2P@12V or 2S@24V will it make a material runtime difference to a dual voltage DC Fridge

[eabyrd]

So I'm building out a Prius V as a basecamp vehicle for some doing some long trail segments and in order to ensure my step-2s or my SPATs are kept at the appropriate temperature while I'm traipsing through the foliage I bought a BougeRV DC fridge. It will run on 12V or 24V DC, and I've more than a couple well matched 12V AGM Deep Cycle batteries around the place so I'm going to bring 2 with me.

I've already experimented and determined that the fridge will run for a full 72 hours on a single 12-volt battery leaving it at ~30% capacity. So I figure 2X12 will be good for a good 144 hours or six days (plenty for a nice 100 mile loop trail through the mid-state. But I got to thinking, would running the fridge at 24V extend that run time in any way.

I'm going to experiment with it, in differing climate conditions to see what impacts that has as well. My 72 hour tests had it running in the basement, which is pretty much as kind as I could be to it. This weekend will see it running off that same batter in the uninsulated shed. It should see temps up about 100F. From there I'll move on to the 2-battery tests.

Why am I posting this you ask yourself (assuming you even made it this far); well I was wondering if any of you yahoos had played with these before and might like to hazard a guess as to the results.

Take care,

Ed

 

[A.Justice]

The short answer is no. When you add batteries together in series, the voltage goes up; in parallel, the amp-hour capacity goes up. In reality, both of your configurations would have the exact same amount of power. 24 volt system might have marginally less loss than a 12 volt due to less voltage drop, but it shouldn't really make that much of a difference, for you, in practice.

Watt hours (wh) are the real measurement of power. Amps and volts are relative, watts aren't.


For example: two 100ah, 12v batteries in series would make 100ah @ 24v (100ah X 24v = 2400wh). In parallel, they would be 12v @ 200ah (12v X 200ah = the same 2400wh).

 

[eabyrd]

Actually the short answer is that 24V provides about 10% additional runtime to shutdown. I've run the test three separate times now, and while it is impossible to duplicate the exact temperatures using a shed over 3-days, and while my timing isn't "to the second", I can say that on average the fridge runs on average 150 hours using 2 Mighty Max 110AH AGM batteries in a 2S configuration and only about 135 hours with those exact same two batteries in a 2P 12-volt configuration. Temperature settings were the same throughout at 36deg and 0deg in the two compartments, and the thermal load was also equivalent with the possible exception of 1 beer removed during a status check after a particularly hot day mowing.

While I agree with the math above, it appears that the fridge itself is more efficient at the higher voltage so does run for a materially longer time.

 

[A.Justice]

Actually the short answer is that 24V provides about 10% additional runtime to shutdown.

It seems like you've answered your own question then. I'm not sure the explanation for why that happened is the "short" answer though. ?

I've run the test three separate times now,

Do you mean you ran 3 tests in each configuration, for 6 discharges total?

and while it is impossible to duplicate the exact temperatures using a shed over 3-days, and while my timing isn't "to the second", I can say that on average the fridge runs on average 150 hours using 2 Mighty Max 110AH AGM batteries in a 2S configuration and only about 135 hours with those exact same two batteries in a 2P 12-volt configuration. Temperature settings were the same throughout at 36deg and 0deg in the two compartments, and the thermal load was also equivalent with the possible exception of 1 beer removed during a status check after a particularly hot day mowing.

While I agree with the math above, it appears that the fridge itself is more efficient at the higher voltage so does run for a materially longer time.

If your wiring is the same for both your 12 and 24 volt setups, then you'll get a higher resistance on the 12 volt system, and more heat loss. The fridge might not be that much more "efficient" at 24v, but rather the wiring is small for 12v. I'd be interested to know the results if you were to double the wiring up with the 12 volt system.