How much solar for small ice-box conversion?

[argosail]

So I have a boat under contract, turns out she does not have a fridge. I was looking at either a ice-box conversion (12V) or portable fridge/cooler (12V). There is only 12V on the vessel.

I've seen various units with draws from 1A to 4A, so lets say an amp draw of 2.5A.

The boat is 35' LOA. Being on the hook in Florida/Caribs, I am wondering if 800W (2x400w panels) and 1 AGM house battery (standard size, that's all I know) will be sufficient. I would rather not change the battery set-up (because $$$).

I will likely go Adler/Barbour for ice-box conversion, or Dometic/Iceco for a fridge cooler.

I don't plan to have any cold alcohol on board, I don't plan to have oodles of frozen/cold food. A minimal set-up to keep fruits and veg, and sandwiches fresh. The icebox itself isn't huge, it's small for a 35'.

There would be no other systems running other than keeping starting battery (another AGM) topped up and radio, this is on the hook, not underway.

I know there is some formula but everyone seems to have their own version of how things work for them based on their unique set-ups??

[Rohan]

Originally Posted by argosail

So I have a boat under contract, turns out she does not have a fridge. I was looking at either a ice-box conversion (12V) or portable fridge/cooler (12V). There is only 12V on the vessel.

One 12v battery on a 35' boat is a little unusual. You may have a separate starting battery and don't realize it. In any case, if you want to take advantage of solar, you first need more battery power. Otherwise your fridge and other things will be charged fine during the day, but your batteries won't be able to keep up at night. You need a way to store the enormous amount of energy that you soak up during the day.

The boat is 35' LOA. Being on the hook in Florida/Caribs, I am wondering if 800W (2x400w panels) and 1 AGM house battery (standard size) will be sufficient. I would rather not change the battery set-up (because $$$).

Honestly, for a single fridge 800w would be insane overkill. A 100w panel would be fine for a 2.5 amp fridge, along with another battery or two.

About 200-300watts would be more typical for your size boat. It would be difficult for you to mount 800w of panels and you don't need that much anyway unless you are running televisions, computers and other energy hogs.

[HeywoodJ]

The question of solar size and battery size are intertwined, but also separate.

1. If your fridge averages 2.5A then it will consume 2.5*24=60Ah/day.

2. Very crude rule of thumb, solar Watts/3 ~= Ah/day you can expect at 12V. So 800/3 = 266Ah/day, well more than enough to run your fridge (W/3 is based on another rule of thumb that says 4 hours of nameplate equivalent production per day, and average of 14V used in charging the batteries).

3. Battery size is all about how long you want to be able to go without sun. One 'night' can be 16 hours, even in Florida, so minimum usable capacity would be 60*16/24 = 40Ah. If you using AGM this means an 80Ah battery minimum (and that will be a pretty deep discharge every night). If you want to be able to go more than one night without sun then you need a bigger battery. How much bigger depends on what you think you need (i.e. when the sun is gone for too long do you fire up a generator, or the engine/alternator, or turn off the fridge?).

[argosail]

Originally Posted by Rohan

One 12v battery on a 35' boat is a little unusual. You may have a separate starting battery and don't realize it. In any case, if you want to take advantage of solar, you first need more battery power. Otherwise your fridge and other things will be charged fine during the day, but your batteries won't be able to keep up at night. You need a way to store the enormous amount of energy that you soak up during the day.

Yes there are two batteries, 1 house and 1 starter, sorry that's buried deeper in my post, both AGM 12V.

[argosail]

Originally Posted by Rohan

One 12v battery on a 35' boat is a little unusual. You may have a separate starting battery and don't realize it. In any case, if you want to take advantage of solar, you first need more battery power. Otherwise your fridge and other things will be charged fine during the day, but your batteries won't be able to keep up at night. You need a way to store the enormous amount of energy that you soak up during the day.




Honestly, for a single fridge 800w would be insane overkill. A 100w panel would be fine for a 2.5 amp fridge, along with another battery or two.

About 200-300watts would be more typical for your size boat. It would be difficult for you to mount 800w of panels and you don't need that much anyway unless you are running televisions, computers and other energy hogs.

Hmm, so maybe better to use money for the extra 500w for a small Lifepo instead?

I was initially thinking because of the max 50% AGM drain that would use more solar to keep battery from going under 50%

[Rohan]

Originally Posted by argosail

Hmm, so maybe better to use money for the extra 500w for a small Lifepo instead?

I was initially thinking because of the max 50% AGM drain that would use more solar to keep battery from going under 50%

You can buy a lithium battery, but it's not good to mix batteries. Either go all lithium or all AGM. If the starter battery is separate, that's fine if that one is different. Yes, lithium effectively gives you twice the running time, since you can run them all the way down (they claim they charge faster too, but in my own experience its the AGMs charge up super quickly)

To give you some comparison, my boat is 36' and I have 4x AGM batteries. I also have a fridge that draws about 5 amps, so twice what you are looking at. When I added 200 watts solar, that was more than enough energy to charge everything on sunny days. Batteries would drain close to 50% during the night and then be topped off again by noon.

This year I upgraded to 350 watts, and that's enough to do the above, even on 100% cloudy days. On sunny days, I have more power than I need, I can plug in computer and other things, no problem and I'm still showing 13 volts until it gets dark.

[argosail]

Originally Posted by Rohan

You can buy a lithium battery, but it's not good to mix batteries. Either go all lithium or all AGM. If the starter battery is separate, that's fine if that one is different. Yes, lithium effectively gives you twice the running time, since you can run them all the way down (they claim they charge faster too, but in my own experience its the AGMs charge up super quickly)

To give you some comparison, my boat is 36' and I have 4x AGM batteries. I also have a fridge that draws about 5 amps, so twice what you are looking at. When I added 200 watts solar, that was more than enough energy to charge everything on sunny days. Batteries would drain close to 50% during the night and then be topped off again by noon.

This year I upgraded to 350 watts, and that's enough to do the above, even on 100% cloudy days. On sunny days, I have more power than I need, I can plug in computer and other things, no problem and I'm still showing 13 volts until it gets dark.

Sounds good and gives me a better idea what I need. Does your fridge also have a freezer (wondering why the bigger draw)?

[Rohan]

Originally Posted by argosail

Sounds good and gives me a better idea what I need. Does your fridge also have a freezer (wondering why the bigger draw)?

It has a tiny freezer built in, it's basically a college dorm fridge, it's probably just a little bigger than yours. Of course, while the refrigerator is the main power draw, it's not the only thing running when anchored. There's the lights, the bilge pump, anchor light, desk fans, the water pump, the electric toilet, etc.

[valhalla360]

You don't need lithium. It will only add complications to your simple system.

Assuming the fridge draws 2.5amps @ 12v = 30watts.

A worst case scenario is it's very hot and the unit struggles to stay cool, so it runs 24hours a day. Result is it will consume 720 watt-hours (30w * 24h) per day.
- In reality, the unit is likely to cycle on and off, which will reduce the number of watt-hours used per day. Duty cycle is the percentage of time the fridge is using power.

Lead-Acid batteries (including AGM) can be drawn down to around 50% state of charge without risk of damaging the battery. At the same time getting to 100% is challenging because as you get above 80-90%, the battery will accept ever decreasing amounts. The result is the usable amount is somewhere on the order of 30-40% of the rating.

Assuming a 100amp-hour battery with 40% usable...that will hold about 480 watt-hours.

As someone else mentioned, there can easily be 16hr per day when solar panels aren't producing and the battery has to cover the consumption. Given a reasonable duty cycle, it may or may not make it thru the night with 1 100amp-hour battery.

But if you get a cloudy day, where the solar doesn't put out, you need to almost double the battery storage to cover it.

Now you have solar output, a good rule of thumb is the panel rating will generate roughly 4 times the rated amount in watt-hours (ie: 800w of panels will generate around 3200watt-hours). It actually slowly ramps up (rarely doesn't it produce 800w) and then gradually drop off.

As you can see, that's a lot more than a single battery is likely to absorb. Even worse, it's likely to top up the battery to 80-90% in the first few hours of the day and then absorb next to nothing for the remainder of the day.

So lets say, you want to cover 2100watt-hours (3 days of battery to cover overcast days), 4 220amp-hr 6v batteries will provide that (2 wired in series creates 220amp-hr @ 12v then doubled for 440amp-hr...40% usable is 2100w-hr of usable power).

To produce 2100w-hr, is about 525w of solar panels. This allows you to largely recharge in a single day under worst case 100% duty cycle.

Now you need to spec what you want:
- I might drop back to 350-400w of solar as the duty cycle of the fridge is unlikely to be 100%, so in reality, you should be able to recover after 3 days most of the time and worst case in 2 days of charging.
- The 4 - 6v golf cart batteries, can't really be changed without cutting capacity in half, so run with that.

Now for the real question, how much will you use for lighting, fans, water pump and other electrical draws? Same basic idea, figure out how many watts they draw and how long they will be on to estimate watt-hours consumed.

PS: I prefer to use watt-hours over amp-hours. This lets you include an inverter with AC power loads. Watts = voltage * amps.

[sailorboy1]

Originally Posted by argosail

So I have a boat under contract, turns out she does not have a fridge. I was looking at either a ice-box conversion (12V) or portable fridge/cooler (12V). There is only 12V on the vessel.

I've seen various units with draws from 1A to 4A, so lets say an amp draw of 2.5A.

200W solar, try to fit at least 200-250AH of battery. But if you don't have the space just be aware a single battery might only last 2-3 years. But that is cheap really.

[Pete7]

Originally Posted by Rohan

You can buy a lithium battery, but it's not good to mix batteries. Either go all lithium or all AGM. If the starter battery is separate, that's fine if that one is different.

I would like to explore why you think this. There are lots of us running hybrid battery systems quite happily, so why not mix LFP and FLA?

Pete

https://www.youtube.com/watch?v=sqT2...k%27sAdventure

[seandepagnier]

Originally Posted by valhalla360

You don't need lithium. It will only add complications to your simple system.

lithium is cheaper. This is because it lasts more cycles. Since it is more efficient you can use smaller solar panels which saves more cost.

Originally Posted by Pete7

I would like to explore why you think this. There are lots of us running hybrid battery systems quite happily, so why not mix LFP and FLA?

You can also mix dacron and dyneema on your rigging. Best efficiency will not use lead acid.

[George_SD]

We sail a 32' that came with a small frigoboat fridge. that used to run from a single lead acid battery. Between electronics and fridge, it got drained quickly.

I've added a second battery and a 100w solar panel from Costco. It cost $99 and came with a basic charge controller. I've added a better controller with a screen and USB port and keeping the other one as an emergency spare. Now electricity is no longer an issue for us. We never run the engine while on anchor, charge house batteries, charge our mobile phones, ipad and run the fridge 24/7.

My recommendation is to get some basic math done to see how much you need in amp hours and then get battery capacity where it lasts you through the day and then match to a panel that can recharge the batteries.

Look in the solar install thread (should be pinned) for examples.

[valhalla360]

Originally Posted by seandepagnier

lithium is cheaper. This is because it lasts more cycles. Since it is more efficient you can use smaller solar panels which saves more cost.

In larger systems, I might agree with you and technically, you might be correct if you look into long term life cycle analysis.

In the OPs case but just not worth the hassle for such a small system.

[newhaul]

Originally Posted by argosail

So I have a boat under contract, turns out she does not have a fridge. I was looking at either a ice-box conversion (12V) or portable fridge/cooler (12V). There is only 12V on the vessel.

I've seen various units with draws from 1A to 4A, so lets say an amp draw of 2.5A.

The boat is 35' LOA. Being on the hook in Florida/Caribs, I am wondering if 800W (2x400w panels) and 1 AGM house battery (standard size, that's all I know) will be sufficient. I would rather not change the battery set-up (because $$$).

I will likely go Adler/Barbour for ice-box conversion, or Dometic/Iceco for a fridge cooler.

I don't plan to have any cold alcohol on board, I don't plan to have oodles of frozen/cold food. A minimal set-up to keep fruits and veg, and sandwiches fresh. The icebox itself isn't huge, it's small for a 35'.

There would be no other systems running other than keeping starting battery (another AGM) topped up and radio, this is on the hook, not underway.

I know there is some formula but everyone seems to have their own version of how things work for them based on their unique set-ups??

Solar is plenty but you need to at least double your house bank size . You need at least 400ah in a lead based bank to cover for the crappy days when solar doesn't charge . .

I run 250ah lfp and 200 watts solar for my 29 ft boat running everything including ( currently a dorm fridge on the inverter) and within the next 2 weeks a cool blue holding plate system in my 3.5 cf fridge with 1.5cf freezer. I use about 40 ah a day in port and will likely use ( experience with the refer system on other boats ) 50 underway . Plan on 3 days with no charging .

One word replace the crap insulation before doing any install of a refrigerator conversion system .