Rez fridge question

naturist wrote:

DrewE wrote:
Hours; maybe 12ish hours on average, if there is such a thing as average.

This is a question that can be answered pretty well, at least well enough for back-of-the-envelope calculations, by comparing the usable energy storage of the battery with the energy consumption of a residential fridge. According to the Energy Star labels I've seen, a typical not huge fridge use around 1 kWh or a bit more per day on average (under their test conditions). A single marine/RV battery has maybe 50 Ah of usable charge, which multiplied by 12V yields 600 Wh or 0.6 kWh of energy.

Well, except that (a) the usual battery on an RV is rated 75 or 80 AH, so the usable is more like 37.5 or 40 AH, and (b) that rating assumes a 20 hour discharge time, and will be less with a shorter discharge time, so that you are not going to get those 12 hours. Probably more like 8 or 9. Into the bargain, the compressor startup surge (as @oldman pointed out) will likely be beyond the single battery to supply; too many amps drawn at once.

Correct.

As Naturists points out and I will point out that a typical standard sized RV "12V battery" that you would get from a RV dealer lot will be of a Group 24 or Group 27 size.

Group 24 might get you 70 Ahr and Group 27 might get you 80 Ahr of which you do not want to draw more than HALF of that capacity which nets you 35Ahr or 40 Ahr respectively.

Oldman points out the startup surge, and he is correct, a single group 24 or 27 most likely will not have enough capacity to handle the startup surge current.

My Res fridge has a running current of .80A at 120V, that roughly translates to 8A at 12V that the inverter will draw from the batteries.

I measured the startup surge of 10A at 120V which translates to 100A at 12 that the inverter will draw from the batteries.

The startup surge is in milliseconds but the battery must have enough capacity to not drop the voltage below 11V DC, otherwise the inverter will simply shut down due to too low of voltage.

In reality you MUST have enough battery capacity to sustain the high startup surge and not drop the battery voltage below 11V after hrs and hrs of operation.

one single Group 24 or 27 is not going to get you more than a few hrs due to the lack of surge capacity.

You really need at a min of 200 Ahr (100Ahr usable) in order to get about 20-24 hrs of fridge operation before needing to recharge.

To get that you will need FIVE group 27 12V batteries OR you could simply bite the bullet and buy two 6V Golf Cart batteries (wire in series plus to negative to get 12V).

2 GC batteries wire in series would cost less than the five group 27 batteries. $70 (group 27) x 5 = $350 OR $90(GC batteries) x 2 = $180..

Something else to consider, you NEED a ROBUST inverter, you are not going to make this work on a 600W or smaller inverter, they simply do not have the surge capacity. Some folks have been able to get way with 700W inverters but some have not. I personally suggest at a minimum go with a 1200W-1500W inverter, those tend to have enough surge capacity to reliably start the compressor.

Additionally, you NEED to use a SHORT HEAVY GA WIRE to feed the 12V to the inverter. The shorter and heavier the wire the better chance you will be successful. I use 1/0 and kept mine under 10 ft total (5ft for pos and 5 ft for the neg).

Cheap out and you will be redoing and redoing and be frustrated..