A lot to unpack here. AH isn't really an "hourly rating". It most definitely is NOT amps PER hour. It is AMPS TIMES HOURS.
When the compressor is running, it sinks some current, say 5Amps, but that is what engineers call an "instantaneous reading", meaning that at the instant you put a meter on the compressor, it's 5A is going from the battery to the motor. If the compressor ran 100% of the time for an hour, then it would be "consuming" 5A * 1H = 5AH from the battery. But fridge compressors don't really run 100% of the time - they cycle on and off. We can confidently state that a fridge uses X-many amps when running, but trying to put a hard number on a fridge for Amp-Hours is difficult because it depends entirely on how much of the time the compressor is actually running. (The duty cycle). If the fridge runs for 50% of the hour, then it "consumes" 2.5AH over that hour. But the duty cycle will vary all over the place depending on factors like insulation, external temperature, how many times the fridge is opened, etc.
Mostly, when people say a fridge "running between XAh to YAh", they're trying to give an average of amp-hours per hour, or an "average instantaneous Amp draw". That's a weird number, but it is meant to encapsulate that (1) not only does the compressor not run 100% of the time, but (2) the duty cycle of the compressor isn't even constant throughout the day. It runs less at night, or in cool conditions, versus when you're parked in midday sun.
ARB, for example, quotes the fridge's current draw as "0.85 amps/hour average power consumption tested at 37.4˚F cabinet temp. 78.8˚F - 87.8˚F ambient temperature range. Test duration of 66 hours and supply voltage of 12V DC "
Even this is a little bit of a weird statement because the units don't track. "amps/hour" is not a power reading (power is watts, or amps-times-volts). Once you involve time you're getting into
work, (power * time), or Joules. I
presume what ARB is indicating is that over 66 hours at the given temperature, 56.1 Amp-Hours were depleted from a battery. (e.g. 56.1 AH / 66H = 0.85 A. This number makes sense in terms of Amp-hours per hour.)
I understand the confusion, believe me. Part of the issue is that all the different companies are very loose with their terminology. Amp-Hours are already an imprecise number. When sizing a battery, what you really want to know is
Joules, which is the total measurement of the energy in the cell. A Joule is a watt-second (one watt for one second, so 60J is a watt-hour). However, since watts are just amps * volts, and the voltage of a battery is "fixed" (i.e. 12V), then battery companies market the batteries as Amp-Hours.
Except that the voltage isn't really fixed, (volts drop as cell is depleted), and cell chemistry also dictates that capacity (Joules) suffers as the current sink goes up- which is why Amp-Hours are expressed as what they call the 20-hour rate, whereby they state that a battery will give a constant rated Amps for 20 hours), e.g. 100AH, calculated that the battery will give 5A for 20 hours. If you draw 10A, for example, you will find that you get less than 10 hours (less than 100AH).
So, you have the following environment:
- A battery holds X Joules, except X varies by temperature (T) and the draw rate (Amps).
- That battery has a cell voltage V, which varies over time (H), and draw rate (Amps)
- A fridge pulls A Amps when running, but only runs D % of the time (duty cycle), which also depends on temperature (T), along with insulation and how you're using it (putting in warm beer every 20 minutes, etc.)
So while theoretically you could say that A * D * V * H = Joules-needed, and just buy battery X > Joules-needed, you can see how complicated it gets.
If you want to see what this looks like in practice, here's a test I ran by powering a fridge on a laboratory power supply for 24 hours.
This has somewhat fewer variables because the voltage was stable, the temperature was stable, etc. What you can see from the raw data though is that for most of the time (84%) the current consumed was 0 because the compressor only ran 16% of the time. During the time the compressor was running, I sampled the Amps every 15 seconds. We can calculate "Amp Hours" used during those 15 seconds by multiplying measured amps (A) * 15s (Then multiplying * 1m/60s * 1h/60m to convert from seconds to hours). That gives Amps times Hours.
As a specific example: at 17:55:16 (page 5), the fridge was pulling 4.708A. So during those 15 seconds, we estimate that the AH consumed was = 4.708 * 15 / 3600 = 0.01961667 AH. If we add up the AH measurement from all the other 15 second intervals, we get the total AH consumed. The data is all summarized on the first page, but I will point out that my measured number was an average of 0.58823 amp-hours per hour, which is not far off from what ARB quotes in their datasheet. My average is lower than their average because I conducted the test with a cooler ambient temperature, but we're in the same ballpark.
