential frig that states it draws 11amps. How do I calculate how long I can run my frig on a fully charged set of batteries? I have the 216 amp hour batteries.

I realize the frig may cycle on and off though out the day, but to keep it simple I am trying to calculate the total run time.

Thanks!

L.
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Old 11-20-2015, 05:07 PM   #2
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You have about 650 Ah of batteries with about 325 Ah of usable capacity (50% SOC). Divide 325 Ah by 11 amps will give you about 30 hours of run time at 100% duty cycle and efficiency. Of course you don't have 100% efficiency (running through the inverter) and the refrigerator won't run 100% of the time. Don't forget many other things run off of the batteries, drawing them down. Even with the above, you should be able to survive for 24 hours before you need to fire up the generator to recharge the batteries. I usually run the generator in the morning for an hour or two and again in the evening during meal prep to get a jump on the recharge cycle.
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Old 11-20-2015, 05:15 PM   #3
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The power consumption of domestic fridges is typically between 100 and 200 watts. Over a full day they are likely to use around 1-2 kilowatt-hours...Quoted from the internet...

Your battery capacity is around 600 amp hours

Theoretical calculations
2000 watts per day
166 watts per hour
Ohms law E x I = Power in Watts or Watts/E = I in amps
166/12 = 13.8 Should average about 14 amp hours.

Does not take in any account for inverter efficiency.

Your battery bank should easily run the fridge for a couple of days.
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Old 11-20-2015, 05:22 PM   #4
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FWIW two other issues. The refrigerator is no the only draw and may not be the biggest. See what else is running. The biggest hit on the refrigerator is probably the automatic defrost heater and ice maker heater. Shut them off if you are dry camping.
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Old 11-20-2015, 05:49 PM   #5
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Quote:
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You have about 650 Ah of batteries with about 325 Ah of usable capacity (50% SOC).
I'm 100% green at the RV stuff. But would there not be roughly 440 Ah? 4- 6v batteries wired parallel/series? I would like to learn this for sure, but I thought parallel/series would double the Ah and double the Volts (2- in series, 2- in parallel)?
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Old 11-20-2015, 06:33 PM   #6
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I'm 100% green at the RV stuff. But would there not be roughly 440 Ah? 4- 6v batteries wired parallel/series? I would like to learn this for sure, but I thought parallel/series would double the Ah and double the Volts (2- in series, 2- in parallel)?
You are correct if you had 4, 6 volt batteries.
The OP said he has 6, 6 volt batteries.
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Old 11-20-2015, 06:56 PM   #7
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Quote:
Originally Posted by gunkis84 View Post
I'm 100% green at the RV stuff. But would there not be roughly 440 Ah? 4- 6v batteries wired parallel/series? I would like to learn this for sure, but I thought parallel/series would double the Ah and double the Volts (2- in series, 2- in parallel)?
Of the 440 Ah available, you can only use about 50% of it without doing long term damage to the batteries. A fully charged "12 volt" battery should have around 12.6 to 12.7 volts. A 1/2 discharged battery will measure around 12.1 volts.
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Old 11-20-2015, 08:01 PM   #8
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Since this is a residential refrigerator, don't forget that the 11 amps rating is at 120 VAC not 12 VDC. The amp rating is the max the refrig will draw which would be 1320 watts. Figure the average usage may be in the 500 watt range. Take the 325 amp hours of 12 volts that your battery bank can safely supply and now determine the inverted theoretical max at 100% efficiency. That works out to 32.5 amp hours of 120 volt AC. 32.5 amp hours at 120 VAC is 3900 watts. So at 500 watts average, you would be able to run the refrig for a little over 7 hours. Depending on your inverter the actual efficiency will be lower than 100% so 90% is a good number to use. That gives you about 6.5 hours.
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Old 11-20-2015, 08:07 PM   #9
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Originally Posted by jumper777 View Post
Greetings all!

I have a 2011 Phaeton. It has 6 - 6 volt batteries. My question is,,,,, I have a residential frig that states it draws 11amps. How do I calculate how long I can run my frig on a fully charged set of batteries? I have the 216 amp hour batteries.

I realize the frig may cycle on and off though out the day, but to keep it simple I am trying to calculate the total run time.

Thanks!

L.
That 11 amp stated draw is only at the split second of compressor startup, the lights on and the ice maker running.

Like someone else mentioned, it will probably draw 2 amps at 120 volt, equaling 20 amps 12 volts DC.

I have tracked my residential 7.5 c.f. frost free, fridge, in my boat, for 5 years, running on a 2000 watt sine wave inverter. It draws 1 amp at 120 volts AC and 9.8 amps, 12 volts DC. At startup, my DC monitor jumps to 30 amps for less then a second.

You can pickup a Kill A Watt meter for around 25 bucks to track and confirm your usage.
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Old 11-20-2015, 08:46 PM   #10
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Battery volt availability

Well okay - I guess I don't understand all the math. Preparing for this winter and when we will be boondocking in warm climates and can cannot run a generator from 11:00 PM - 7:00 AM. I was concerned about running our refrig, plus I need to run a CPAP. Did a lot of looking and found the two attached matrixes. I have 4, 6 volt batteries running 2 in series and then combining the 2 parallel. To test the math, I left my refrig on in warm weather it ran for 22 hours and showed 11.2 volts on my inverter read out.

Hopefully that is a data point for you.
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Old 11-20-2015, 09:49 PM   #11
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Ahh. I missed this. :-)
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Old 11-20-2015, 10:48 PM   #12
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The OP never indicated whether the amp rating he stated was 120V or 12V. However I have measured my refrig, and it draws from 120VAC 1amp while the compressor and fan are running, and 8amps while the defroster is working.

"IF" an inverter was 100% efficient, then a 120Volt draw of 1amp would equate to a 10amp draw @ 12v from the batteries. So his statement of drawing 11amps I equated to being a statement of 12volt draw.

At that point it is a simple calculation to get from a 12v battery amp-hr capacity to theoretical hours of refrigerator usage.
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Old 11-21-2015, 06:44 AM   #13
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If you are off shore power while using a residential fridge and other loads you are going to be consuming at least 15 to 20 amp/hours.
After one day you are still going to put back 300 to 500 amp/hours back into that battery bank.
That will be several hours running the engine, generator or on shore power.
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Old 11-21-2015, 06:59 AM   #14
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X2 for the killawatt. Best way to find actual usage over time.
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