Not a Self-build, but a Tweaker

[wildebus]

More down the the refrigerants ability to move heat from one place to another and the condenser coils ability to dissipate it at given temperatures....
On fan cooled condenser coils you can use head pressure control to prevent over condensing at low ambients....

Not really necessary on small domestic fridges with the time spans the compressor runs for.

Give this is a fairly small (135L?) domestic (LEC) Fridge/Freezer, any suggestions for best operating when in a cold unheated (rear to back of thin motorhome wall) environment?
I am thinking currently to go with the option of controlling the AC supply to it based on the temp of the Freezer compartment (initially switch on power until temp drops to -18C; then turning off AC supply, and not turning back on until temp has gone up to -14C and repeating On/Off cycle).

 

[mistericeman]

Sounds to me like you've got thermostat issues....

(It's not one of the modern ones that have external temp sensing as well internal (reduces the duty cycle of the compressor to maximise efficiency)

The outside temperature really shouldn't cause great problems for a small domestic combined fridge/freezer ....

You set the temp to (roughly) what you want.... The rotary bimetallic stats really aren't THAT accurate....
And the compressor runs or doesn't, low ambient will mean that it finds it easier to get rid of the heat from inside the fridge than in warmer weather...
In our UK temps it's unlikely to overcondense massively (there's no fan blasting air at it AND once down to temp shouldn't really run for too long to cause any issues...)
Low ambient around the fridge should mean it works less hard than in higher ambients anyway....

Big chest freezer in a cold garage constantly loaded/unloaded OR set ludicrously low.... Then yes I've seen ice from freezer to floor OVER time.

 

[wildebus]

Issues as in a fault? Or just a design limitation/'feature'?
How does a Fridge/Freezer work with a single control but two zones that have to operate at significantly different temperatures? It is an A+ rated unit and has proved to be very economical in use.
I have setup a routine that can use the Victron Temp Sensor in the Freezer to determine if the AC to the fridge is on or not rather than having the fridge start the compressor. Prefer not to have to do that, but least got that option (need to rejig a cable first if I use this option and also maybe bypass the thermostat control in the Fridge itself?). Will see in the next couple of days....

 

[mistericeman]

Issues as in a fault? Or just a design limitation/'feature'?
How does a Fridge/Freezer work with a single control but two zones that have to operate at significantly different temperatures? It is an A+ rated unit and has proved to be very economical in use.
I have setup a routine that can use the Victron Temp Sensor in the Freezer to determine if the AC to the fridge is on or not rather than having the fridge start the compressor. Prefer not to have to do that, but least got that option (need to rejig a cable first if I use this option and also maybe bypass the thermostat control in the Fridge itself?). Will see in the next couple of days....

Often by measuring the fridge temp and allowing cold air to cascade to the fridge section (colder air heavier)from the freezer
And hoping for the best....if its a fridge freezer combined (ie not separate doors)

Ones with fully separate freezer compartments will still usually read off the fridge (most safety critical bit temp wise) and there will be less/differently sized evaporator coils in the fridge part so receives less cooling proportionally

Most folks will never have a thermometer near to a fridge so it's frankly a bit of a cheap way of doing it rather than separate compartments with twin compressors.

 

[wildebus]

Often by measuring the fridge temp and allowing cold air to cascade to the fridge section (colder air heavier)from the freezer
And hoping for the best....if its a fridge freezer combined (ie not separate doors)

Ones with fully separate freezer compartments will still usually read off the fridge (most safety critical bit temp wise) and there will be less/differently sized evaporator coils in the fridge part so receives less cooling proportionally

Most folks will never have a thermometer near to a fridge so it's frankly a bit of a cheap way of doing it rather than separate compartments with twin compressors.

Mine has separate external doors. So all based on the designer working out the coils sizes based on size, insulation and temp requirements then? It does have quite large coils in the freezer on three different levels and certainly cools fast as mentioned.
In my case I keep no meat in fridge or freezer so a little less critical I guess?, but I don't like the food in the freezer going soft so I think I might get a better result using a more programmable control then anyway?

Thanks for the info

 

[mistericeman]

Mine has separate external doors. So all based on the designer working out the coils sizes based on size, insulation and temp requirements then? It does have quite large coils in the freezer on three different levels and certainly cools fast as mentioned.
In my case I keep no meat in fridge or freezer so a little less critical I guess?, but I don't like the food in the freezer going soft so I think I might get a better result using a more programmable control then anyway?

Thanks for the info

You could always fit an electronic stat to give closer control by reading the temp in the freezer compartment...switching power to the compressor

And deal with the fridge stuff being 'cold'

Frankly domestic fridges (certainly none twin electronic stat/twin compressor ones) are always a bit of a compromise.
And very often built down to a price....

Bit like years ago when aircon had a fan/live feed via a c17 stat....
Reliable as hell BUT temps could be a tad hit and miss (comfort cooling is @ + or - 2 degrees...
But varied wildly....

Modern comfort cooling is often close to what would've been considered 'close control' computer room spec + or minus 1 degree...

 

[wildebus]

You could always fit an electronic stat to give closer control by reading the temp in the freezer compartment...switching power to the compressor

That is essentially what I have prepared I think with some coding in NodeRed for use on the Cerbo ...
I set what temp is "too warm" and what temp is "too cold" and the Relay turns on as appropriate, in turn switching on an SSR to control AC supply to the Fridge.

And deal with the fridge stuff being 'cold'

Frankly domestic fridges (certainly none twin electronic stat/twin compressor ones) are always a bit of a compromise.
And very often built down to a price....

Bit like years ago when aircon had a fan/live feed via a c17 stat....
Reliable as hell BUT temps could be a tad hit and miss (comfort cooling is @ + or - 2 degrees...
But varied wildly....

Modern comfort cooling is often close to what would've been considered 'close control' computer room spec + or minus 1 degree...

When I was monitoring the Fridge part last year, I was seeing quite a tight range of just 2 degrees which I thought very good for a basic Fridge/Freezer. (The Compressor Fridge/Freezer I was thinking of buying has separate controls for Fridge and Freezer and was very good but aorund 4 x the price).

I wonder if with this more electronical control, I would be able to use the Fridge/Freezer in a more flexible way? ie. if I don't need the Freezer to be live and want to save power, I could just set the "too warm" and "too cold" parameters much higher. (Currently I am thinking "too warm" of > -14C and "too cold" to be < -19C for a 4* Freezer.)

 

[mistericeman]

That is essentially what I have prepared I think with some coding in NodeRed for use on the Cerbo ...
I set what temp is "too warm" and what temp is "too cold" and the Relay turns on as appropriate, in turn switching on an SSR to control AC supply to the Fridge.
View attachment 5012


When I was monitoring the Fridge part last year, I was seeing quite a tight range of just 2 degrees which I thought very good for a basic Fridge/Freezer. (The Compressor Fridge/Freezer I was thinking of buying has separate controls for Fridge and Freezer and was very good but aorund 4 x the price).

I wonder if with this more electronical control, I would be able to use the Fridge/Freezer in a more flexible way? ie. if I don't need the Freezer to be live and want to save power, I could just set the "too warm" and "too cold" parameters much higher. (Currently I am thinking "too warm" of > -14C and "too cold" to be < -19C for a 4* Freezer.)

Possibly BUT it will depend on how the evaporator is configured (different manufacturers lay it out in different ways)
So it's possible that you will be able to fine tune it to just cycle the running times on a reduced basis so only cool the fridge (and freezer) rather than it running long enough to freeze.

 

[wildebus]

Bit purplexed with this as the Fridge/Freezer seems to operate well i.e. when on it cools fast, but the thermostat just doesn't seem to kick back in properly now (as mentioned, ok last autumn).
I remembered this morning that when I had the Thetford 3-way Fridge/Freezer installed, I planned to set it to AES mode and use the Relay in the Victron BMV-712 to connect the 240V AC when the Battery State of Charge was high enough. I also added a physical rocker switch to let me bypass that relay (I like manual overrides ). But this meant I could use the programme in Venus OS with the Temperature Sensor without any rewiring needed to the fridge

But first, needed to check if the "MAX" setting on the Fridge would make it stay on ....
Set to Max just before 9AM and it just kept cooling, not stopping at -18C as usual, but continuing. The temp got down to a recorded -25C (seen on a different view, not this chart) and the sensor actually stopped working. Supposedly they have an operational range of -40C to 100C, but I found previouslyt that around 55C was the upper range, so crapping out at below -20C I am disappointed at but not surprised.

Opening the Freezer door and pressing on the sensor to warm it up a little brought it back to life (and the Temperature status of "Unknown" reverted to "OK" again)
I don't want to run below -18C anyway so the limit of not working below -20C is not that important. But confirming the "MAX" setting does seem to keep the fridge on *should* mean I can use the MAX setting with the programme in the Venus OS unit to control the power.

First thing is to change the setting in the BMV-712... In the Relay menu in the settings, there are different modes available. The nomal one is "default", logically enough and that is what you leave it at to use the various settings within the BMV to control the relay. I want to control it via the Victron system remotely, so I need to change to "Remote"

Then in the NodeRed Fridge Control Programme (which is accessed when you have the 'Venus OS Large' firmware running on your Venus OS device, such as the Cerbo GX), I simply changed the Relay I was controlling from the Cerbo (Venus device) Relay 2 to the BMV-712 (400Ah Hybrid Bank) Relay

And relabelled the Victron nodes accordingly

Then flicking the override switch I added last year back to off so the AC Power to the Fridge is controlled via the Relay, the Program above will now take affect.
To illustrate how that will work, got a series of screenshots of when the relay will come on and go off....

The actual temp points at which I will set the relay to go off may be different though, as I have noticed after the Fridge compressor goes off there is still residual cooling happening of upto 3C, so to get a minimum temp goal of -18C, I might need to set the Relay off temp (number 5. in the picture above) to -15C rather than -18C for example (to start with for now, I have set the "Fridge too warm" and "Fridge too cold" values at -14C and -15.5C respectively and will see what tweaking to do)

A comparision:
Using the built-in Fridge Thermostat control, it was basically going down to around -18C, and then defrosting. This is yesterday evening/overnight and then just before 8AM I turn the 'stat down to "MAX" and it went to maximum coldness.

After activating the Programme, using the Victron Temperature Sensor, there is quite a different pattern seen (note the scale is different in this graph)

And below you can see the AC supply being turned on to the Fridge via the relay (bottom section) and the power consumption when on (top section)

With a current duty cycle of 20 minutes on and 120 minutes off (note times are determined by the Freezer Temperature sensor values, and not by any timer mechanism), this gives a projected power consumption of 190Watt-Hours over 24Hrs, or in Amp-Hours (for 12V system), 16Ah. Add in Inverter overhead and still under an average of 1Ah/Hr out the battery.

The "right" value in terms of what temps to turn the power ON and OFF may well depend on what you are using the freezer for. The Fridge/Freezer I have is a 4* Rating one, but maybe don't need that low a rating for a freezer that is used in a Motorhome for a week or so away? See the explanations below for more info:

• *One star (-6 degrees centigrade) is perfect for making ice cubes and is sufficiently cold to keep food for three or four days.
• ** Two stars (-12 degrees centigrade) stores food for fifteen to twenty days.
• *** Three stars (-18 degrees centigrade) will keep food safe for up to three months.
• **** Four stars (below 18 degrees centigrade) means that the compartment provides the ideal conditions for freezing down fresh and pre-cooked foods

There could be further power savings to be made by running the Freezer at a slightly higher temp, but that would need to be balanced with making sure the Fridge part is not allowed to get too warm of course. I'd need to get a Temp sensor on the fridge really to monitor that in parallel.
It will be interesting to see how adaptable this programmable setup could prove to be

 

[wildebus]

Got a full day and a half with the new setup and think it is working really well

Nice and tight control there of the Freezer temperature. I need to put my smart thermometer in the fridge so I can track how the upper section is running still. I'll do that this evening and will have a good view in the morning if the fridge is within the required range (recommended between +3C to +5C).

 

[wildebus]

After monitoring the Fridge Temp with a smart thermometer (this one - Govee Room Thermometer Hygrometer ), I downloaded the info for the full 24 hours for the device, and also the same time period from Victrons VRM for the Sensor in the Freezer and combined them in Excel (bit awkward as both systems log the time in fractions of a second so matching the data involved rounding the time to the nearest 10,000th of a Day! (don't need anything like that level of accuracy, but just how the systems save the data).
But the end result is a good picture of how the Freezer and Fridge work together. I think the Fridge actually gets too cold really when the freezer is working as a 4* unit. It could be due to both compartments being totally empty though, but the fridge, like many budget Fridge/Freezers, does ice up, so it would be good to try and minimize that.

But anyway, controlling the Unit using the temperature of the Freezer section as the guide, and adjusting the points when I supply AC power and disconnect AC power to give the best overall result - which is a Freezer operating at least as a 2* Freezer (so suitable for storing food safely for a 2-3 week trip) and a Fridge that doesn't freeze food but keeps it below 5C. I raised the thresholds by 1C around Lunchtime yesterday and and you can see the results in the data below.

(The difference in the smoothness of the two lines is down to the different products - the VRM saves the temp in full degrees; the Gove saves in 10ths of a degree).
Freezer has a 5C degree variation, with an average of -12.5C with the post-noon settings, and the fridge has a 1C variation, and with the final settings of the day runs between 1C to 2C. This looks pretty good.

The variation on the Freezer of 5C may seem quite a lot, but it will be just the way the Fridge works - it cools down extremely rapidly and I actually only have 1 degree difference in the programme to tell it to switch on and switch off:

This alternative way I looked into to stop the fridge/freezer defrosting in the winter has actually resulted in a setup which I think will be better in just about every way, from how the food and drink is stored to how much power is used. A Happy Happenstance

 

[PeteS]

The control looks really impressive but I cant help that think controlling the fridge via the freezer may have different characteristics when its loaded up. It would be good to see the graphs with a full freezer and part loaded fridge just using water perhaps, just to give it some load.
I wonder if the freezer section will hang on to temperature longer due to it being smaller and allow the fridge temperature to rise too high.

 

[wildebus]

The control looks really impressive but I cant help that think controlling the fridge via the freezer may have different characteristics when its loaded up. It would be good to see the graphs with a full freezer and part loaded fridge just using water perhaps, just to give it some load.
I wonder if the freezer section will hang on to temperature longer due to it being smaller and allow the fridge temperature to rise too high.

Well, there is only one control for both. And for me, controlling the freezer temp is more important than the fridge, so basing the power on the freezer is more logical.
I have to assume that the designers of the fridge/freezer worked out the sizes of the cooling coils on the basis of it in use to provide a balanced result suitable within constraints for both parts?
I guess if the freezer does cool more slowly when full and the fridge temp rises too much (it is actually at the lower end of the recommended range anyway, so an increase of a couple of degrees is no problem), I can lower both temps and even have the "AC On" Temp to match the "AC Off" Temp. That would start the cycle faster. As it is, the freezer is a little higher than it could be anyway so if the differences increase in use and the fridge starts to go a little higher , that is fine as can just lower the parameters to bring both back in line.
Takes just a minute to do

 

[MarkJ]

I think fridge technology is surprisingly basic. The amount of food you have in the fridge is definitely a factor in how much cooling is needed, not so much from the thermal capacity of the items but the airflow. I think the manufacturers just go for a middle ground probably.

 

[mistericeman]

I think fridge technology is surprisingly basic. The amount of food you have in the fridge is definitely a factor in how much cooling is needed, not so much from the thermal capacity of the items but the airflow. I think the manufacturers just go for a middle ground probably.

Only usually a factor as far as initial loading or taking items out then replacing them....

Other than that,
energy use between full/empty is frankly negligible.

it's a static load as food in there itself doesn't actually generate any heat load.

 

[wildebus]

The control looks really impressive but I cant help that think controlling the fridge via the freezer may have different characteristics when its loaded up. It would be good to see the graphs with a full freezer and part loaded fridge just using water perhaps, just to give it some load.
I wonder if the freezer section will hang on to temperature longer due to it being smaller and allow the fridge temperature to rise too high.

Went shopping yesterday lunchtime and bought a fair bit of frozen food so decided to whack it in the Motorhome Fridge and have a gander at how/if it impacted the operation.
It was interesting.

I put the food in the Freezer (now around 3/4 full) and a couple of small things in the fridge around 14:30 on the 1st Feb.
The Fridge temp is monitored via the Gove kit and this time I haven't combined into a single chart but am using the identical time line - 1st Feb 00:00 until 2nd Feb 12:00.

Firstly the Freezer - it is the temp sensor in this which controls the power on & off to the Fridge/Freezer....

So temp increases around the time I am filling it up. Typical as the door is opened and shut, even for short durations. What is interesting here is that with the freezer mostly full, the temp cycle becomes tighter. It doesn't warm up quite as much (only slight difference) and it cools down by one degree less than when it was empty.

Moving to the Fridge ...

Seeing that the Fridge Temp cycle generally is much wider than the Freezer time wise (that is not picked up on the timeline shown above) and goes between 2 to 6C. After the Freezer has been filled however, the temperature of the Fridge is now significantly tighter in terms of the min/max range, as can be seen above, with a range of 4 to 5C.

This is actually very good for both Freezer and Fridge sections as I can lower the program thresholds to reduce both temps to have them cooler. As they stand now they are perfectly safe but at the upper end of desirable. Around just 1-2C lower would be ideal. I will adjust the program in around an hour (giving 24 hours since filled).

As far as power usage goes, made no real difference once you take the "door opened" time out the picture (this reinforces why Fridge and Freezer doors need to be closed as quickly as possible and opened as infrequently as possible.)

('Closed' = Power ON; 'Open' = Power OFF)

 

[wildebus]

24 hours after loading the fridge and seeing the change in the behaviour (for the even better!), I dropped the On and Off thresholds by one degree at 2PM yesterday

The change in the programme was quickly seen in the Fridge/Freezers readings ...

The Fridge lower temp dropped by 1C and the upper by 0.5C. The recommended range is 3 to 5C, so we are spot on here.

And the freezer followed the same pattern as the previous 24 hours, but one degree lower, which again is excellent.

And again, these changes made no difference to the power consumption of the Fridge/Freezer unit, which currently has a duty cycle of under 15% (on average on for 7.8 Minutes each hour, drawing 45W when on, which gives an consumption currently of an amazing 7Ah/Day!
This is in a currently unheated (room temp average 7C) motorhome in winter of course and in summer, it will need to work a fair bit harder no doubt, but even so, maintaining a Freezer of food at an average of -13C at an average power draw of 3.5W is pretty good even in Winter


That concludes my Fridge tweaking I think. When on the road again, I will be keeping an eye on the temps and may have to alter the threshold possibly according to the seasons and fridge use, but if so I would think it will be a very minor adjustment needed

 

[wildebus]

Something I am not that keen on with Motorhomes, Campers, etc, is the "12V" supply - as it is NOT 12V, but anything between 15V (if battery charger on on a very cold day) down to maybe below 11V (if Inverter pulling a lot of amps).
I have a bunch of 12V regulators that will bring the higher voltage down to ~12V (but not boost if lower than 12V), plus a couple of better buck-boost devices that provide a steady 12V, and they work quite well, but they are dotted all over the shop for the various 'domestic' 12V devices I have (those that are designed to be powered by a 12V DC regulated supply via a 240V AC Transformer).

As well as the Regulators, I also tried a DC-DC converter I had kicking around. This was not a very expensive item for the specs (not throwaway money either however).

This is a 248W 12V regulator, but I don't think it is really designed to be on 24/7 and it failed after a few months of constant use.

It is, I guess, a matter of 'you get what you pay for', so I decided to bite the bullet and go for another Blue Box from Victron.
So now to compare the 220W Victron DC-DC Converter with the generic 248W DC-DC Converter, there is a slight difference in the size ...

I am totally confident that the Victron Orion will be happy to be always on without a problem (and of course, there is a 5 year warranty if not!).
This is the Orion-Tr Smart, so is generally more usually found working as a Smart B2B Charger, but I elected to go for this rather than the 'normal' Orion-Tr straight DC-DC converter as it is a lot easier to configure and adjust via the Victron Smartphone App and you just set it into Power Supply Mode in order to use as a fixed output DC-DC converter.

This is connected in-between the Battery Bank and the Sargent EC325, so it ensures all the standard Habitation Electrics are supplied with a fixed 12V output (I haven't put a meter on the supply at the point it enters the Sargent unit yet. It might be with voltage drops on the run from Orion to Sargent that I need to up the 12.0V to maybe 12.1V say to give 12V at distribution point?).
The Sargent Hab electrics are protected by a 20A fuse and is limited to no greater than 20A, so an18A converter is just about perfect (I never see more than around 8-9A DC from the DC Habitation demand).

 

[SquirrellCook]

Something I am not that keen on with Motorhomes, Campers, etc, is the "12V" supply - as it is NOT 12V, but anything between 15V (if battery charger on on a very cold day) down to maybe below 11V (if Inverter pulling a lot of amps).
I have a bunch of 12V regulators that will bring the higher voltage down to ~12V (but not boost if lower than 12V), plus a couple of better buck-boost devices that provide a steady 12V, and they work quite well, but they are dotted all over the shop for the various 'domestic' 12V devices I have (those that are designed to be powered by a 12V DC regulated supply via a 240V AC Transformer).

As well as the Regulators, I also tried a DC-DC converter I had kicking around. This was not a very expensive item for the specs (not throwaway money either however).
View attachment 5122
This is a 248W 12V regulator, but I don't think it is really designed to be on 24/7 and it failed after a few months of constant use.

It is, I guess, a matter of 'you get what you pay for', so I decided to bite the bullet and go for another Blue Box from Victron.
So now to compare the 220W Victron DC-DC Converter with the generic 248W DC-DC Converter, there is a slight difference in the size ...
View attachment 5123

I am totally confident that the Victron Orion will be happy to be always on without a problem (and of course, there is a 5 year warranty if not!).
This is the Orion-Tr Smart, so is generally more usually found working as a Smart B2B Charger, but I elected to go for this rather than the 'normal' Orion-Tr straight DC-DC converter as it is a lot easier to configure and adjust via the Victron Smartphone App and you just set it into Power Supply Mode in order to use as a fixed output DC-DC converter.
View attachment 5124
This is connected in-between the Battery Bank and the Sargent EC325, so it ensures all the standard Habitation Electrics are supplied with a fixed 12V output (I haven't put a meter on the supply at the point it enters the Sargent unit yet. It might be with voltage drops on the run from Orion to Sargent that I need to up the 12.0V to maybe 12.1V say to give 12V at distribution point?).
The Sargent Hab electrics are protected by a 20A fuse and is limited to no greater than 20A, so an18A converter is just about perfect (I never see more than around 8-9A DC from the DC Habitation demand).

I have a 24 12 in Betty that must be running unattended for over 6 months now. As you have mentioned, with the possible massive voltage shifts it would make sense in all Motorhomes.

 

[PeteS]

I guess thats one of the advantages of running Lithium, the 12v supply is always going to be limited to a max of 14.2V and with most series regulators now fitted to equipment having a max input voltage of 16v its not really a problem for me.
The current shunt on my Bailey control panel is very poor and has a significant volts drop in my opinion. The wiring is also definitely underrated so I used to see voltages less than 12v before I switched to Lithium.
A good idea nevertheless if your running Lead Acid.