PV in a RV running a refrigerator with some SLA batteries - sanity check

[DTJ]

I've got a project I'm working on and need a sanity check.

I want to run a 12V refrigerator in a vehicle from an independent 12V solar backed SLA supply.
I've already got a 20Ahr SLA battery in the vehicle for charging phones and running a LED light occaisionally.

The plan is to put a large PV panel on the roof. This can run the fridge and charge a SLA battery during the day. Overnight the battery can run the fridge on its own.

Fridge Specs: Waeco CF-35. Power requirement for 24 hours is 6A at 17% duty.
Lets say this is 24hrs x 6A x 20% = ~30Ahr.
I guess from this we can say the 'off solar' requirement will be 20Ahr.

Solar panel specs:  195W panel, 46V o/c 5.1A s/c
Comparing this to the output of my house PV system I should be able to get well over 500Whrs per day from the panel.

Solar reg specs: I've ordered a 20A PV regulator on eBay.
Like this one https://www.ebay.com.au/itm/12V-24V-20A-30A-Solar-Panel-Battery-Regulator-Charge-Controller-PWM-LCD-Display/282950032611?hash=item41e122dce3:m:mCFFAyhDXjqVnZqZ8y4zZ2g:rk:2:pf:1&frcectupt=true

My issue is what to do for a battery. I already have a near new 20Ahr 12V SLA, if I added a second one in parallel I think it will do the job.
An additional single 20Ahr SLA will cost me $76 versus around $200 for a deep cycle lead marine or automotive lead acid battery.


So the set up would be:

195W PV panel ==> Solar Regulator  -------------- 30Ahr / day fridge load
                                                                    |
                                                                     ------- 2 x 12V 20Ahr SLA batteries in //


I don't think the battery charge rate will be excessive - its limited by the panel out out of 5A

What do you think - will adding another 20Ahr SLA to the one that I've already got do the job?

[capt bullshot]

The solar panel / regulator / battery voltage combination doesn't look good to me.

Your typical "outdoor fun" solar battery charger just limits the end of charge voltage (by shorting out the panel using an PWM controller), and otherwise just passes the current from the panel to the battery. So your 195W panel will charge the battery at peak (14.4V * 5.1A) 74W, not the theoretically available peak 195W, reducing your harvest to 185Wh, that's roughly 13Ah, compared to roughly 35Ah at full panel power.

So redo your numbers when you compare this panels output to your home solar system.
To increase the output, look for a MPP tracking type charger that has a built-in step-down converter to maximize the output from the panel.

Using a 20Ah battery to cyclically supply 20Ah also is a bad idea, so you're on the right way by adding a second battery, reducing the discharge cycle to 50%. Look up the battery specs for cycle life vs. cycle depth.

195W peak power from your panel would result in 13A peak charge current (using the MPP regulator), so check your batteries for that, 6.5A charge current is pretty large for a 20Ah SLA, most probably it won't take that. So add more batteries.

[NiHaoMike]

If you can use conventional deep cycle instead of sealed lead acid, you can cut down on the cost tremendously to the order of about $100 (after refundable core charge) for an 80Ah or larger battery.

[Ian.M]

The fridge's power requirements are given at an ambient temperature that's favourable to the manufacturer, in free air.  Put it in an enclosed space without forced ventilation and it will certainly be worse - I don't know if that model has a variable speed compressor, but if it does the run current may go up as the condenser temperature increases.  Its duty cycle will certainly increase.   It will also be worse with higher ambient temperatures so if that side of your RV is south-facing with no shade and you are on tarmac on a calm sunny summer day,  you can expect the duty cycle to go as high as 100% andd the fridge fails to hold temperture, as the interior of the RV becomes a solar oven!

Also unless you have a tilting popup frame for the panels they'll be less effective due to the poor incidence angle.  Its not a big problem up the top end but if you are right down in Tasmania you'll loose around 30% of their output if they aren't optimally tilted.

A 20A 'house' battery would be regarded as a joke on any boat big enough to do more than rough it overnight occasionally.   100AH is about where it starts for boats you might spend a bank holiday weekend on.   Figure that unless you've had a *long* drive or been hooked up to site power overnight, a lead acid battery is unlikely to get much past 90% charged, and you never want to go below 40% for cyclic use or the battery life will be affected, so the rule of thumb is you get about half the rated capacity as usable cyclic capacity.   That also gives you a bit of margin to cope with reducing capacity as it ages, which makes the difference between having to replace the battery more or less annually or getting a proper five or more years life out of it.

Finlly, if its going to be charged a lot at high temperatures, for <DEITY>'s sake get a wet cell battery that isn't maintenance free and mount it somwhere accssible as keeping it topped up will vastly extend its life

[Zero999]

The fridge's power requirements are given at an ambient temperature that's favourable to the manufacturer, in free air.  Put it in an enclosed space without forced ventilation and it will certainly be worse - I don't know if that model has a variable speed compressor, but if it does the run current may go up as the condenser temperature increases.  Its duty cycle will certainly increase.   It will also be worse with higher ambient temperatures so if that side of your RV is south-facing with no shade and you are on tarmac on a calm sunny summer day,  you can expect the duty cycle to go as high as 100% andd the fridge fails to hold temperture, as the interior of the RV becomes a solar oven!

Also unless you have a tilting popup frame for the panels they'll be less effective due to the poor incidence angle.  Its not a big problem up the top end but if you are right down in Tasmania you'll loose around 30% of their output if they aren't optimally tilted.

A 20A 'house' battery would be regarded as a joke on any boat big enough to do more than rough it overnight occasionally.   100AH is about where it starts for boats you might spend a bank holiday weekend on.   Figure that unless you've had a *long* drive or been hooked up to site power overnight, a lead acid battery is unlikely to get much past 90% charged, and you never want to go below 40% for cyclic use or the battery life will be affected, so the rule of thumb is you get about half the rated capacity as usable cyclic capacity.   That also gives you a bit of margin to cope with reducing capacity as it ages, which makes the difference between having to replace the battery more or less annually or getting a proper five or more years life out of it.

Finlly, if its going to be charged a lot at high temperatures, for <DEITY>'s sake get a wet cell battery that isn't maintenance free and mount it somwhere accssible as keeping it topped up will vastly extend its life

The original poster is in Australia, so a south-facing aspect is going to be cool, compared to north facing.

The fridge has a variable speed compressor and is specified for 0.71 Ah/h at +32°C ambient temperature.
https://www.waecofridges.co.uk/freezer/cf35.htm

It uses a Danfoss BD35F compressor, which is very efficient and has a built-in VFD, which probably has a regulated output voltage.
http://files.danfoss.com/TechnicalInfo/Dila/06/bd_compressors_04-2007_pk100c802.pdf

[Ian.M]

Aagh!!! You can tell I'm from the Northern hemisphere 
It is of course the NORTH facing side that gets the sun in Australia.  Thanks for the correction. 

With a bit of extrapolation from the published figures, it looks like it will probably need about 0.8Ah/h in +40°C ambient, and above that, it isn't going to hold a low enough interior temperature to be useful as a fridge without some help.  The best help one could give it would be a thick quilted insulation jacket with aluminised fabric on the outside, covering everything except its vents, and an inch thick slab of foil faced insulation foam under it.

It also needs to be in a well ventilated compartment to keep the ambient temperature down, which is problematic if you've got all vents closed up to minimise the dust intake for dry season dirt road travel.

[jmelson]

There are all sorts of scrambled units in the OP.  6 A x 12 V * .17 duty factor is 12.24 W average, * 24 = 294 Watt-Hr per day.  Assuming some of the load is powered direct (without storage) then maybe that needs 100 - 150 W-Hr of storage.  150 W-Hr storage at 12 V is 12.5 A-Hr.  Now, of course, most batteries are rated to near zero, so you don't want to drain your battery that far down.  Depending on how good the MPPT tracking works on your charge controller will determine whether you can charge the batteries fully every sunny day.  Don't forget to include a fudge factor for cloudy days.

And, a 6 A x 12 V refrigerator is going to be a Peltier cooler, not a compressor type.  Look for the most efficient one you can get, it will save you a lot of problems making this all work.

Jon

[Zero999]

And, a 6 A x 12 V refrigerator is going to be a Peltier cooler, not a compressor type.  Look for the most efficient one you can get, it will save you a lot of problems making this all work.

No, the one described in the original post is a compressor type, which is much more efficient than the crappy Peltier coolers found in cheap fridges.

[jmelson]

And, a 6 A x 12 V refrigerator is going to be a Peltier cooler, not a compressor type.  Look for the most efficient one you can get, it will save you a lot of problems making this all work.

No, the one described in the original post is a compressor type, which is much more efficient than the crappy Peltier coolers found in cheap fridges.

Yes, indeed, a compressor would have to be really AWFUL to be worse than Peltier.  I'd never heard of compressor refrigerators with only a 72 W power draw!
Great stuff is being made.

Jon

[DTJ]

The solar panel / regulator / battery voltage combination doesn't look good to me.

Your typical "outdoor fun" solar battery charger just limits the end of charge voltage (by shorting out the panel using an PWM controller), and otherwise just passes the current from the panel to the battery. So your 195W panel will charge the battery at peak (14.4V * 5.1A) 74W, not the theoretically available peak 195W, reducing your harvest to 185Wh, that's roughly 13Ah, compared to roughly 35Ah at full panel power.

So redo your numbers when you compare this panels output to your home solar system.
To increase the output, look for a MPP tracking type charger that has a built-in step-down converter to maximize the output from the panel.

Using a 20Ah battery to cyclically supply 20Ah also is a bad idea, so you're on the right way by adding a second battery, reducing the discharge cycle to 50%. Look up the battery specs for cycle life vs. cycle depth.

195W peak power from your panel would result in 13A peak charge current (using the MPP regulator), so check your batteries for that, 6.5A charge current is pretty large for a 20Ah SLA, most probably it won't take that. So add more batteries.

Great point about the power utilisation of MPPT v PWM regulators - it's pretty obvious once pointed out! I was quite conservative in the 500Whr figure for the panel production - the peak value I got was closer to 900Whr. I dropped it back to 500 to allow for a poor day. I might just scrape by with the single panel and the PWM regulator.

I don't want to spend too much money on this - the MPPT regulator would probably mean bigger batteries that could cope with the higher charge rate. Lots more to think about! Thanks.

[DTJ]

There are all sorts of scrambled units in the OP.  6 A x 12 V * .17 duty factor is 12.24 W average, * 24 = 294 Watt-Hr per day.  Assuming some of the load is powered direct (without storage) then maybe that needs 100 - 150 W-Hr of storage.  150 W-Hr storage at 12 V is 12.5 A-Hr.  Now, of course, most batteries are rated to near zero, so you don't want to drain your battery that far down.  Depending on how good the MPPT tracking works on your charge controller will determine whether you can charge the batteries fully every sunny day.  Don't forget to include a fudge factor for cloudy days.

And, a 6 A x 12 V refrigerator is going to be a Peltier cooler, not a compressor type.  Look for the most efficient one you can get, it will save you a lot of problems making this all work.

Jon

Hi Jon - it's definitely a compressor unit. From my limited experience the Peltier units are little more than coolers and don't work well as a refrigerator.

To clarify, from the Waeco spec sheet the power needs are "6A @ 17% duty". This gives 6A x 24 hr x 0.17 = 24.8Ahr per day.

[james_s]

Yes, indeed, a compressor would have to be really AWFUL to be worse than Peltier.  I'd never heard of compressor refrigerators with only a 72 W power draw!
Great stuff is being made.

Jon

My fullsized domestic fridge only draws 140W (measured) while it's running, I'd expect most small dorm and RV fridges to draw considerably less than that, 72W doesn't sound too unreasonable. Fridge efficiency went up quite a bit at some point due largely to improved insulation.

[georges80]

I run a 50qt ARB danfoss based fridge. Previously (for 30+years I'd run a Engel). Fed from a dual battery system in my 4wd which charges both batteries while driving and splits to a main and aux when parked. The aux powers the fridge. And yeah, a modern ARB draws maybe around 4A while running, so around 50 - 60W.

I first bought a solar panel in the early 80's charge the aux for fridge use. So, quite few years experience with this kind of usage.

I run a nominal 80W panel that I can position to face the sun. I run a Victron mmpt (bluetooth paired) controller (check them out - fairly inexpensive and good quality/performance).

I've camped in 37C (100F) and higher temps and the panel + 80ahr lead acid can easily keep up with 24 hour demand. One of these days I'll upgrade to a 120-140W panel (they are so cheap in oz).

These modern fridges are very efficient even in hot climate (just keep the fridge in the shade and provide a decent airflow path for the compressor radiator).

This kind of setup is very popular in oz and panels are quite inexpensive. Put some money into the controller and into the battery. Consider that if your panel is in the 100W (or more range) then you will collect around 6A or more per hour during sunshine part of the day (call it 8 hours). So even on an average day you are in the 50a.hr range being collected.

Seems nuts to have a 20a.hr or even 40a.hr battery with near 200W for the OP's setup. Very unbalanced system.

In cooler weather a danfoss equipment compressor will be running maybe 1/6 or even less of the time, so less solar collection due to some cloud cover will be compensated by lower a.hr draw each 24 hours.

Anyhow, I recommend you do some more calcs to figure out real a.hr usage in the weather you plan to camp and match the battery capacity to make good use of the harvested a.hr from the solar panel.

Finally, keep the fridge full (more efficient), don't open it unnecessarily, refill drinks etc early in the morning (or before driving) so that it can be cooled while the engine/alternator/solar is running - versus refilling in the evening and camped.

cheers,
george.

[Teledog]

Off topic perhaps..?
Why not a propane fridge?
KISS?

[NiHaoMike]

Off topic perhaps..?
Why not a propane fridge?
KISS?

The absorption cycle is something like 1/5 the efficiency of a good compressor refrigeration cycle.

Now I wonder if it might be possible to take advantage of thermal storage. Maybe keep bottles of very pure (distilled) water inside and adjust the temperature so it just barely freezes during the day, relying on the fact that the water in the food is going to be much less pure and would freeze at a lower temperature.

[james_s]

Propane powered refrigerators are neat, but efficient they are not. I remember my grandparents had a 1968 motorhome that had a propane refrigerator and the exhaust on that thing got ferociously hot. As a kid it was a complete mystery to me how a gas burner could be used to make something cold.

[soldar]

I'd never heard of compressor refrigerators with only a 72 W power draw!
Great stuff is being made.

My ancient mid -sized draws 100 W when running. I just bought a new full size and it also draws 100W and with lower duty cycle so it is more efficient and I am using less energy than before.

75 W or less for a smaller fridge sounds about right.

[station240]

It uses a Danfoss BD35F compressor, which is very efficient and has a built-in VFD, which probably has a regulated output voltage.
http://files.danfoss.com/TechnicalInfo/Dila/06/bd_compressors_04-2007_pk100c802.pdf

Yes, internally it boosts the input voltage to ~48VDC, the compressor is BLDC without hall effect sensors.

Word of warning, don't get the compressor wet, the controller electronics have conformal coating, but no protection from short circuits.
The pins on the compressor itself can allow water in, which fries the BLDC drive mosfets and something in the supply.

[DTJ]

The follow up post:

I tried the system with  the PWM charge controller below.

https://www.ebay.com.au/itm/30A-12V-24V-Solar-Panel-Battery-Regulator-Charge-Controller-LCD-USB-30AMP-PWM/322520750752?epid=676327152&hash=item4b17bc1aa0:g:U3wAAOSwfRdZHXsV:rk:6:pf:0

It worked but would sometimes kick out and disconnect the solar panel. Cycling the power reset it. It turned out the maximum PV input voltage was 24V.

I replaced it with a simpler regulator with a higher voltage spec for the PV input.
https://www.jaycar.com.au/12-24v-10a-solar-charge-controller-with-usb/p/MP3750


Its now been running a full 24 hours with the single 195W panel and a single 20Ah SLA. I think I'll add an additional 20Ahr SLA anyway to cover for poor solar days.

The Waeco 31 litre fridge only draws 4.1A when running. I have not measured the duty cycle but it sounds like the manufacturers spec of around 20% is reasonable.