My Real Life 12 Volt AC Experience

[twelve]

Before buying the DC air conditioner I did give strong consideration to going with an 110VAC unit/dedicated inverter set up. The thought was that I could get a smaller unit--thinking that 5000 BTUs or so would work as well as 6000 BTUs and there could be a net power savings. But I gave up the thought because there was no good location for the inverter and I feared the added electronics would make the system less reliable. In fact, I had one discussion with a technician for a highend powerboat manufacturer who designed AC systems for small cutty cabins. He swore that dedicated inverters just led to problems and that his company only used DC units. I never did a serious comparison of the relative battery drains.

OK, about two hour alternator charging: Some details need to be straightened out before getting to Mainesail’s question about how I’ve done that. My minimum system requirement was no more than 50% discharge for a 500 amp-hour battery bank, but overnight consumption has actually been better than what is needed (for those who had trouble with the math, it’s about 180 amp-hours, including the refrigerator and anchor light). The batteries were new when the AC was installed before the start of summer, and have always been kept topped up with a shore power charger, so they were probably at or close to their rating when the AC was first used. The alternator should then have started it’s work on a bank that was over 60% full.

As Mainesail notes, alternator regulation and measurements on the boat while cruising are anything but laboratory quality, and no one is even there to log values at set intervals. I do pay attention to current though, and am confident that the Blue Seas digital meter on the boat is pretty good. Bulk charging starts at about 120 amps, and stays in that vicinity for about a half hour, but the trend is down. For the next hour and a half the amperage continues to fall, but seems to do so at a faster rate. I do not think the decline is from the Balmar M614 temperature sensors (my understanding is that the battery probe adjusts voltage, and the alternator probe, if it does trigger a programed response, would put the alternator in "small boat" mode and amperage would drop like a stone). Unless I’ve got the science totally wrong, the current decline results from the bank’s falling acceptance rate due, in turn, from the increase in charge state.

So, after about two hours the current reaches 5 amps, and then further decline is very slow. It is at this point that I would turn off the engine and let the panels do the rest of the work (several hours at 5 or more amps is an easy output for 300+ watts of panels on a summer day in South Florida). But I don’t have the panels yet, so this past summer, to bring the state of charge up further for the next night (and to keep the batteries healthy) I kept the engine on until it looks like the current flow to the bank is near zero. As Mainesail points out, this takes a couple of hours more time, and that’s why I put quotes around the word "full."

All of this is pretty close to what Lifeline says about charging in their tech manual. Download it at:
http://lifelinebatteries.com/wp-cont.../12/manual.pdf
In section 5.4 Lifeline says that "the battery is fully charged when the current drops below .5% of the battery’s rated capacity." So, yes, I do cheat a little and call "full" at 1%–because passive charging (solar) is all that’s then required to both keep the batteries in good condition and get them ready for the next cycle. Sorry for using my own shorthand in the first post without a "full" explanation.

I hadn’t thought about Mainesail’s concern regarding the high discharge rate (40 amps) leading to otherwise unexpected "walkdown" of the battery bank’s capacity. His suggestion to monitor voltage is, of course, the accepted method for measuring the amount of discharge and keeping such at 50%. But measurement is accurate after the load is removed and the batteries have "rested." Yes there are battery monitors with algorithms, but I don’t know that I’ll ever have the patience to set one up and the confidence to rely upon it. But I do watch voltage before charging to see if there is any change since the last cycle. Just before the AC is shut down in the morning the system voltage is 11.8 or higher (but not 12). The voltmeter shoots up as soon as the AC is turn off, generally to about 12.1. The engine is immediately turned on (using the same battery bank with no difficulty), and a minute later the alternator starts doing it’s thing. Can it be agreed that this battery behavior shows less than 50% discharge?

It really comes down to my sense that even if one discharge was greater than 50%, I’d let the system keep cycling until the batteries die (after as well, trying Lifeline’s recommended conditioning and equalization had been done). If there is some "walkdown" in battery capacity I’m hopeful I’ll get at least a couple of summers before it’s a problem. And assuming 10 to 20 nights of AC use per summer, if the batteries go three years, I’ll be satisfied.

[Colin A]

Originally Posted by a64pilot

I think, but cannot verify that the module through an inverter converts to three phase AC and that is what runs the compressor

Point is though, that a DC airconditioner will not necessarily draw less power than an AC one. It may be that even with conversion losses an AC one is more efficient, certainly more available.
Best way to make air conditioning off of your battery bank viable is by finding one that is very efficient, not necessarily DC.

Just as Solar propulsion is possible, Solar air conditioning is too, both likely requiring large banks of batteries and panels and air conditioning would benefit from efficient insulation and shading. If I had to guess I'd guess that very effective insulation and blocking heat gain would be key to making it work, cause if you insulate well enough and block heat gain, maybe a 5K would cool the whole boat? Maybe 2K would the sleeping area? If you could get it down to 2K, than that looks a lot more easily done off of a battery bank.


Starting to sound a lot like making your fridge work.




On edit, I suppose it is possible that the Danforth is pure DC, but pulsed DC. I'm not smart enough on brushless speed controllers even though I have flown many a model airplane and dove with a Scotter that was Brushless DC with a speed controller.

Well I think it is like a giant fridge. My guess would be up here in New England with water temps in the 70's most of the summer insulating below the waterline might not help you at all or make it worse. But if your conditioning below the water temp I imagine it would make quite a bit of difference. I would think having a smaller 2,500 BTU unit for just a small sleeping cabin should be possible if you have the insulation.