2000 watt inverter

[wrenchited]

I have 4 6 volt battery's for my 2000 watt inverter. They are 115 mins. @ 75 amps. I'm wondering how long I could keep my 19 cubic foot, 110 vac refrigerator cold when motorhome is sitting in approximately 80 degree weather?
Thank you in advance.

[Kiawah]

Think in the neighborhood of a day.

We've found a reasonable approach when camping somewhere without power, is to run the generator for about an hour when we wake, heats the hot water for shower, coffee pot, breakfast, hair dryer, and puts some charge to the batteries.

Run the generator again in the evening while cooking dinner, makes hot water, charges batteries. Run the A/C if needed to cool the coach down, or fireplace if want to warm the place up.

Sometimes we'll just run the water heater of propane, but if going to run the generator anyhow for the batteries and high current devices, then might as well let it make the hot water also.

We've got 6 house batteries. Auto Generator Start is set up to come on and charge the house if the batteries ever get to low, as a safety net.


If we're driving during the day and just stopping for the night, then don't run the generator at all, as the engine has the batteries all charged up.

[vito.a]

The answer is "That depends". We need to know how many amps your fridge consumes when the compressor is running and approximately how much time per day it runs?
Of course this depends on how much the door is opened and if it is one of the new Energy Star+ models.

Also, what voltage does your inverter charge your batteries up to. Some only charge to 13.2v which leaves a portion of your battery capacity unused, but helps them last longer.

Our Windsor has a 13CF Energy Star+ fridge that uses .8 amps at 120v when the compressor is running. It will go several days on four 6v golf cart batteries as long as you are careful with door opening. You can generally multiply the .8 amps at 120v by a factor of 10 to get amps at 12v. This does not take into account losses in the inverter, but is close enough.
Our Executive has a 20CF fridge that uses 10-20 amps at 12v to power it when the compressor is running. The compressor on it tends to run more often and it also runs a frost free cycle as well as an ice maker that consumes power. Most of the time we can get at least a day out of the batteries, but again there are a lot of variables here.

One thing that also helps is the engine alternator charges the house batteries after the chassis battery is full. This really helps going down the road. We also have one 100w solar panel that charges about 6 amps during the day. To be completely generator free, we would need over 1000 watts of solar.

Hope this helps you answer your question.

[Gary RVRoamer]

12-24 hours for a typical residential fridge (an Energy Star rated model). Most newer fridges use only 1-2 amps @ 120v, which translates to 10-20 amps from the batteries whenever the compressor is operating. But how often the fridge runs to stay cold can vary widely. Some major factors are how often (besides the RV's interior temperature) include you open the fridge door, the fridge temp setting, and whether or not the sun is shining directly on the wall behind the fridge.

[jelag]

Go do some research on OHM's LAW, and study that.... once you have a handle on Ohm's law you can take either the wattage or amperage that your refrigerator draws when its running.... that information will either be in the owners manual or the placard on the unit next to where the power cord exits the refrigerator.... once you know this value you can determine the power usage.... assume that the unit runs about 20-30% of the time based on how often you open the door and let out the cold air... of course overnight this run time will decrease as the door is closed for longer periods of time....

lets assume that the draw is 8 amps.... or 8 amps X 120 v = 960 watts....

You have 2 batteries connected in series and 2 sets of these.... so your 75 amps x 2 = 150 amps for 115 minutes or about 300 amps for 60 min.... @ 12VDC if I do the math correct that's 30 amps for 60 minutes @ 120VAC across a 100% efficient device... you unit in my 8 amp (GUESS) would mean that you'd run about 4 hours continuously or about 12 hours if the unit runs at a 25% duty cycle... I did the above in my head rounding off so my math might not be correct but I hope you see the process.... NOW assume your unit only draws 2 amps @ 120VAC, that would mean close to 48 hours of OFF/ON use (as suggested by Gary above).... I've never looked at the label of one of these units so I'm guessing

I'm very surprised, as usually a battery is rated in amp hour rating.... not the kind of rating your suggesting... but the math is still the same... the size of your inverter has no input into this process unless your unit has less than a 95% efficiency rating... and its large enough to handle the load....

Plus my math has nothing to do with any other load (12 volt DC or 120 V AC) on the circuit.... remember that if the unit draws 1 amp at 120 volts that means it draws 10 amps at 12 volts allowing nothing for electrical loss across a device that doesn't have a high efficiency rating..

One last thought... MY 6 Volt Batteries (6 of them) are rated at 235 amp hours... that's 235 amps for one set of batteries for 1 hour... or 235 hours with a 1 amp draw... I have 3 sets... so my total capacity is close to 650 amp hours.... I'm thinking either you have your battery rating wrong.... or you have very small 6 volt batteries.... mine are golf cart batteries....

when the batteries are in series... the capacity of the battery is not added together as your using the same capacity at 2x the voltage... going from 6 to 12, but when you put 2 sets of these (4 batteries) 2 in series with 2 sets in parallel you than double the capacity... in my case with 6 batteries its 3x's the capacity...

[twinboat]

L.p.

Originally Posted by jelag

One last thought... MY 6 Volt Batteries (6 of them) are rated at 235 amp hours... that's 235 amps for one set of batteries for 1 hour... or 235 hours with a 1 amp draw... I have 3 sets... so my total capacity is close to 650 amp hours.... I'm thinking either you have your battery rating wrong.... or you have very small 6 volt batteries.... mine are golf cart batteries..

Your 235 AH batteries are rated at the 20 hour rate, you divide the AH by 20.

That means you can draw 11.75 amps for 20 hours, before they are dead.

If you have 3 strings of 6 volt, you can draw 35.25 amps for 20 hours.

If you draw more then the 20 hour rating, your capacity drops. A little research will give you the 100, 20, 5 or 1 hour rating.

Here is an example of a 105 AH battery. You can see that capacity drops as amp draw increases.

Capacity also increases as amp draw drops.

27TMX*Deep-Cycle Flooded
Capacity Minutes

Capacity Amp-Hours

5-Hr Rate 85

10-Hr Rate 97

20-Hr Rate 105
The typical rating batteries are marketed at.

100-Hr Rate 117


Minutes in reserve capacity, RC, is a measurement using a fixed amp draw.

They draw 25 or 75 amps, and then time how many minutes before dead.

These figures are just for calculating battery capacity at a fixed amp draw. Using them for our purpose, we will never exactly match them.

[twinboat]

Originally Posted by wrenchited

I have 4 6 volt battery's for my 2000 watt inverter. They are 115 mins. @ 75 amps. I'm wondering how long I could keep my 19 cubic foot, 110 vac refrigerator cold when motorhome is sitting in approximately 80 degree weather?
Thank you in advance.

Your batteries are about 225 AH each.

Having 2, series strings, gives you 225 AH usable capacity, until drawn down to 50% capacity.

If your fridge draws 15 amps DC, 1/3 of the time, thru the inverter, you are near the 20 hour rating, so your batteries should last about 48+ hours, running the fridge ONLY, if fully charged when started.

Knock off 5 hours for ineffencies of the inverter and wiring.

If the fridge is opened and closed or it draws more then 15 amps, the compressor run time will increase, use more power, and the batteries will last less time.

This is with close to new batteries, with a few deep cycles, under their belt.

As batteries age, if treated correctly, they start dropping capacity, until your not satisfied with their performance.

If you abuse them, the just fail quickly.