Fridge time-on measurement
- Thread starter Mark Maulden
- Start date
The refrigerator will always be drawing some current when it is on. The thermostat will draw some trivial amount of current, on the order of a few milliamps. When the T-stat calls for cooling the compressor will start and draw a non-trivial amount of current, which I think is the current of interest. True, the initial surge to kick the compressor on will only last for fractions of a second and amounts to a trivial amount power used.
There are 2 measurements needed, average amp draw while the compressor is running and the time the compressor is running. If there is a decent digital ammeter in the system (and there should be) getting the current draw will be easy: Record the current draw with the refrigerator turned off then record the current draw with the refrigerator up and running. Get your phone out and do the math. The current draw will be the regardless of the ambient temperature or refrigerator temperature. (There may be some fancy refrigerator that vary the cooling with internal temperature, if you have one, this won't work.)
A Hobbs meter will record the time the refrigerator is running if it is wired correctly, i.e. it needs to be wired to the compressor or fan or water pump (if water cooled). In other words it must be wired after the thermostat so it is not recording time the thermostat is drawing power. To find out how much time the refrigerator is running, record the hours at some point in the day and exactly (or pretty close) record the hours 24 later. Get the phone out and do the math. Hours X Current = Amphours in one day. All in all, pretty simple.
From a purely logical point of view they are equivalent, much as DOD and SOC are equivalent but the inverse of each other.
There are 2 measurements needed, average amp draw while the compressor is running and the time the compressor is running. If there is a decent digital ammeter in the system (and there should be) getting the current draw will be easy: Record the current draw with the refrigerator turned off then record the current draw with the refrigerator up and running. Get your phone out and do the math. The current draw will be the regardless of the ambient temperature or refrigerator temperature. (There may be some fancy refrigerator that vary the cooling with internal temperature, if you have one, this won't work.)
A Hobbs meter will record the time the refrigerator is running if it is wired correctly, i.e. it needs to be wired to the compressor or fan or water pump (if water cooled). In other words it must be wired after the thermostat so it is not recording time the thermostat is drawing power. To find out how much time the refrigerator is running, record the hours at some point in the day and exactly (or pretty close) record the hours 24 later. Get the phone out and do the math. Hours X Current = Amphours in one day. All in all, pretty simple.
I'm not sure I understand this statement. Measuring the non-occurance of something is much trickier than measuring the occurrence of something. In this instance measuring the occurrence of the compressor running is easier than measuring the non-occurrence. Measuring the occurrence is also the circumstance the OP wants to measure, how much is it running.
From a purely logical point of view they are equivalent, much as DOD and SOC are equivalent but the inverse of each other.
Curious if what you said about the current is in theory or have you actually observed this? If you did actually observe this (a spike and then I assume nearly constant current the whole time the compressor is running) what type of fridge? Maybe some fridges do this but its not at all what my Isotherm with Isec does. My fridge may start up with something like a steady 5 or 6 amps and then ramp down to I believe even as low as 2 amps. Going from memory so could be off a little but the time frame for this might be 10 minutes or more depending on ambient. If I were to pick an "average" in say the first 20 seconds, I would way over estimate what the actual average is. Its certainly possibly that the Isec controller is why I see something so different and you dont have this.. but still wondering if you have actually watched the current say for 10 minutes or more? I suspect that the current to the compressor will vary depending on how hard the compressor is working and that will ramp down for each cycle.
FYI, the battery monitor very nicely and accurately does all this current averaging for you if its actual amp hours used over some time period that you want to know (not what was originally asked).
Yep.. the ambient temperature is accounted for by the duty cycle of how long the compressor is running to how long its off. As ambient temperature varies, the duty cycle will vary.
My fridge is an Adler Barbour with a Danfoss compressor, but I have an upgraded, "smart" controller and a variable speed compressor, so it "learns" the 'burn time' and reduces compressor speed to run longer on/off cycles, which are apparently more efficient. So, current draw will vary.
I have the parts, and have been meaning to build my own "Power Duct," to suck cooler air into the condenser, which can make a big difference in efficiency.
I have the parts, and have been meaning to build my own "Power Duct," to suck cooler air into the condenser, which can make a big difference in efficiency.
The current draw at any given time may be lower, however in a 24 hour period how much more efficient is your system? Does it simply draw less current for more hours? Or does it draw less current over all?
Refrigeration is about moving heat and using energy to move the heat. There is no free lunch here. Refrigeration can be made more efficient by reducing motor losses and improving condenser cooling.
My ancient water cooled refrigeration draws a ton of current (~28 amps) when running. However it runs for about 30-40 minutes 2 to 3 times a day. The overall efficiency is pretty good and comparable to modern refrigeration.
And apparently by running longer cycles at lower compressor speeds. (So sayeth Danfoss, anyway.)
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Found this online claims to be good for AC or DC up to 200 volts if you just wanted to find out how many hours the refrigerator spent running or did you want to know how much power it used in 24 hours? For a different 20 bucks you cook get a similar kind of plug in device to tell you how much electricity you used.
Runleader Digital LCD Hour Meter
Runleader Digital LCD Hour Meter
The compressor is one part of the cooling equation. Another part is the evaporator/holding plate. Our refrigerator, a Grunnert, has 2 very large holding plates. These will chill down into the 20° range and keep the refrigerator cold, much as a block of ice. It can be run when the engine is running so it is not draining the battery as the alternator supplies the power, thus it has little drain on the batteries.
Being the OP, some interesting discussion here. I have an AB refer that draws up to approx 7a. It is variable. Any device to measure this would have to be connected somewhere after the compressor or the fan or whatever. Otherwise the analyzer starts marking time when the breaker is on. I cannot find an analyzer that measures current either by series or clamp on methods. Then there would be the time issue of when the analyzer turns on/off especially with different amperage draws which would require settable trip points. I would already know the V…
There are two reasons I'm looking at this. One to determine if I should spend the effort for more insulation which would be on the inside and I really don’t want to that. Second is sizing a solar panel..I do not want a solar farm on the boat. I thought I would get a portable suitcase panel and open it up on deck (not underway) and run the wiring into a cigarette lighter which currently exists right to the battery bank. Thinking maybe a 120W panel that could get me 6a (Full daylight) and forget the refer analysis…I will get what I get for not much money.
There are two reasons I'm looking at this. One to determine if I should spend the effort for more insulation which would be on the inside and I really don’t want to that. Second is sizing a solar panel..I do not want a solar farm on the boat. I thought I would get a portable suitcase panel and open it up on deck (not underway) and run the wiring into a cigarette lighter which currently exists right to the battery bank. Thinking maybe a 120W panel that could get me 6a (Full daylight) and forget the refer analysis…I will get what I get for not much money.
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A battery monitor may be the only way to measure the consumption if the compressor has variable loading.
A data logger would be useful here, and one with enough channels to also measure outside temp, cabin temp, and fridge temp in addition to voltage and current at the supply, and thermostat closures. A sensor for fridge door openings would be useful, too. I've seen such loggers under $100 with lots of channels. You'd have to have some programming acumen.
Nor will it give you accurate information. I had reason to test a similar product. It was around 20% fast.This gets cheaper all the time.
View attachment 206597
You can now measure you refrigerator's TIME ON during the day for only $3.41 CAN.
At that low price, better get several because the meter isn't going to last for very long!
This is might be the easiest way to go. What you proposed doing is possible, however the data set would be quite large and the calculations to get really good insight in to the factors involved would require some statistics that I have long since forgotten how to calculate.
A rough idea could be obtained by randomly checking the current draw. You won't capture minute detail, however you will get a good approximation, the more data points you have the more accurate the approximation will be.
You will need an digital ammeter in the system or a good battery monitor. Record the refrigerators power draw and if the refrigerator is not running, record it as 0. Do this 6 to 10 times a day for 5 days. Find the median value (there is spreadsheet function for this), that would be your best estimate of the average current draw at any one time. Multiply that number by 24 to get a daily draw.
The median is the most appropriate measure here because it is not affected by the data range, whereas the mean or arithmetic average is.
Do not be misled by the nominal rating of the panel. You will seldom if ever get the full 120 wats for any length of time. The panels are rated under controlled conditions which are rarely seen on a boat. If the panel is out all day, expect 3 times the nominal capacity. A 120 watt panel can be expected to produce 360 watt hours (30ah) a day on average. That is enough power to run the refrigerator at full capacity (7a) for about 4 continuous hours allowing for some charging inefficiencies.
Mark,
how often (and for how long) do you plan to be away from power? I ask because I tried putting a gallon frozen water jug in my top-loading fridge last year as a way to reduce power consumption on a 2-night, 3 day sail away from shore power.
That gallon jug substantially reduced my need to run the fridge compressor… I just turned it off and checked the temp a few times a day (getting food out). Also, I had the advantage of ice cold water as the jug of ice melted.
So if you aren planning to be off-grid for long periods of time, you may be able to get away with your suitcase solar and a jug of ice!
Greg
In the PNW, the possibility of being “off grid” is days or a week or weeks. Depends on the cruising habits.
There was a time when 12 volt Refrigeration compressors where not not available.
just about any and all 12/24 volt Refrigeration systems are nothing less than a modern-day miracle.
Many people overlook obsess about how long the compressor is running,
When the compressor is running is when the true miracle is evident by how little current they actually draw.
What many also don't understand is the small btu capacity works because of long Cycles not short Cycles from equipment.
Best thing anyone can do is make sure the box is well insulated the lid or doors are as airtight as possible, electrical wiring and connections are proper and sized according to the loads
Most people can get by with a two battery two bank system
liveaboards off the grid will probably be adding solar or wind to assist the battery banks but, no matter how you look at it 12/24 volt Refrigeration is incredibly efficient! The more it runs the more efficient it is when it starts is when it draws the most current not when it's running.
This is not only electrical reasoning, it's also good because of saturated vs. unsaturated refrigerant gas moving through the system and becoming
more saturated during the off cycles think of saturation as density,
Saturated IE dense refrigerant is one of the reasons air conditioning compressors sometimes will not start at the beginning of the season but that perfectly good compressor will last for many years after being kicked started with a hard start device.
Most all Refrigeration compressors have built-in start relays or start device because hard starts are normal for refrigeration compressors except in the case of the 12/24 volt systems which again through this modern Miracle called engineering has made low torque startups possible.
just about any and all 12/24 volt Refrigeration systems are nothing less than a modern-day miracle.
Many people overlook obsess about how long the compressor is running,
When the compressor is running is when the true miracle is evident by how little current they actually draw.
What many also don't understand is the small btu capacity works because of long Cycles not short Cycles from equipment.
Best thing anyone can do is make sure the box is well insulated the lid or doors are as airtight as possible, electrical wiring and connections are proper and sized according to the loads
Most people can get by with a two battery two bank system
liveaboards off the grid will probably be adding solar or wind to assist the battery banks but, no matter how you look at it 12/24 volt Refrigeration is incredibly efficient! The more it runs the more efficient it is when it starts is when it draws the most current not when it's running.
This is not only electrical reasoning, it's also good because of saturated vs. unsaturated refrigerant gas moving through the system and becoming
more saturated during the off cycles think of saturation as density,
Saturated IE dense refrigerant is one of the reasons air conditioning compressors sometimes will not start at the beginning of the season but that perfectly good compressor will last for many years after being kicked started with a hard start device.
Most all Refrigeration compressors have built-in start relays or start device because hard starts are normal for refrigeration compressors except in the case of the 12/24 volt systems which again through this modern Miracle called engineering has made low torque startups possible.
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Compared to what? Mine draws 5A. My previous one drew 5A. It might dip a bit below that when the compressor is run at lower speeds, but not enough to make a 5A estimate significantly less accurate.
I agree. What has made a huge difference is the modern materials, amongst them being magnetic components that produce much higher flux densities with lower losses than was ever possible in previous generations of machines. These advances are showing up in cordless tools, electric bicycles and numerous other applications. Higher tech insulation also contributes.
When I was a kid the family fridge could not keep ice cream hard. Now my boat and trailer fridges do it on 1/3 of a Kw-Hr per day.