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

--- Quote from: Ian.M on January 10, 2019, 02:31:02 pm ---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

--- End quote ---

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
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