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Old 11-21-2015, 06:13 AM   #15
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FWIW the math for the batteries is simple.

Two batteries in series have the capacity of the smallest battery. With his matched pair each string is 216 Amp hour.

Batteries in parallel add capacity so each string of 216 AH = 216+216+216= 648 AH

For best life one stays above 50% charge or 324 AH discharge.

The 324 AH is not a hard number. It is measured at a fixed rate for a long period on a deep cycle house battery. Usually 20 Amps. Increasing the draw reduces the capacity while reducing the draw increases the capacity. His bank is designed around a 3 x 20 = 60 AH draw at that capacity. As long as one stays around that the number is good.

The issue for Inverter produced AC is efficiency and maintaining power. Load power stays the same plus there is an added loss for efficiency. The nominal voltage shift is 10:1 but the real voltage shift is really a bit lower because the battery is higher than 12 VDC. That lets use 10:1 as a reasonable estimate. In this case his 11.6 Amp @ 120 VAC needs 116 A @ 12 VDC so the load is theoretically hitting the battery bank hard thus lowering the capacity by a bit. How much might or might not be on the manufacturers page. It also defines the worst case as 324 AH / 116 A = 2.79 H. run time.

Don't give up yet:

The refrigerator is the really mushy number. Here is the link to the Samsung RF197:


If you look up the specifircations the motor draw is listed as 130 W, the ice maker heater at 350 W, defrost heaters total 360 W. That still does not come to the 11.6 Amp or ~1400 W specified by Samsung. What that is really telling us is that there are several things that draw power intermittently with the compressor motor probably the low draw but possibly the longest running.

What that really tells us is that it pays to minimize power use in the refrigerator. Get the defrost cycle off or timed for daytime, stay out of the refrigerator before bedtime so it is cooled down and stable and make sure the ice is full all while on generator power after dinner. That should set up for a few hours of only the 120 W compressor running at night. That would be roughly 10 A on the battery bank or 324/10=32.4 hours run time so one can coast through the night *as long as the other stuff does not come on*!

It also leaves a chunk of capacity for the CPAP and water pump and some lights and TV. This should also show why some folks cruise and some complain. If you like to kill the genset at 7 so dinner will be quietwith the movie on the big screen while cleaning up and putting away the food then popping out a few cold one's etc your batteries will be half way down before bedtime because the refrigerator will be busy making ice and defrosting. OTOH if you keep the genset on through dinner and getting your evening set the shut it down after giving everything time to cool off then stay out of it things will probably work through the night.

This is also a good example of why I would not put a residential refrigerator in any unit I expected to dry camp with in anything not capable of handling 6-8 batteries in the house battery bank and probably solar on the roof to start taking up the slack when the sun came up. YMMV.

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Old 11-21-2015, 07:33 AM   #16
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I guess I must have a very low consumption fridge or I am getting something extra out of my batteries. Based on my experience I get way better use than would be suggested by some of the math.

PS: Second test I ran was boon docking at Tamarack Rest Stop. We watch TV in both the bedroom and living room, ran the cpap used the refrig and incidental lights from 10:30PM until 7:00AM (about 8.5 hours). Battery status indicator still showed 11.9.

This and my previous anecdotal example are based on my recent experiences.

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Old 11-21-2015, 11:43 AM   #17
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I looked up the energy rating for the Samsung RF197 mentioned above. The annual power consumption estimate is only 445 kilowatts. So Harris 155 was pretty close with his statement of 100 to 200 watts per hour and my estimate of 500 was way high. The actual per day would be 445/365 = 1219 watts or only 51 watts per hour. With this data, the refrig should run for 60+ hours before depleting the batteries to 50%. That's pretty darn good.
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Old 11-21-2015, 12:12 PM   #18
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I have six 12volt lifeline batteries, we have a Samsung 197. June we operated off the batteries for 36hrs , no solar, two tv's in afternoon, LED lights, ceiling fan on both nights. Ice maker was off with no problems. Plus water pump, charging two tablets, two cell phones. We dry camp often for several nights easily, we crank main engine to recharge battery's when we can.
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Old 11-21-2015, 02:44 PM   #19
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Ok you have six Amp Hour six volt batteries arrainged like this

into 3 216 amp hour 12 volt batteries.. Total 648 Amp hours 324 usable

your fridge draws 11 amps at 120 volts (OUCH that' over 1300 watts as much as a Microwave.. most Residential fridges are closer to 1.1 amp (130 watts range) Assuming one of us missed a decmil point

324/11 and factor in some conversion loss.. Around 25 to 30 horus would be my best guess.

Of course this assumes NO OTHER LOADS. but if you run your generator for 8 hours a day you will likely be good for years.
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Old 11-22-2015, 05:18 AM   #20
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For fellow learn by pictures

I understand better with pictures and thought this shows it fairly clear. Then you can do the math for your coach.

Note: Thanks to the folks at Trojan Batteries for this.
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Old 11-22-2015, 06:38 AM   #21
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FWIW - You folks are missing the point of the math.

The first point is that 11.9 VDC is below the 50% point for maximum life thus you are hurting your batteries at that voltage.

Second - the problem with a refrigerator is that there are several systems running concurrently. If all it is doing is cooling then all that is running is the compressor and a couple of fans or less than 200 watts thus ~16 hours of run time before the 50% point and closer to 30 hours if you want to kill your batteries. On the other end if the defrost cycle kicks in and the icemaker makes a new tray you are drawing the rated ~1400 Watts thus hitting the 50 % point in ~3 hr and draining the batteries in ~6 hours.

That is why there are all the anecdotal long run times. They all fall within theoretical limits. The issue is what you are willing to deal with in short battery life and trying to control usage.
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Old 11-22-2015, 06:57 AM   #22
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I have a amp hour meter I my coach. The last time we dry camped I checked the meter just before bed.

With 3 TV's/direct tv receivers, microwave, all on stand by (not in use), a 10.3 cf residential fridge on, (but doors not opened), all run with a 2000 watt Xantrex inverter/charger, I used 80 amp hours in a 8 hour period.

I won't go into outside/inside temperatures, defrost on/off, misc. lights etc.

With 4 golf cart batteries, with the above conditions I figure I can go approximately 20 hours (400 amp hours/200 amp hours available for use)before running the generator.

With all the variables with each coach/conditions I hope this gives a general idea of my amp hour usage.

Your usage will vary.
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Old 11-22-2015, 09:12 AM   #23
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More Pictures

Here is a chart on use of batteries and useful %s that I've used. While it is based on 12v, fairly applicable to 6 volt batteries.. Hope it helps others as did us as we were doing our computations. Note that the closer you got to 11 volts after passing 11.7 the more subject you are to shortening your battery life. I use the Trojan T105 batteries that cost around $125.

The info is taken from this site: RV 12v Information - Everything You Need to Know
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Old 11-22-2015, 09:15 AM   #24
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Simple math is to use power as the common variable so first convert all variables to that.

Formula is simple.

Volts times amps.

For anything related to the inverter add 10% for overhead.

To determine drain on battery convert back to amps at 12 volts but we add a twist.

Whatever your low voltage is use that.

So if you use 11.5 volts then divide your watts by that number.

That will be the highest amp drain so calculate based on that.

To complicate matters intermittent loads like fridge can be placed in a spreadsheet to determine an average load for run time and maximum load for capacity of equipment.
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Old 11-22-2015, 09:53 AM   #25
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OP - If you start the night with a 98-100% charged bank, you should be OK overnight.

Good input to turn off and control the times as much of the fridges features as possible, ice making, set defrost for during the day.

Also even better input that you have many more systems drawing power then the fridge, which is additive to the both the CPAP and existing power draws. And it is the 'total draw down' from the battery bank, that matters - not just the new fridge. (Which also remember, is replacing an existing fridge, probably LP/120 fridge, which was also drawing at 12V while in the LP mode. So, say conservatively it draws 25% of what a residential fridge will draw overnight, which means your new fridge is actually about 75% additive draw down.)

If you are planning extensive boon docking, have a shunt valve and power reading meeter added to your system. Know and monitor and take sometime to understand what this meter is showing. SOC (State of Charge) is the easiest to understand. Trimetric is one of the most popular meters.

Trimetric Battery Monitoring System

As I was adding an all new Magnum MS2812 system, I elected to use the Magnum BMK Meter, which is a good one too.

Many factors to maximizing the life of your battery bank, but one key habit is to stay above the 50% SOC level draw down. Battery life, is all about total cycles - and simple math of battery life, is that the lower your regular discharge, and then recovery back to 100% SOC, the faster you use up the available total life cycles of any battery bank. (We try to keep our night discharge to between the 70-75% SOC level when boon docking, and have added battery bank capacity to do this. Lifelines a expensive, so maximizing battery life is facilitated by oversizing your battery bank a bit.)

Without a meter, you're running blind! But again, simply put, if you are close to fully 100% at the start of the evening, you should be fine in the AM.

Best of luck to you, and have fun,
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Old 11-23-2015, 05:11 AM   #26
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So Jumper777 - is that enough info? Ask a question around here and the data avalanche begins - which is a good thing.

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Old 11-23-2015, 04:42 PM   #27
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Battery rating are be to be compared at the same rate and the would be the 20 Hour to equal 225 amp hr.

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