7.2 chest freezer conversion problems

sichan86 · February 2017

Howdi all. I converted a chest freezer to a fridge with a Johnson controls relay. I have it plugged into a micro solar pure sine wave 1000w/2000w peak inverter. My problem is when the motor for the fridge kicks on the inverter beeps as if the voltage is too low or some other problem I can't define. After a 10 second beep, the motor continues to operate. I am running two 6v 210ah golf cart batteries as 12v. I find that running this way for a 24 hour period drains the batteries down to 40% or so. I might have accidentally let the batteries get below 40% 3 days ago before I hooked up the fridge. Have I ruined my batteries? Is the inverter too small? I'm stumped and frustrated. Any help appreciated!

Estragon · February 2017

Looking for that inverter I found one on Amazon which seems to come with 7ga battery cables. At 12v you could be pulling 100a or more through those fairly light cables during the starting surge, especially at the lower voltages (higher amps) as the batteries are drawn down. I would have expected it to at least sometimes pull voltage down far enough to fault (10v?) though, if it's consistantly pulling down to warning range (10.6?).

It could also be that it beeps in the surge range from 1000w to 2000w to warn you to reduce load to below surge range. If you have a multimeter you could watch voltage at the 12v inverter terminals to figure out which it is.

Pulling batteries down below 40% SOC regularly will shorten their life some. Leaving them at a low SOC for long will kill them, quickly if they're warm, more slowly if cool.

Johann · February 2017

How are the batteries charged up?
What size wires did you use and how long are they on the dc side?
Got any power info on the chest-freezer. Amps, volts, watts or kwh used etc?

BB. · February 2017

Your inverter just about on the smallest range that will work for a typical refrigeration compressor (typically recommend around a 1,200-1,500 watt inverter to run a single refrigerator/freezer).

The battery bank is also very small. The amount of load 2x 6 volt @ 210 AH batteries (12 volts @ 210 AH with batteries in series) would be:

12 volts * 210 AH * 0.85 AC inverter eff * 1/20 hour discharge = 107 Watt load (for example) 5 hours per night, 2 nights with 50% maximum discharge (for longer battery life)
12 volts * 210 AH * 0.85 AC inverter eff * 1/8 hour discharge =268 Watt load maximum for ~4 hours of load (and 50% max discharge
12 volts * 210 AH * 0.85 AC inverter eff * 1/5 hour discharge =428 Watt max continuous load for minutes to ~1 hour or so
12 volts * 210 AH * 0.85 AC inverter eff * 1/2.5 hour discharge =857 Watt max surge (seconds--Motor starting)

Maximum inverter for that size battery bank would be around ~428 Watts (with 857 Watt surge).

If you wanted to run the refrigerator for 2 days of "no sun", 50% max discharge, on a 24 volt battery bank with a 1,200+ Watt AC inverter, the system would look something like (this would be a full time off grid system sizing):

120 Watt load * 0.50 duty cycle for freezer * 48 hours of run time * 1/0.85 AC inverter eff * 1/24 volt battery bank * 1/0.5 max discharge = 282 AH @ 24 volt battery bank

4x 6 volt * 210 AH battery bank would be 24 volts @ 210 AH -- Still a bit small for this size system. 8x 6 volt @ 210 AH batteries in 4x series by 2x parallel strings would be 24 volts @ 420 AH battery bank

To charge an 8x battery bank, you would need solar panels rated for:

420 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 791 Watt array minimum (weekend/seasonal cabin/emergency backup)
420 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,582 Watt array nominal (full time off grid)
420 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,056 Watt array "cost effective" maximum

And then we need to size the system to the amount of load and average amount of sun per day. Say you are in Los Angeles California:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Los Angeles
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 56° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.50	4.82	6.05	6.78	6.83	6.80
Jul	Aug	Sep	Oct	Nov	Dec
6.69	6.67	6.40	5.85	5.07	4.41

You have good sun and, on average, you could run a freezer 12 months a year with only using a genset during periods of bad weather:

120 Watts * 0.50 freezer compressor duty cycle * 24 hours per day = 1,440 Watt*Hours per day (guessing)
1,440 WH per day * 1/0.52 off grid system eff * 1/4.41 December average hours of sun = 628 Watt array minimum

Lots of guesses, but the recommended array for 8x golf cart batteries, guessing at your load and location would be 791 Watt array minimum and a 1,582 Watt array would be really nice (plus you could run other loads during the day time).

You should use a Kill-a-Watt type power meter to measure your AC loads--This would give us a better idea of your load(s).

Also--where you will install the system and how you want to use it will help us to better design your system.

-Bill

sichan86 · February 2017

Thanks for the responses guys. I think I have a little more info to bring to the problem. I charged the batteries to about 85% (12.6v) using a generator because I'm still waiting on two 235watt solar panels and a 40amp mppt charge controller. When the inverter is wired to the charged batteries the multimeter says 12.45. when the freezer motor kicks on it briefly dips to 11.58! Then settles around 12.35. I checked the batteries this morning about 12 hours after charge and without the freezer motor running at that moment ( just the inverter wired in) the batteries read 11.89! The only other thing I use is a 3gpm 6 amp max pump for showers and dishes and it maybe runs 10 minutes max a day. So wtf. How am I burning through 100amps in 12 hours. I was under the impression that converting a chest freezer to a fridge is Uber efficient. Using it as a freezer alone says it will use 228 kWh a year, that's 6 amps a day? (224 kWh/365 days =620 watts a day / 120 volts = 6 amps) supposedly this method of converting to fridge makes it even more efficient. The motor kicks on maybe once an hour for five minutes. What am I doing wrong? Thanks in advance

Estragon · February 2017

[email protected][email protected]

BB. · February 2017

It sounds like you are not getting good charging yet (but you know this). As long as the batteries are cycling (50-80% or so), you only need a "good charge (>90% SOC) once a week. That may mean setting the voltage higher on the AC to DC battery charger (until you get your new panels/controller setup correctly) and/or longer charging time. Roughly, you need at least 2 hours of ~14.75 VDC when the batteries are mostly charged, to ~4-6 hours at 14.75 VDC if the batteries are deeply cycled (50% or less state of charge).

And be careful about Amp and Amp*Hours. Power (Watts) and Energy (Watt*Hours or kWatt*Hours) are "fully" defined units. I.e., that is "all we need to know" when doing the electrical math.

Amps is a "partially defined unit"--We also need to know at what voltage...

Power = Volts * Amps = Watts (a rate like miles per hour)
Energy = Volts * Amps * Hours = Watt*Hours (like total miles driven)

So--A very efficient freezer to refrigerator conversion may use ~230 Watt*Hours per day

230 WH * 365 days = 83,950 WH per year = 84 kWH per year (1kWH = 1,000 WH)

A typical (pretty good) refrigerator or freezer may use around 1,000 to 2,000 WH per day. Or 365 to 730 kWH per year.

Using it as a freezer alone says it will use 228 kWh a year, that's 6 amps a day? (224 kWh/365 days =620 watts a day / 120 volts = 6 amps) supposedly this method of converting to fridge makes it even more efficient. The motor kicks on maybe once an hour for five minutes. What am I doing wrong? Thanks in advance

Units matter here... You "lost an "h" symbol:

224,000 WH (per year) * 1/365 days per year = 614 Watt*Hours per day
614 WH per day / 120 VAC = 5.1 AH @ 120 VAC per day
614 WH per day * 1/0.85 AC inverter eff * 1/12 VDC battery bank = 60.2 AH @ 12 VDC per day

So--The AH at 12 VDC are roughly 10x that at 120 VAC...

Assuming your freezer draws ~120 Watts when running, and you are seeing 5 minutes of runtime per 60 minutes, the estimated AC energy usage per day would be:

120 Watts * 24 hours per day * 5 minutes run * 1/60 minutes duty cycle = 240 WH per day

Which is what it should be running (note that fridge and freezers run longer at 90F than at 70F room temperature).

Now--You can see that running your fridge takes a 120 Watts, and starting it draws >~600 Watts (typically starting load for induction motors is something like >5x running power/current).

Your battery bank is really too small to support a 600 Watt starting load--At 12 volts for a flooded cell battery bank (using our rule of thumb of 400 AH per 1,000 Watts of starting load) need around 266 AH minimum.

Realistically, the suggestion is a 1,200 Watt minimum AC inverter (supports ~2,400 Watts starting surge) and a:

2,400 Watt surge * 400 AH (at 12 volts) * 1/1,000 Watt surge load = 960 AH @ 12 volts battery bank

If the refrigerator is your only load--You can probably get away with 2x 6 volt @ 200 AH * 2x series string or 4x batteries at 12 volts @ 400 AH battery bank.

The killer with refrigerators/freezers and standard induction motors is the starting surge--That drives your minimum AC inverter and Battery bank sizing. Typically, I would suggest that adding a fridge/freezer to an off grid power system takes it from a a small to a medium size system--And that means a 24 volt battery bank+inverter (for reliable operation)--24 volts * 200 AH (4x 6 volt @ 200 AH batteries).

And a solar array to support that size battery bank.

-Bill

sichan86 · February 2017

> @Estragon said:
> [email protected][email protected]

Oops. I didn't realize I had to do the conversion through 12v. I thought that was redundant because I was going through the inverter. Still seems like I'm using way more than it should though. Any suggestions for this little system? Do you see any wasteful problems? More batteries? Larger cables like you said? Thanks

littleharbor2 · February 2017

Sounds like you're undercharging and at a low state of charge your start up surge is pulling your battery voltage low enough to set off the low battery alarm in the inverter. When your batteries are already low and you put a heavy load on them the voltage will drop briefly while the inverter is trying to put out high enough amperage to start the compressor in the fridge/freezer. Once you correct the charging problem you should be good to go. A 24 volt system would help but unless you can reliably replace the daily power draw you will continue to have this problem.

sichan86 · February 2017

> @littleharbor2 said:
> Sounds like you're undercharging and at a low state of charge your start up surge is pulling your battery voltage low enough to set off the low battery alarm in the inverter. When your batteries are already low and you put a heavy load on them the voltage will drop briefly while the inverter is trying to put out high enough amperage to start the compressor in the fridge/freezer. Once you correct the charging problem you should be good to go. A 24 volt system would help but unless you can reliably replace the daily power draw you will continue to have this problem.

When you say under charge, is that because I am not topping them off? The battery charger I have says that I am getting them to almost 90% which may not be completely accurate but it also says atleast 12.6v with the multimeter. I'm still confused why this "efficient" method of refrigerator is still pulling so many total amps throughout the day. I guess I'm not getting it

Estragon · February 2017

12.6 could be way under 90%. You would need to wait a while with no charge source to get a "resting" voltage. Depending on the battery, temperature, etc. it would be around 80-85% at about 14.5v while charging.

Read Bill's post through a few more times. Lots of good info but may take a few reads.

westbranch · February 2017

Your battery charger should be showing up into the 13.2 to14.4 volt range for a full charge, depending on how long after the cables are disconnected that you make your multi-meter measurement...ie it should be 3 hrs or more after charger is disconnected to do the measurement...

Thom · February 2017

What is the battery charger you are using how many amps is it ?

thom

7.2 chest freezer conversion problems

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Los AngelesAverage Solar Insolation figures

Los Angeles
Average Solar Insolation figures