Actual Ah demand for AirCon

[sailingharry]

First, a warning -- my questions are beginning! For the last 3 years, I've been aware my 800Ah AGM bank was dying, and LFP was in my future. In fact, starting in 2022, I stopped making "get to 100% SOC" my daily priority, and recognized that the end was near. Yesterday, with a fully charged battery, I turned on my inverter to run my heat gun -- and in under 5 minutes I had a low voltage alarm. UGH. The admiral will NOT be happy when she can't run the toaster! So, an LFP build is no longer "in the works" and is a NOW, or sooner!


First note -- I'm a 12V boat. I fully understand the benefits of 24V, but all my equipment is 12V. To get any benefit of 24V, I'd have to replace my 2 year old inverter, as well as my windlass and power winch. Not happening!



So, first question -- capacity. I have had very little problem running above the 50% mark, so 400Ah of actual use. Improved solar (already in the works) and improved charge profile with LFP, will make this even easier. This leads to a bank of maybe 600Ah, although even 400Ah would probably be sufficient.


BUT. I don't have Air installed -- but might. When not on shore power, it would be a fairly rare event to run, but I could see some night time use. I was in Isla Mujeres (Cancun) in December on an identical boat (Saga 43) and the A/C ran maybe a 20% duty cycle. It was definitely off more than on. Not sure the BTU/etc.


Would it be reasonable to expect that a smallish A/C, set to a moderate temp, could be run overnight on say 300Ah? A 10K BTU unit has a full load of 7A ac, or ~70A dc, or under 100A total dc load. Running 8 hours at a 25% duty cycle is 200Ah, leaving room for a full hour at 100% duty cycle to cool it down initially. My LFP design choices currently allow for 600Ah or 900Ah, and the extra 300Ah adds $500* to the bill -- the thought of A/C makes that small price seem tempting.



I know -- LFP doesn't "make" power, it only stores it. Every Ah that comes out has to be put back the next day, and my 1kW solar array will need some dino-assistance!


* 300Ah for $500? In a bare cell build, most (much?) of the cost is the setup, and is a fixed cost. BMS, controls, disconnects, fuses, wiring, etc. The cell part is sort of a "black box" in the design, and the size of the battery has little impact on the supporting infrastructure. To go from a 2S4P (600Ah) to a 3S4P (900Ah) requires buying 4 more cells. Currently, decent 300Ah cells are $125 each delivered.

[SailingHarmonie]

If you have a solar bank that falls short of supplying everything you need, and will be supplying power from a genset or alternator to make up the balance, I would make the argument that the size of the battery bank is (almost) irrelevant. Once you get to the point when in normal conditions the generator needs to run no more than once a day, what’s the advantage of a bigger bank?

A bigger Li bank doesn’t save generator run time. If you run a generator once a day for an hour or every other day for 2 hours, who cares? Why spend them money for more?

Our solar system supplies about half our energy needs. The rest comes from a DC genset that auto starts based on battery SOC and time of day. For us it is very much like driving a Prius. The engine comes on when needed and shuts off when not. Our generator runs about 60 minutes a day on average, and is pretty transparent to us almost all the time.

[Pete7]

Originally Posted by sailingharry

* 300Ah for $500? In a bare cell build, most (much?) of the cost is the setup, and is a fixed cost. BMS, controls, disconnects, fuses, wiring, etc. The cell part is sort of a "black box" in the design, and the size of the battery has little impact on the supporting infrastructure. To go from a 2S4P (600Ah) to a 3S4P (900Ah) requires buying 4 more cells. Currently, decent 300Ah cells are $125 each delivered.

Surely the cells should be the other way around, 4S3P ?

If you are using the 300A for 1 hour fast cooling and 8 hours at 25%, this leaves you 300A for the rest of the yacht if you opt for the 600Ah bank. That's quite a bit, we can manage on 120Ah but added another 100Ah for good measure during inclement weather. There is also 85Ah of FLA just in case because we don't have a genny and only use the alternator occasionally to charge the domestic bank.

So I think you are in the ball park. Therefore, whilst the extra 300Ah for $500 isn't a big cost in the project, its still a cost. Also how much room to do you have on 34ft? Okay the existing SLA bank is going, but space is really valuable on a smaller yacht.

How about going for 600Ah but designing and wiring for an increase to 900Ah if needed?

Oops, just seen you have sold the 34 and are now using a 43ft yacht. In that case sod it go for 900Ah. You will find a use for it somewhere like electric cooking, bread making and cakes. Wifey can use the hair dryer without worry too, which will likely sell the project to her.

[sailingharry]

Originally Posted by Pete7

Surely the cells should be the other way around, 4S3P ?

No, 3P4S. It's a built bank (not drop in). By setting each three parallel, each 3 looks like a single cell. Then put them in series to get 12V.


Going with 4S3P means you need 3 individual BMS, and then the logic for controlling the charges and loads gets nasty. At the very least, you need 3 separate main contactors. Although, I guess that since any BMS intervention is a true rarity, there would be no problem paralleling them -- any high/low cell, or any other excursion, results in "the action" on all 3 banks.


And, yes, I have 43 feet. Even the 900Ah is half the size of the existing AGM bank, in a location where I really can't use the space anyway!

[sailingharry]

Originally Posted by SailingHarmonie

If you have a solar bank that falls short of supplying everything you need, and will be supplying power from a genset or alternator to make up the balance, I would make the argument that the size of the battery bank is (almost) irrelevant. Once you get to the point when in normal conditions the generator needs to run no more than once a day, what’s the advantage of a bigger bank?

A bigger Li bank doesn’t save generator run time. If you run a generator once a day for an hour or every other day for 2 hours, who cares? Why spend them money for more?

Our solar system supplies about half our energy needs. The rest comes from a DC genset that auto starts based on battery SOC and time of day. For us it is very much like driving a Prius. The engine comes on when needed and shuts off when not. Our generator runs about 60 minutes a day on average, and is pretty transparent to us almost all the time.

I agree 100% with what you wrote. The problem is I don't have a genset....LOL. With your system, the LFP only has to be large enough for the genset to run a decent runtime. Assuming you can cram 200A into the bank (only limited to how many/how big your chargers are!), a 200Ah battery gives you an hour runtime on the genset.


My charge (after solar) is only my alternators (well, also 40A from a Watt and Sea, but that only works when on transit as well). While I have 270A of alternators (when hot, closer to 150A), it still is annoying to run the main engine for charge. A larger bank allows the ability to wait for the next transit. We rarely spend 2 nights in one place (I can't recall every spending 4 nights in one place!), and unfortunately motoring is a large percentage of transits. If we can delay a recharge for 1-2 days, there is a good chance that it will be 100% handled by the engine on transit.


And all that leads to my air conditioning question. Unless I want to run the engine right before bed, or in the middle of the night, I need enough Ah to get through the afternoon/evening, and then A/C through the night, and then morning cooking (electric toaster and coffee), before either transit or at least some solar.


I have no history on A/C power requirements, as I don't have one on the boat. Nice diesel heat! But we have yet to go south.

[SailingHarmonie]

Originally Posted by sailingharry

I agree 100% with what you wrote. The problem is I don't have a genset....LOL. With your system, the LFP only has to be large enough for the genset to run a decent runtime. Assuming you can cram 200A into the bank (only limited to how many/how big your chargers are!), a 200Ah battery gives you an hour runtime on the genset.


My charge (after solar) is only my alternators (well, also 40A from a Watt and Sea, but that only works when on transit as well). While I have 270A of alternators (when hot, closer to 150A), it still is annoying to run the main engine for charge. A larger bank allows the ability to wait for the next transit. We rarely spend 2 nights in one place (I can't recall every spending 4 nights in one place!), and unfortunately motoring is a large percentage of transits. If we can delay a recharge for 1-2 days, there is a good chance that it will be 100% handled by the engine on transit.


And all that leads to my air conditioning question. Unless I want to run the engine right before bed, or in the middle of the night, I need enough Ah to get through the afternoon/evening, and then A/C through the night, and then morning cooking (electric toaster and coffee), before either transit or at least some solar.


I have no history on A/C power requirements, as I don't have one on the boat. Nice diesel heat! But we have yet to go south.

I am sure you’ve seen this before, but my experience with AC is (I believe) pretty typical, and this includes summer cruising in the tropics. We hardly ever use it at anchor. And by hardly ever, I mean once in 7 years. Now in a marina it’s a different story, but when you are anchored out where there is a breeze, and the boat can orient it self into the wind, the cabin is comfortable pretty much all the time. This is very different if your boat is a natural greenhouse like moat catamarans and deck salons.

There can be exceptions! If you’re in the Chesapeake in the high summer there is no wind, and the bugs can be ferocious, you might want AC then. The hot season in Thailand might be another time/place a night time AC would be a nice to have.

We find our 6000 btu unit in our aft cabin pulls about 650 watts. At night it runs about a 30% duty cycle. Sun load is the HUGE factor. Putting up a shade cloth over the deck will easily drop the interior temperature 10F and greatly reduce the load on your AC.

Finally, don’t think a smaller AC than recommended will save energy. They all have about the same efficiency, so pumping a BTU out of the cabin these the same energy if you do it with a 6000 or a 12000 btu unit. The smaller one just runs longer to get the same temperature. The recommended size will give you the best performance.

[rslifkin]

Some of the (expensive) newer VSD units spec significantly lower power draw relative to BTU rating than many of the conventional marine A/Cs (which have improved surprisingly little in efficiency). So those might be worth a look for running on battery power.

I've been giving similar thoughts to sizing my next battery upgrade around the idea of running A/C in just the aft cabin on a really hot night, but I haven't settled on what it'll take or whether it's worth it yet.

[sailingharry]

Originally Posted by SailingHarmonie

I am sure you’ve seen this before, but my experience with AC is (I believe) pretty typical, and this includes summer cruising in the tropics. We hardly ever use it at anchor. And by hardly ever, I mean once in 7 years. Now in a marina it’s a different story, but when you are anchored out where there is a breeze, and the boat can orient it self into the wind, the cabin is comfortable pretty much all the time. This is very different if your boat is a natural greenhouse like moat catamarans and deck salons.

There can be exceptions! If you’re in the Chesapeake in the high summer there is no wind, and the bugs can be ferocious, you might want AC then. The hot season in Thailand might be another time/place a night time AC would be a nice to have.

We find our 6000 btu unit in our aft cabin pulls about 650 watts. At night it runs about a 30% duty cycle. Sun load is the HUGE factor. Putting up a shade cloth over the deck will easily drop the interior temperature 10F and greatly reduce the load on your AC.

Finally, don’t think a smaller AC than recommended will save energy. They all have about the same efficiency, so pumping a BTU out of the cabin these the same energy if you do it with a 6000 or a 12000 btu unit. The smaller one just runs longer to get the same temperature. The recommended size will give you the best performance.

Thanks, good stuff.


First, at anchor, to date, we have used it 0.000% of the time -- 'cause we don't have it! Our home waters are the Chesapeake (where I've cruised for almost 60 years), and I've never had AC. So your comment is what I've seen.


I do understand that the efficiency curve is a constant. I also have noticed that size and cost are nearly a constant (a 12K isn't anywhere near twice the price or space of a 6K! For example, a 6K on Defender is $1960, a 16K is $2250).


Your comment on needing 650W on a 30% duty cycle (or an average of 200W) is very helpful. That's a net of about 20A, or under 200Ah for an overnight -- consistent with my thoughts.

[thinwater]

I think it will be very tough (also a Chesapeake sailor) but not impssible.


I had a small AC (10,000 BTU?) in my PDQ32 cat. If I cooled the boat down first, as you say, it would run a very low duty cycle. I could easily run for 4-6 hours on 100 Ah. In fact, I commonly left the AC on when leaving marinas, so that I could get out in the wind before opening windows. You know how harbors can be. No strain at all. Very low duty cycle, even with the sun up (but low), so long as I left the window covers on.



But the unit would run HARD for several hours to cool the boat down (80-100% duty cycle, which is 7 A at 120 or about 150 Ah, even if I waited until the sun was down. Part of the load is drying the air, which is very humid. I fear the several hours of cool down and drying will blow your plan. The rest of the night will probably only be another 100-150 Ah. This depends on insulation, temperatures, and all that.



Good luck!

[GordMay]

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[Sailorlady2022]

I am not an expert and I am not sure if this will help you or not, but real world experience for our particular use.


We have a 40 ft catamaran with 1200 amp hr LiFePO4 battery bank. We have installed 3 Mabru DC Air Con units. The unit closest to your specification is the 12000 BTU (Model SC12DC). We are cruising the Bahamas currently and have used it sporadically on anchor more to decrease the humidity than for cooling. Our overnight battery consumption is approximately 30% of our battery capacity which equates to 360 amp hours for all the usual appliances PLUS this air con unit.



That being said, we cannot isolate the usage of the Air Con unit only as other devices are also running (DC items include: fridge, freezer, fans, NAS device and AC items: Starlink and router).


The variability of the items included and how much they run makes this difficult to narrow down specifically; however, we estimate the Air Con is approximately half of this total overnight usage. (15% of our battery bank =180 amp hrs).

[CaptainRivet]

Originally Posted by sailingharry

First, a warning -- my questions are beginning! For the last 3 years, I've been aware my 800Ah AGM bank was dying, and LFP was in my future. In fact, starting in 2022, I stopped making "get to 100% SOC" my daily priority, and recognized that the end was near. Yesterday, with a fully charged battery, I turned on my inverter to run my heat gun -- and in under 5 minutes I had a low voltage alarm. UGH. The admiral will NOT be happy when she can't run the toaster! So, an LFP build is no longer "in the works" and is a NOW, or sooner!


First note -- I'm a 12V boat. I fully understand the benefits of 24V, but all my equipment is 12V. To get any benefit of 24V, I'd have to replace my 2 year old inverter, as well as my windlass and power winch. Not happening!



So, first question -- capacity. I have had very little problem running above the 50% mark, so 400Ah of actual use. Improved solar (already in the works) and improved charge profile with LFP, will make this even easier. This leads to a bank of maybe 600Ah, although even 400Ah would probably be sufficient.


BUT. I don't have Air installed -- but might. When not on shore power, it would be a fairly rare event to run, but I could see some night time use. I was in Isla Mujeres (Cancun) in December on an identical boat (Saga 43) and the A/C ran maybe a 20% duty cycle. It was definitely off more than on. Not sure the BTU/etc.


Would it be reasonable to expect that a smallish A/C, set to a moderate temp, could be run overnight on say 300Ah? A 10K BTU unit has a full load of 7A ac, or ~70A dc, or under 100A total dc load. Running 8 hours at a 25% duty cycle is 200Ah, leaving room for a full hour at 100% duty cycle to cool it down initially. My LFP design choices currently allow for 600Ah or 900Ah, and the extra 300Ah adds $500* to the bill -- the thought of A/C makes that small price seem tempting.



I know -- LFP doesn't "make" power, it only stores it. Every Ah that comes out has to be put back the next day, and my 1kW solar array will need some dino-assistance!


* 300Ah for $500? In a bare cell build, most (much?) of the cost is the setup, and is a fixed cost. BMS, controls, disconnects, fuses, wiring, etc. The cell part is sort of a "black box" in the design, and the size of the battery has little impact on the supporting infrastructure. To go from a 2S4P (600Ah) to a 3S4P (900Ah) requires buying 4 more cells. Currently, decent 300Ah cells are $125 each delivered.

As a newbie never ever build 2P or 3p batteries, you build 1p4S batteries with each a BMS and put that in parallel. And no nasty things going on, you obviously have no glue how to design such a system but that’s how all dropins are build, work and installed. As you will most likely use mosfet BMS each battery must have a battery fuse that limits the current to what your BMS can do and a BMV712 battery monitor+disconnect relay as independent last resort cut off…all that around will cost the same as the cells to make it safe.

2nd only 12V water cooled Marbu AC will have a chance to be efficient enough, they are expensive…
As other mentioned is more the dehumidifier then the cooling down you will need.

Below real 1500W solar (means you see 1500W on the MPPTs, not WP) don‘t start thinking about running AC of the batteries and that’s challenging and expensive on mono,not the panels but the arch and bimini mounting of them. Below that you need gen or high output alternators and run engine at anchor.

I agree 900AH is minimum, plan for 1200AH with your mentioned usage…that gives you buffer and as said gives you possibility to wait till next transit or survive 2-3 days with bad solar harvest.

[s/v Jedi]

We really need to know what you have now and intend to keep using: what shore power, solar, which solar controllers, which alternator with which regulator, which AC charger, inverter or inverter/charger etc.

Note: you don’t have to change the windlass, powered winch etc. when you change the house battery to 24V or even 48V.

Also, I believe you wrote you don’t have a generator. Do you intend to buy a portable generator? How do you plan to generate the daily energy that is needed?

[CaptainRivet]

Originally Posted by s/v Jedi

We really need to know what you have now and intend to keep using: what shore power, solar, which solar controllers, which alternator with which regulator, which AC charger, inverter or inverter/charger etc.

Note: you don’t have to change the windlass, powered winch etc. when you change the house battery to 24V or even 48V.

Also, I believe you wrote you don’t have a generator. Do you intend to buy a portable generator? How do you plan to generate the daily energy that is needed?

Agree we need to know in detail what you have but AC of battery most likely won‘t happen or cost you a fortune on a 43ft Mono.

If you power the windlass and powered whinch from the big expensive 12V starter yes you can go 24V or 48V house and why should he go 24 or 48V?
You add a lot complexity and transformers and have big loads running of starters. On green field, yes no doubt make use of the higher efficiency of 24 or 48V but not on a 12V vessel where he wanna add AC that must run on DC to be effective enough.
Till 3000W on inverter 12V is totally fine, just place it very close to bank. If it’s within 1,5m of the bank you just run 4x70 or even 95mm2 to inverter, yes maybe cost you 100Euro more then 24V cables but you have very little voltage drop like with 24V.
To have a chance on a 43ft mono you need to use the most efficient AC that’s around…Marbu and that runs on 12V and not 230V AC.

[witzgall]

Chris from Balmar here. We are owned by Dometic, so I have run some numbers on our new Voyager variable speed AC units. The great thing about variable speed units is that when you slow a compressor down, the watts used drop at a greater percentage than the drop. For example, the running watts of a TX10 10k BTU unit, is 723 watts at 100%, but only 276 at 50% compressor speed.

So if you are running one at 100% compressor speed for 6 hours at 50% on time, the total battery power used would be 2385.9
wh, assuming a 90% efficient inverter. for a 12v system, that would be ~186ah, exclusive of a water pump.

Run the same system 100% of the time at 50% speed, and you consume 1821wh, 142ah.

Both examples give the same total BTUs of cooling over time, but you save about 23% in energy running at 50% compressor speed 100% of the time.

Chris