Inverter needed for SMALL dorm fridge?

[Electric Personality]

Hey all, we are taking delivery of a small trailer that has two refrigerators, one a traditional RV 2 way 6 cubic ft, and the other in the outside kitchen that is a cube maybe 1.5' in every direction. Google says it should draw 50 to 65 watts when cycling, but 3 to 4 times that on startup. From what I understand, it will run much better on a true sine way vs. modified. I am considering also a 2KW inverter for bigger stuff like microwave, TV, wife's hair dryer, etc. But those will be mostly temporary loads, and will shut it down to avoid parasitic loads while idle. So my thought was a smaller inverter that might be more efficient to drive this tiny fridge. I know there are small 12 volt refrigerators. but want to price out what it would cost to run the one the trailer came with.

 

[MisterSandals]

I am considering also a 2KW inverter for bigger stuff like microwave, TV, wife's hair dryer, etc.

My initial thought is about whether you have enough battery to supply this amount of power.

Google says it should draw 50 to 65 watts

Lets run some numbers. Lets guess fridge runs 50% of the time. And lets take a middle-ish power number of 60w.

60w x 24h x .5 = 720wh daily.

A 2kw inverter sounds about right, and yes, pure sine wave will be pretty much required for refrigerators, microwaves and appliances with motors.
The startup surge current could be 5-10x the amps for a few seconds, my thought is that 2kw should be fine, doing the math or running real tests would be best if possible.

Yes, inverters use power, even when idling. Bigger ones more wasteful. As for actually efficiency, you'd have to look at each inverters' specs as they vary mostly by, you guessed it, how much they cost.

 

[Electric Personality]

Planning a 4c 280 ah build per the current thread in the diy section. So they should be able to handle a 2k inverter. (the 6mm terminal screws are concerning however, but no one seems to be having issues so far). By the math, there is about 3.6 kwh in a 4 cell 280 ah setup. So 3 to 4 days, minus lights and fan. Two days maybe between charges. Solar won’t be big in the woods. I was hoping a much smaller inverter could power a baby fridge.

 

[MisterSandals]

I was hoping a much smaller inverter could power a baby fridge.

There are a bunch of folks that swear by them... Ped comes to mind, he's a full time RVer. Do a search on this site for "dorm fridge" or "dorm refrigerator", lots of results.

If you do not see exactly what you need, you could probably reach out to those with experience for more info. Everybody is willing to help from my experience, just for the polite asking.

And what the heck, it sounds like you are FAR more knowledgeable than your original post implies... :*)

 

[tictag]

A 2kW inverter will draw 166A from a 12V battery, now, whilst LiFePO4 can safely handle up to 1C discharge rates (e.g. 280A from a 280AH battery), quite often its BMS will be the limiting factor. Definitely worth checking. If you have a choice over the BMS, I would under-rate it by at least 30% if buying cheap e.g. Daly. For example, you need 166A, buy a 215A or more capable BMS.

There was a thread on here recently about a very low power fridge, sorry, I forget the brand but it was designed for RVs. The thread was about whether its stated power was correct because it was so low, turned out it was.

There's nothing stopping you buying a smaller inverter for this fridge but DC-AC-DC conversions are never something to boast about at dinner parties, stick with DC power.

 

[Electric Personality]

A 2kW inverter will draw 166A from a 12V battery, now, whilst LiFePO4 can safely handle up to 1C discharge rates (e.g. 280A from a 280AH battery), quite often its BMS will be the limiting factor. Definitely worth checking. If you have a choice over the BMS, I would under-rate it by at least 30% if buying cheap e.g. Daly. For example, you need 166A, buy a 215A or more capable BMS.

There was a thread on here recently about a very low power fridge, sorry, I forget the brand but it was designed for RVs. The thread was about whether its stated power was correct because it was so low, turned out it was.

There's nothing stopping you buying a smaller inverter for this fridge but DC-AC-DC conversions are never something to boast about at dinner parties, stick with DC power.

I got the 200a daly for the bms. While the inverter can draw that much, it would only do so at peak loads. Once the fridge is running a second later, it should only be drawing 60 or 70 watts plus inverter losses from inefficiency.

NEW QUESTION: With a better inverter with a sleep or eco, or search mode, I am wondering if the dorm fridge cycling on to a sleeping inverter is enough to activate the inverter? That would solve my issue right there, instead of setting up a controller/ relay to turn the inverter on or off.

 

[MisterSandals]

I am wondering if the dorm fridge cycling on to a sleeping inverter

The fridge has to use SOME power while idling. If the inverter went to sleep, the fridge could not have any power to cycle on.

 

[tictag]

While the inverter can draw that much, it would only do so at peak loads.

It is a good practice to design your system for rated use; whilst you might only use 60-70W now, that might not always be the case. An inverter capable of supplying 2,000W should be wired and protected as if it was supplying 2,000W.

I am wondering if the dorm fridge cycling on to a sleeping inverter is enough to activate the inverter?

Depends on the inverter, of course, but most that have such features will have the ability to configure the 'wakeup' power required. For example, on the Victron MultiPlus you can set the 'wakeup' power anywhere from 1W upwards, defaults to 75W if memory serves.

 

[tictag]

The fridge has to use SOME power while idling.

If the fridge is anything like mine, the control electronics are powered from a separate, always-on, source to the heater*.

*Always freaks me out that my fridge is cooled by a heater

 

[Electric Personality]

It is a good practice to design your system for rated use; whilst you might only use 60-70W now, that might not always be the case. An inverter capable of supplying 2,000W should be wired and protected as if it was supplying 2,000W.

Depends on the inverter, of course, but most that have such features will have the ability to configure the 'wakeup' power required. For example, on the Victron MultiPlus you can set the 'wakeup' power anywhere from 1W upwards, defaults to 75W if memory serves.

Oh absolutely. It will be wired to handle full load +.

 

[MisterSandals]

If the fridge is anything like mine, the control electronics are powered from a separate, always-on, source to the heater.

I did not know that such a thing existed.

I learned something today! Thanks.

 

[Electric Personality]

If the fridge is anything like mine, the control electronics are powered from a separate, always-on, source to the heater*.

*Always freaks me out that my fridge is cooled by a heater

I would think (and I am assuming here, could be completely wrong) that a cheap $79 dorm fridge would have a mechanical thermostat so binary on or off... Maybe I am wrong. Wondering if anyone operates in this mode...

 

[tictag]

I did not know that such a thing existed.

Those designed for RVs, and in my case a caravan, can be powered by DC, AC or LPG. It has to have a stable DC source to power the control circuitry so that it can switch between heater sources depending on the situation e.g. if AC is present, use that, if DC is available, switch off gas valve and use DC etc. My caravan has a 5A fused, always on, supply for the fridge, the gas hob igniter (don't know!) and the alarm.

 

[Electric Personality]

This fridge that I am talking about is a cheap, 120 v cube, maybe 2 cu ft, P.O.S., strictly to keep my beer and water cold in the outdoor kitchen.

EDIT: Looks exactly like this, and says in this one, it has a mechanical thermostat. Perfect! I bet it would play nice with an inverter in sleep mode, likely the 2000va multiplus (or 3000 va if it will fit). https://www.walmart.com/ip/Galanz-1-7-Cu-Ft-Single-Door-Mini-Fridge-GL17BK-Black/40685158

 

[tictag]

If you just want cool beer, wouldn't a cheap 12V peltier-effect unit not be suitable?

 

[PHoganDive]

FYI, Peltiers are MASSIVELY inefficient compared to compressor type, phase change refrigeration. They can only get 40-50 degrees below ambient cooling, and use several times as much power in the process.

 

[tictag]

FYI, Peltiers are MASSIVELY inefficient compared to compressor type, phase change refrigeration.

I don't disagree, but so is converting DC to AC just to run a tiny fridge, together with the added cost of an inverter where low(ish) power DC devices exist. We're talking about cooling beer here, which, whilst obviously important, shouldn't have to break the bank or have an overly complex solution.

 

[Fred S]

Electric Personality, Yes your small fridge should wake up the inverter just fine, if the inverter is in standby mode. I have 2 small fridges and a chest freezer, running 50w, 55w and 65w respectively. All are cheap, so as you said, a simple mechanical thermostat cycles on the power, there is no continuous draw.

 

[Electric Personality]

I don't disagree, but so is converting DC to AC just to run a tiny fridge, together with the added cost of an inverter where low(ish) power DC devices exist. We're talking about cooling beer here, which, whilst obviously important, shouldn't have to break the bank or have an overly complex solution.

I want the inverter for more than just cold beer. Microwave, tv, and wife’s hairdryer.

 

[tictag]

I want the inverter for more than just cold beer. Microwave, tv, and wife’s hairdryer.

Apologies, @Electric Personality, your original post mentioned, "So my thought was a smaller inverter that might be more efficient to drive this tiny fridge.", I interpreted this as one always-on inverter to power the fridge, one on-demand inverter for higher loads. My point was that you'd be better off powering this fridge by DC, then just having one inverter for your larger loads.