I just looked up Peltier cooling, I think its the same tech used in cheaper 12v fridges, yeah? Any idea what sort of wattage would be required to keep your batteries 20 or so degrees below ambient (i.e. 80f at 100f ambient)? From what I remember fridges built using this tech struggle to keep things cool and definitely don't keep them cold in a hot car. But for batteries 'cool' would be more than enough, so the question becomes how much energy does it take.
12V minifridges - yes, some of them use peltier. The fancy / good 12V/24V fridges that Will P. recommends on his other website are NOT peltier coolers to my understanding. They I think actually have vapor-compression kind of technology - just as a traditional fridge would - and I think the reasoning for this is because it's more energy efficient than peltier. Peltiers are fundamentally limited I do believe more so than vap-compression in that they can only achieve a certain delta T - so you are limited to cooling only X or Y degrees below ambient, whereas vap-compression you could get down to below freezing when it's 110 F outside. Peltiers I think are limited to 20 or 40 F, depending on the actual ambient temperature (different peltier performance at different ambient temps). You can stack peltiers (multiple peltiers) to achieve more cooling, however.
I took a cheap 12V peltier fridge and rigged up a compartment out of an old styrofoam cooler. 12V fridge was affixed to the top of the cooler and sealed-up such that the 12V peltier could cool the approximately 1 cubic foot volume that was the styrofoam cooler. styrofoam cooler then placed inside of an insulated dog house where I used hair driver to keep ambient temp (in the dog house) 110 F+. I gave power to the peltier with temp control loop to try to get to 25 C / 77 F and watched the temps as hairdrier was applied. An hour after starting to heat-up the doghouse, the temp increased inside of the 1 cubic foot volume to 27 C, after which the temp controller kicked-on and stayed on, trying to cool the volume with peltier back to 25 C / 77 F. For the next 6 hours, dog house temp I kept at 110 F+, and the 1 ft cubed volume stayed at 80 - 81 F for the duration. I think the system was at steady state, so I concluded that it was possible to cool such a volume to ~ 81 F when it's 110 F outside. Peltier power consumption I measured to be 35W. This was 1 peltier with heat sink and fan - 35W to run it all.
So I concluded 35W of peltier could likely adequately cool such a 1 cubic foot volume to 25 C / 77 F, if it was 100F - 105F outside.
I did something similar with a mini-fridge (but ambient temps were ~70 F instead of 110 F. I think fridge is about 4 cubic feet in volume. Inside and outside of fridge started at 70 F. After starting the fridge, the compressor ran for 27 min to get the temp in the fridge down to 49 F. While compressor on, fridge consumed ~ 115W. After the initial cool-down to get the fridge to setpoint, I just let it cycle on and off as-needed to maintain the temperature inside of the fridge. The next few hours it cycled a handful of times, and I concluded I could maintain about a 20 F dT (fridge 49 F ambient 69-70 F) with 15 W on-average of power. So the fridge would actually only run for like 5 minutes, then it would hit setpoint and turn off for like 30 minutes plus. 1 complete cycle I said was about 37 min. 5 min/37 min = 0.13 and some change. So that's 13% or whatever it was that the fridge was actually on and consuming power. 115W * 0.13 and some change gave me 15W.
Insulation I'm sure was better in the fridge than my cheap styrofoam cooler. And ambient temperatures were different between the 2 trials. But the test confirms what I've been reading - and that's that vap-compression is more energy efficient than peltier. Surge loads are of course to be considered - no surge loads for peltier, which is a plus if system size constraints.
so in summary, this is what I concluded / learned:
peltier consuming 35W continuous can maintain 1 cubic foot of volume to 80-81 F with 110F ambient temps. peltier had heat sink and fan too. 35W accounts for all of it.
compressor/mini-fridge can maintain 49 F with 69-70 F ambient temps of a 4 - 5 cubic foot volume with 15W on-average power. Fridge only needed to run a few minutes per hour (5 minutes out of every 37 minutes) to maintain temps.
THEREFORE - fridge is probably better in-terms of energy efficiency.
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