Running freezer off the batteries

Hi,

I would like to get some advisory opinion on a project my little brother is working on. He wants to sell some ice cream at the beach this summer and asked me if I can help him to build a simple and cheap portable freezing equipment to carry around on a bike or cart.

Since I have only basic understanding of electronics, I'm afraid to make some dumb mistakes.

Let's say we have a small 220V nominal voltage freezer consuming 1 kWh power in 24 hours and we want to run it off 12V car batteries - about 8 hours a day, then recharging from a grid.
For 8 hours we'd need 1000Wh/24h*8h = 333Wh of power. It would be a capacity of 27Ah car battery. Since I want to keep battery charged more than 50% I'd need it double, plus I'm planning to use 12vdc/230vac inverter with effectivity of 70%, so final battery capacity would be about 77...80Ah.

Am I missing out something or would it (at least theoretically) work like I described? Could a car battery handle such constant draw for a long time? Anything else You can think of?

Best regards,

Andry

Hi there,

The setup looks correct but keep some things in mind for the power calculations.
1.) The 1kWH is nominal and doesn't involve transients that can affect both the inverter and the battery discharge so be sure to leave a buffer in for that or have a soft start to prevent damage and increase longevity. Transients are mainly for the motor (compressor).
2.) The battery may be able to handle that much but you would have to look at the "C" rate of the battery. That is the rated discharge rate for performance. So you chose a battery that was rated at the correct Ah rating but not the correct C rating it wouldn't be a stable output and the performance wouldn't be predictable
3.) For all of that work, you could just look at creating a peltier module cooler that is made for DC power so you wouldn't need all the conversions and have better efficiency and control
4.) Keep in mind the power fluctuation also. When the freezer first kicks on to lower the temperature it is going to run longer to get to the set temperature but once it is down to that temperature it will just have to do enough to maintain that temperature. So this may save you some power but better to oversize your system.

Those are just my opinions. Hopes it helps.

I would try to get a freezer with 12V or 24V compressor, as it's used in boots or caravans.

The freezer in my home does not run all the time, it runs only long enough to stay cold, it has insulation.
BUT:
1) It is not outdoors on a hot day in the sunshine.
2) It is not opened every few minutes to sell an ice cream cone.

Maybe a freezer cannot work properly outdoors on a hot day in the sunshine being opened frequently.

Another option could be "Peltier element based portable freezer/cooler" ... just an idea you can study more on it and Google more on this.

I think you want to look carefully at the freezer power
draw (esp. starting) against the inverter and battery
ratings. You might be money ahead buying a newer
better efficiency freezer, than overengineering the
power source.

Peltier coolers, the cheap ones, cannot make it all
the way down to freezing. A single stage cooler does
not have the delta-T you need (unless you're under
maybe 25C) and they are very wasteful at high stage
delta-T.

Dry ice is tried, true, cheap and lightweight (certainly,
compared to a car battery, inverter and compression
refrigeration) leaving you more room for frozen treats
and less backache dragging the cart over soft sand.
You could have a good enough cooler for the cost of
a sheet of foam insulation, and a box of any sort.
In a 1/2" styrofoam box, shipments have made it cross-
country to me with the dry ice still half intact - only
the opening and closing of the chest, really costs the
dry ice. Certainly no problem for it to outlast an afternoon.
You could even have a reserve chamber for a second
bag (or more) and be able to keep just enough in play
at any given time, extending the range.

Thank you all very much for your ideas, there is a lot to think about!

@dick_freebird: I will consider dry ice definitely, too!

Ordinary freezer compressor kicking in was certainly something I didn't think of. I see you all recommend something else than messing around with inverter.

I should take some time to study more about peltier elements and consult someone on the solutions.
I took a look at those elements on Ebay and in local shops and found out that there is a really huge price range... as I'd like to do the project on lowest possible budget, I can consider only cheaper ones.

Here, in the edge of Northern-Europe, summers rarely exceed 23 degrees C. Ice cream should be kept at -17...-20C which may be considerable range.

I found what seems to be a trustworthy graph of 12V peltier performance here: http://www.heatsink-guide.com/peltier.htm
As much as I understand those graphs, I could tell it might even come into consideration. But the Delta-T given there isn't really difference between outside air temp and ice cream temp inside, is it? How should I find Delta-T I need?
I think I could get kind of low wattage peltiers, even if I'd add 25% efficiency reserve + container heat loss, which can be calculated.

Best,

Andry

Have you considered a small propane (LP) powered freezer?

You should be able to find a used but functional unit which fits your needs and a small propane (LP) tank as well.

Some models can even switch between LP/AC/DC as a power source.


BigDog

Propane freezer was my first consideration, but it turned out to be almost impossible to find used one around here, unless I want to buy a whole RV or yacht...

Best,
Andry

The compressor motor would draw at least 5 times its running current every time it starts. As start I would just 3/4 fill old plastic bottle with water and freeze them in a domestic freezer with its thermostat turned right down to -15 degrees C. Use them as your cold store in a box lined with 100mm of insulating foam.
Frank

Water doesn't work to maintain -20° C. You need a phase change material with -20..-25 °C melting point, e.g. saturated saline solution.

No you use 80 cals/cc for the latent heat of fusion of ice + 2 cals/cc/deg C for cooling the ice, the thing is that its easy to do. I once filled the space in a cooler box with the frozen water bottles, two days later in France there was still some mushy ice left. A very cheap way to keep the beer and wine cool, en-route
Frank

Yes, water is fine to keep goods near zero degree, but the OP wants -17 to -20 degree C.