Refrigeration installation

[Ken Pole]

Considering installation of a Dometic CD20 or 30 drawer fridge, I’m uncertain whether I have enough battery capacity. My closest Dometic distributor says either model draws 2.9 amps when the compressor is in “maintenance” mode.
I have two 100W bimini-mounted semi-flexible panels, charging two new Fullriver DC85-12 AGM batteries via an MPPT controller. The panels easily maintain the batteries at 13.6V. Their capacity is 85 amphours each at a 20-hour rate while reserve capacity is 145 minutes at a 25a discharge rate.
I have a Blue Sea 1-2-both switch which enables me to use either as a starter and the other as a house battery; this kind of lets me keep usage much the same on each – if that makes sense.
The engine is an Atomic 4 gas; with one-litre total displacement, it does not require a ton of cranking. Other power requirements are minimal (all interior and exterior lighting is LED) and I have Raymarine ST60 depth, log and wind, and an infrequently-used ST-4000 wheel pilot.
There is space to add battery capacity (likely two more DC-85 12s) but don’t want to unless it’s really necessary.
Any advice would be appreciated – except the one tht I can buy a lot of ice with the money I’d spend on the fridge. Thanks in advance

[roverhi]

Refrigeration is an energy hog. Most people go with 400 amp/hr batteries as a minimum for cruising with a fridge. Would recommend 6v golf cart batteries wired series/parallel. The GC batteries are built to take abuse and are cheap. Would add a group 24 engine start battery just in case the GC batteries get accidentally run down. AGM's are expensive and don't take kindly to chronic under charging which the norm for a cruiser unless they have a bunch of charging options.

If you are just daysailing with very occasional weekends your battery capacity will probably work. You didn't give the wattage of your panels. If you have 200 watts or so of solar that isn't shaded for most of the day, it will help but doubt whether it will keep up with the refrigeration usage.

Don't know how well insulated the Dometics are but 4" of blueboard insulation for a fridge and 6" for a freezer on the sides and bottom is reccomended.

[rgleason]

This appears to be an AC cooler. That is very unusual and will not be compatible with a DC system unless you are going to invert with a resultant loss of efficiency. --Unless I have the wrong model.



https://www.dometic.com/en-us/us/pro...cd-20-_-137247

[Ken Pole]

Dometic's tech manual indicates 12 or 24V and that it also can be AC powered from shoreside. As for a suggestion that I go 6V for added battery capacity, the general wisdom is that it's best to stick with one battery type. My highly-rated Fullrivers are less than three months old and I can easily purchase two more which I'd paid (170ah) as well as pairing the two I have. I should have explained in my initial outreach that I mostly daysail and have ready access to shorepower. That said, all input is welcome. Cheers.

[sdj]

You have plenty of battery. You have borderline PV but it will work. You should make sure the low-voltage cutout is set correctly and that fridge is wired to use this cutout correctly and it will be fine.

edit: If you could be persuaded to add two more identical batteries I'd wire them series-parallel for 24V operation and double the size of your solar. This would work real well. Of course, you have all your your loads to swap to 24V would likely be untenable. Four 12V batteries in parallel is a bad idea.

[Ken Pole]

Thanks for the feedback. Not planning to parallel four but two sets of two. Cheers. Looking into more PV options.

[Pete7]

Originally Posted by sdj

edit: If you could be persuaded to add two more identical batteries I'd wire them series-parallel for 24V operation and double the size of your solar. This would work real well. Of course, you have all your your loads to swap to 24V would likely be untenable. Four 12V batteries in parallel is a bad idea.

The problem will be the size of the boat. Doubling the batteries might be possible but space is at a premium, doubling the solar to 400w also tricky. I don't understand the value of going up to a 24v house bank voltage, can you explain this.

Ken, we are in a similar position, 180w of solar and 2 x 85A batteries for the house bank. We also have a smaller 60A engine start battery. We have no problem running a Isotherm 50 kit in what is probably a 2cuft cool box. The draw is about 3A and it runs about 2 minutes ever 10.

So I think it will work. Waeco also do a draw fridge if you wanted a 12v version btw. The risk is several straight days of fog and rain and then you have a problem. Happens to us occasionally, the answer is not to worry and run the engine or if you are going into a harbour / marina use the time to charge up the bank.

If there is room for a third battery, then make that the engine battery and join the two existing together would be good. Also you might take the opportunity to go from a Group 24 to a larger Group 31 next time they need replacing.

Pete

[sdj]

Originally Posted by Pete7

The problem will be the size of the boat. Doubling the batteries might be possible but space is at a premium, doubling the solar to 400w also tricky. I don't understand the value of going up to a 24v house bank voltage, can you explain this.

The main questions to be answered about any power setup, is how to fit the most in, and is it safe to be left unattended.

Maximised upgrades will then require a rethink from scratch, in terms of solar panel rack size and location, and same with the batteries.

There are always moments of 'unattended' with remote area power installations, whether you are physically away or preoccupied. In any case the system should preferably not destroy part of itself or burn the yacht to the waterline.

24V advantage - In your situation, four 12V batteries in parallel is a risky charging arrangement - they are rarely charged correctly, and also any cell problem in any battery will quickly disable the entire battery bank in terms of usable electrical capacity AND in terms of battery life. That is, one faulty battery will destroy the others in parallel, and also drop the overall storage capacity by three quarters at least. 24V or 48V series battery systems are a far less risky charging proposition, and also solar controller and mains inverter requirements are physically smaller and less costly.

Remember, while the starting battery is the most important 'battery' item in terms of safety, it's not the most important item in terms of power consumption. That is, if you didn't charge it for a whole month, chances are you wouldn't even know - not so for the house batteries. So the starting batterys' charging requirements are very small - it's far more important to have a correct float charge than a powerful solar charging system for it - which woud be better directed at a large and efficient house battery, which may in turn charge the start battery, usually via the inverter.

If you can tolerate it in the immediate short term, I would suggest you begin on a complete engineering redesign from scratch using 'best practices vs cost' approach, and leave it as long as possible to do the upgrade. At the end of that process, choose whatever options you felt were either affordable or indispensable. This is a lot of research though.

Have fun. Building power systems is very interesting.

[Pete7]

Ok, I see what your saying about 24v. The problem will be though all the domestics are currently 12v. Add another widget to supply them at 12v? well you could. Then there is the alternator, bet that is 12v so how is it going to charge the house bank, or the house bank start the engine in an emergency, more widgets?

This is a 27 foot yacht, so probably has a quite simple electrical system and that is great, adding complication may not increase safety. I do like the KISS approach.

Pete

[rgleason]

Pete, I considered 24vdc too, because of smaller wire sizes, more efficient.alternator and battery considerations but concluded it added more complexity on a small boat.

It certainly is not worth it unless you are changing alternators, sarters and moving battery locations and thus rewiring and even then you are loosing some emergency redundancies.

24vdc and 48vdc are for big boats, IMHO.

[sdj]

Originally Posted by Pete7

This is a 27 foot yacht

Yup, it's all about what you can physically fit in there. I'm sure you could put a full kitchen or a stand up bathroom in there, but not much else..

[Pete7]

True, having just taken a wardrobe out of our stern cabin to see if a mini freezer will fit, after all you can't have Gin without ice

Of course, that will mean doubling the solar. Also is the house bank still big enough? questions and more questions as the KISS approach slowly slips away

Pete

[sdj]

Originally Posted by Pete7

True, having just taken a wardrobe out of our stern cabin to see if a mini freezer will fit, after all you can't have Gin without ice

Of course, that will mean doubling the solar. Also is the house bank still big enough? questions and more questions as the KISS approach slowly slips away

Pete

Is it bigger boat time mate?

I'm building a 40ft sloop with accomodation for one! Imagine all the toys!