So I decided to do another one of these tests, probably out of boredom, possibly just to stretch my batteries legs. So, lets get right into it:
Fridge: ARB 50qt (with wireless control)
Battery: NAPA group 24 deep cycle, 12V 140 RC @ 25 ah/r rate (58 Ahr calculated capacity)
Ambient air temps: 40-50 F
Fridge temperature setting: 0F
Starting fridge temperature reading: -3F
Test started: Feb 26 @ 1:40 PM
Test Starting open voltage: 12.78V
Test ended: March 1st 8:00 AM
Test ending open voltage: 11.45V
Ending fridge temperature reading: 8F
Duration of test: 67 hours.
Duration before cutoff: ~60 hours.
Draw average: .88 A/hr (53 amps over 60 hours)
Given that the temp was 8F I would guess the limit was reached about 60 hours in.
Test procedure was not formal, I opened the freezer at least once a day, and took IR temperature readings, or got me some frozen soup.
NOTES ON VOLTAGE
The Freezer was first set to Medium voltage cut, but that was too conservative and it cut voltage at 11.96V leaving a lot on the table for charge. So it was set to Low after reaching the medium limit.
At first I thought the low setting was reckless (10.1V is a DEAD, non-recoverable battery) I found that depending on the state of the battery, startup load would drop voltage between .5 and 1 volt even with a very short run of 12 gauge cable. With that in mind I think these settings are fine. I think the restart voltage for “low” is a little low, but it doesn’t matter, since at that state of charge the startup load caused the voltage to drop to just under 9.35V with an open resting voltage of 11.45.
Consulting the ever caveated voltage percentage chart,
we see that at the medium setting you are looking at about 45-50% capacity remaining...which ought to be enough to start a car or truck. Low is about the limit you want to push any battery, even deep cycle. Frankly I wouldn’t want to my batteries to be going below 11.6 regularly. It would be nice to have something between medium, which leaves a little too much on the table and low which takes a little too much. Frankly, I plan on leaving mine on Med when I have it in my truck (partly because my wiring have a voltage drop of ~.8 under startup load)
CONCLUSIONS
At an average of .88 Ahr thats a good baseline for a delta T of 45 degrees and gives me a nice datapoint alongside the other test where I got a rate of .575 ahr over 87 hours with a Delta T of about the same 45 degrees. Its about what I would except to see as well given the additional energy required to freeze vs cool.
As soon as it warms up I want to do these tests again to find out whats actually reasonable to expect in real world conditions. The truth is that no one needs a freezer if its 45 out except in a power out situation.
The good news is that even with a pansy group 24 I could reasonably expect to ride out a power outage of 48 hours or so and keep foods frozen.