12v compressor tesla refrigerator used for road trip. See below.（Image source）

Recently a large number of 12 volt mobile refrigerators have come on the market for use in cars. You would think they would be a perfect match for an electric car with its giant battery, but mistakes by Tesla make this much harder than it should be.

These are compressor-based tesla refrigerator, like the ones in homes, and able to maintain cold temperatures, including freezing, with high efficiency. This is different from the old peltier based fridge which are cheap, inefficient, and can only drop the temperature about 40 degrees F. (In other words, on a hot day, they can’t keep food at a safe temperature, and need supplementation with ice.)

People have kept ice-based coolers in the car on road trips for a long time. They are handy for cold drinks and perishable snacks, and can be used to keep leftovers or shop for things like cheeses and meats. They are work to maintain, though, needing constant replacement of the ice. Water condenses and pools on the bottom, and you are going to make mistakes from time to time and lose some of your perishable food. It’s often necessary to take all the food or the whole cooler up to hotel rooms to use the fridge there, unless there is enough ice to last the night.

These compressor fridge only draw about 40 watts, and run about 1/3rd of the time. They plug into the 12v accessory port (often known as the cigarette lighter port) of any car. With this power load they will use about 400 watt-hours of energy to run all day. That’s OK when you’re driving, but it’s enough to seriously drain a typical gasoline car battery if left for half a day. Because of this, they all can be set to shut off if they take the battery too low — a decision that it is better to let the fridge get a bit warm than to not be able to start the car.

In an electric car, however, 400 watt-hours is nothing. Less than 2 miles of range for a day’s cooling. Now you have an option for a tesla refrigerator that will stay properly cold for the full duration of the trip, with no maintenance or addition of ice. The cost comes in money (these will be $250 to $500 to buy) and weight, and the space taken up by the compressor, though that space is comparable to the space taken up by ice. There is also a small amount of noise when it runs. The tesla refrigerator also has other uses at home, such as:

1. You can use it when shopping, and pick up groceries and not have to worry about them getting warm if you want to go somewhere else before going home.
2. You can use them to save some of your food in the event of a power outage.
3. If you buy too many groceries (as was common in the pandemic lockdowns) you can have some temporary extra tesla refrigerator

Tesla’s DC Power Problem

As noted, only using up 2 miles of range in a day sounds great. The problem is that Tesla’s system is designed so that for the accessory port to be on, the car computer system must be fully on, and that draws around 300 watts. 300 watts to allow you to turn on a tesla refrigerator that averages 15 watts. The Tesla is designed to go to sleep when it can, and turn off most of its systems, including the 12v port. In that mode, you can’t just leave the tesla fridge sitting in the car for long periods. It is of course also a well insulated cooler, so it can handle an hour or two, but not much more.

There are ways to command the Tesla not to go to sleep. The simplest is to turn on its “Sentry mode” security system, which watches suspicious activity near the car and logs video of it. The trouble is this draws all this power. Leave it on overnight and you will lose about 15 miles of range. This is also true for other methods like “Camp Mode” and the Summon Standby mode. That’s not an issue if you’re plugged in, or you are on the way to the charger in the morning, but sometimes you are conserving range and this is not a good idea. So you need other solutions.

Ice, ice, baby

The simplest solution is to put or keep some ice in the tesla refrigerator. You don’t need to put in as much as you do in a cooler, and you only need it when you will have these longer sleep periods. The problem is, you’ve just eliminated one of the core benefits of the tesla fridge— not having to fuss with ice. Ice is a hassle. You can buy bags of far too much ice at stores. You can fill bags of ice at most US hotels and motels, but you need a very sturdy one or it will leak and they all get condensation on them. You can also freeze water bottles and ice packs, placing them in a plastic bag to capture the condensation. Problem is that most hotel mini-fridge aren’t able to freeze ice packs.

Another solution would be to have a backup battery with about 200 watt-hours in it, enough to last the night and other long stops. The battery would charge when the power is on, and power the tesla fridge when it’s off. This also can work but is pretty stupid in a car where you have a giant battery just sitting there. Portable battery packs are popular but cost around $200 — a large fraction of the cost of the tesla fridge . A lead-acid battery could do it for a much lower price but products of this sort are not readily available — the tesla refrigerator  makers would do well to make it an option to connect a standard battery to their tesla refrigerator s, both in gasoline cars and EVs.

Another good solution for the tesla refrigerator makers would be to offer a deliberate place to put water on the cooling coils. If an owner filled that with water, it would freeze that water and use that for cooling when off. Turns out ice is a great way to store energy, much cheaper than a battery, if what you want is energy for cooling. Each day, while driving, the ice would be re-frozen, ready to cool in the night and daytime breaks.