Electric math problem

I am no math wiz, and never will be.

I want a refrigerator for a small diesel Blazer mini-motorhome I'm building. Special purpose small DC refrigerators cost $1000 or more.

So, I got wondering just how much power the cheap little AC dorm-sized refrigerators use. Keep in mind that my Blazer has 3000 watts of AC power available.

The Blazer has dual deep-cycle storage batteries with a 3000 watt inverter. They provide a total of 225 amp hours at 12.5 volts.

I've tested the refrigerator for two days on 120 VAC and it is using an average of 24 WH per hour (WH being watt/hours). The actual reading I got is - in exactly 12 hours the frig used .27 KWH at 120 VAC. So, I'm figuring .27 KWH = 270 watt hours. 270 watt hours divided by 12.5 volts DC = 21.6 amp hours - correct?

So, I have a pair of deep cycle 6 volt batteries rated at 225 amp hours each. Run in series to make 12.5 volts, the total sum is still 225 amp hours.

So, they can supply 1 amp steadily for 225 hours at 12.5 volts. Also, that means they can supply 21.6 amps for 10.4 hours? Is that correct?

My problem is - every time I sit down and try to figure this out, I get a different answer - so I must me making some mistakes.

I also realize I'll have around 5-10% loss with the inverter, but forget that for now.

Please correct me if you see errors.

Approx 3-1/2 days given that the compression is likely running about 6 minutes per hour.
Open and close that fridge and make the compressor run 30 minutes per hour. The batteries will be run down in 1/2 a day.
While the batteries may figure to 225amp hours. It's best even with deep discharge batteries to not take them below 1/2 charge.
With the roof mounted 240W solar panel. The outfit would almost work if the panel was aimed directly towards the sun when parked.
This project is making one of those ultra quiet inverter generators look good for a couple of hours morning and night.

We've got a large DC chest fridge at our camp. One 120 watt solar panel and a pair of batteries is more than enough to run it anytime we've been there. A few sunny days, and rainy days, it all balances out. But the DC Sundanzer chest fridge is pricey and supposed to be one the most efficent on the market for off-grid use. It's 8 cubic feet, 4 feet long by 26" wide and 3 feet high. On a 70F day it uses 140 watt hours, and if it's 90F it uses 300 watt hours.
Amazing machine, but too big and too expensive for this mini-motorhome project. All we need is something that can stay cold for maybe up to 12 hours at at time. We never go any longer than that before we start the engine and drive. Then, the batteries get recharged. I don't want a gas/electric fridge because they draw too much current in electic mode. Most just use the electric to run a heater, that in turn, acts like a propane flame. Rube Goldgerg and power hungry.
A cooler with ice also does the job. Just means we've got to keep buying ice, and having ice leaves less room for food.

Just a few ramblings in general

Be very careful about efficiency. I haven't tested this, but you can, I think. The draw of the refer unit may well be different on the inverter because of differences in sine wave purity, actual output voltage, etc. so you need to measure that under those conditions, and average over probably several days

Also, The starting load may affect the amp hours rating somewhat, especially if for some reason the thing short cycles I can imagine that inverter "droop" could cause significant higher starting loads on the inverter than on goo solid AC, especially if the batteries are down.

Last is your amp-hour rating. What conditions are those batteries rated for, I.E, the old so called "20 hour" rating? You DO realize that you cannot simply use the mathematical product of an amp hour rating and come up with the true capacity? That is, these ratings are sort of averaged at best, a guesstimate in the middle, and sometimes an outright lie by the battery supplier at worst.

On another note, Be very careful using a Dorm style refrigerator in a motor home. They don't like ANY movement. I have seen some go bad - broken soldering on the tubing - just carrying in and out of dorms.

A couple suggestions. Have you looked at the refrigerators made for over the road trucks? Depending on how big you need, I think some of them are a lot less than $1,000, and would do a much better job of managing load than a normal household refrigerator running on an inverter.

You might also try poking around some of the RV mailing lists like RV.net or escapees.com. I haven't paid a lot of attention, but I know there have been some threads on replacing RV refrigerators with 120 volt household ones.

Finally, you might try watching places like Craigslist for a used camper or RV refrigerator (or a camper/trailer with one in it). I bought a working one once for $40, and there's currently a couple on the local craigslist in the $200 - $300 range.

Keith

Amp-hour ratings of batteries are the "area under the curve". That is, when rating your batteries, they probably didn't discharge them at a fixed currect, but rather adjusted the load for a fixed voltage as the battery discharged. Hence they are rated at "12.5 volts", which might be 20 amps when fully charged and 100 milliamps when nearly discharged. They plot the resulting current versus time curve, and measure the area under that curve. The area equals the amp-hour rating for the battery.

Now for low-current loads you'll probably get very close the the rated amp-hours. But for a high current load, the voltage will quickly drop off causing the current demand to increase and a further drop in voltage. The result is that you'll be discharging the battery much faster than the rate at which it was rated, so you can't expect to get the full rated amp-hours. I'd suggest you assume you can get fifty percent of the rated amp-hours. You might do a little better, but you probably won't do any worse.

Yes, I've looked and many and used a few. Anything that is truly a refrigerator with a compressor and thermostat is expensive. The "cooler/heaters" are much cheaper, but only cool at best to 40 degrees below whatever ambient temps are - and also run constantly.

As far as RV stuff goes, I have many. The three-ways are very inefficient when used in electric mode. Most do not use compressors and are meant to be used on propane when parked, and electric only when the engine is running - or - when grid power is available. They can be converted to run on diesel or kerosene also.

(quoted from post at 02:55:38 04/29/09)

......for low-current loads you'll probably get very close the the rated amp-hours. But for a high current load, the voltage will quickly drop off causing the current demand to increase and a further drop in voltage......

The result is that you'll be discharging the battery much faster than the rate at which it was rated, so you can't expect to get the full rated amp-hours. I'd suggest you assume you can get fifty percent of the rated amp-hours. You might do a little better, but you probably won't do any worse.

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Read that again. It's important, and is the point I was trying to make. Amp hour ratings are not an exact science, and depend on how the battery was rated, the honesty of the published ratings, and the load that you are applying.

You don't say where you are from, but I seem to kind of remember somewhere in Missouri. But, doesn't really matter, if you were near Denver, CO I would tell you to go to an RV salvage yard there and buy a 110V/LP powered used unit.

I would start watching your local Craigslist, you just might be able to pick up an ugly old camper with a functional refrigerator for next to nothing. You would then have other stuff you can use in yours also. DOUG