I drsigned two power centers in the Prius. The one I have been using is off the 12 volt Prius battery that is resupplied from the Prius high voltage drive battery. On this setup I also can turn on a pure signwave inverter for powering a few 120 volt items that I could not convert to 12 Volts for the trip.
On the opposite side of the rear of the Prius is what is called a house battery. In my case I use two 12 volt 7 amp/hour gel cell batteries in parallel for 12 volts at 14 amp hours .
I had yet to use this house battery setup as I have just let the Prius stay in "Ready" mode and the Prius would turn on and off about once an hour and run for two minutes. This works good when Boondocking in the desert, but is not good for stealthing for the night in parking lots as the Prius, although quite quiet, does make some noise when it runs.
The cooler I have that operates like a fridge as it has a true compressor. It is so high tech it only uses about 2 amp hours to run. So my calculations gave me 7 hours of operation based on the 14 amp hours I have in battery power. That assumes how often the fridge runs based on how hot it is in the car.
I have gotten by with not running the car overnight from 10:00 pm to 6:00 am as it has been cool out at night and the fridge holds the temperature inside quite well. Since it is getting warmer now I need to try out the house battery design.
The house battery needs to be recharged but since it is a gel cell and the car has a wet cell I couldn't charge it with the 12 volts from the Prius battery. Luckily there is a charger that will charge the different technologies and I installed this before I left and what is nice I can charge the car 12 volt battery and run everything off of that if I have shore power (120 volts) like when I was at my aunts in Leesburg, FL.
I use an outside the house wireless thermometer mounted to my dash to monitor the temperature in the fridge. It was only $10.00 at Walmart.
My cooler is an Engel refrigerator/freezer and that cost $800.00 plus $60.00 shipping. This is my second year with it and it is great as it runs off of 12 volts or 120 volts. I use this cooler at home to for special events where extra fridge space is needed.
The controls are simple but there is no specific temperature setting so this is the reason for the wireless thermometer.
The way I normally run the cooler is to plug it into the ignition controlled 12 volt outlet up front in the car. Plugging it into the Prius 12 volt battery directly without the Prius running may result in damaging the Prius battery if I forget and leave the fridge on. Using an ignition power source for the fridge the worse that happens is potentially spoiling food vs being stranded with a dead Prius battery boondocking in the desert.
From the first photo you see a red 12 volt extension cord coming from the house battery panel. This wire extends up front. When I unplug the fridge from 12 volt Prius ignition outlet and plug it into the red wire from the house battery I eliminate any risk to the Prius battery. A drained house battery can be charged again. This way the Prius does not to stay on to run and power the fridge either so I can better stealth and not spoil the food in the fridge.
These are the two 12 volt gel cells wired in parallel to give 14 amp hours.
My test the other night when I stayed at the Colorado Belle I switched the fridge into the house battery to maintain the temperature of 37 deg inside. This also alliedmme to leave the fridge in the car and didn't have to lug it to my room.
The fridge ran on the house batteries for 12 hours (8:00 pm to 8:00 am). This is well more than the 7 hours I estimated, but it wasn't that hot out either. Once it gets hotter outside I will do another test. The information from Engel suggests that 2 amp hours draw is kind of a max draw.
I know the Peius 12 volt battery could also handle this draw without leaving the Prius in ready mode but it is not worth damaging the 12 volt battery as they cost $130.00 for a new battery.