A question about Air Conditioner runtime on batteries.

[WaltCh]

I have a 2017 21EC and had a question about the Air Conditioner runtime on batteries. I was wondering about how long the Air Conditioner (Dometic, 12 Volt, 3500 BTU) should run on battery power before the two batteries are too low to operate it. Mine will run about 30 - 45 minutes (without the engine running / alternator turning, and no shore power) before the batteries are too low to power it anymore. Seems like this isn't very long, but I don't know how everybody else is doing for runtime with the A/C on battery power (on the hook, no shore power, engine not running). I recently had to replace the engine/starter battery because it had a bad cell in it, and I was thinking that I may need to replace the A/C batteries too, but they still pass a load test. Any thoughts / opinions would be greatly appreciated, thanks all.

 

[Crewdog]

The Dometic web page for the Cuddy II 12v a/c unit shows that the unit draws 29.2 amps.

That's a pretty big power draw, and IMHO I wouldn't use that on battery unless the admiral was steaming mad.

I think if you did run on battery for very long, and for many cycles, it would increase the chances of having to replace the battery before the natural end of its service life.

Good luck.

 

[Capt’nKarl]

The 12 volt systems draw substantial power, amps, from the two batteries. These A/C units were not designed to be run without another power source such as shore power, generator or high amp alternator.

 

[lproulx]

I have been thinking about and playing with my R21 AC for a while now. (New boat to me last year.) I agree with both previous posts. I'm not sure what they were thinking when they put in those two batteries. Your run time is compatible with batteries in decent condition. One thing I did that improved the situation considerably was to install a 40 amp Pro Mariner charger in place of the factory 20 amp unit. It is exactly the same physical footprint of the 20 amp one, so it's an easy replacement. I got mine used on e-bay for a reasonable price. That helps, but still won't quite keep up with the AC running continuously. It runs for a couple of hours, then shuts down to cool off or something, and starts up again. When Dometic says the AC draws 30 amps I think they are not counting the seawater pump, which draws another 8 amps or so. What I think would really solve the problem would be a 50 or 60 amp charger. These are the same width as the smaller capacity units, but about two inches longer (higher, when installed vertically in the boat.) I wasn't sure I could make it fit.

My comments apply to my kind of installation, which is a 110-volt unit run by an inverter, which steals some efficiency. I have removed the system, (more or less end of real heat in New England) and am going to try to wire up an arrangement to run it either from the inverter when under way, or directly from 110 volt shore power when docked. In your case, regardless of whether you have a true 12-volt system, or my kind, I think the bigger battery charge would solve your problem.

 

[lproulx]

Sorry for two posts in a row, but another potentially useful move, if you get enough battery charging capacity installed is to buy a Honda 1000 watt generator and put it on the swim platform if you want to stay on the hook. Just connect it to the existing charging receptacle. They are amazingly quiet and should run all your systems just fine.

 

[WaltCh]

The Dometic web page for the Cuddy II 12v a/c unit shows that the unit draws 29.2 amps.

That's a pretty big power draw, and IMHO I wouldn't use that on battery unless the admiral was steaming mad.

I think if you did run on battery for very long, and for many cycles, it would increase the chances of having to replace the battery before the natural end of its service life.

Good luck.

Thanks for the input, it sheds light on the problem!

 

[WaltCh]

Sorry for two posts in a row, but another potentially useful move, if you get enough battery charging capacity installed is to buy a Honda 1000 watt generator and put it on the swim platform if you want to stay on the hook. Just connect it to the existing charging receptacle. They are amazingly quiet and should run all your systems just fine.

That is a really good idea, as I did plan to spend some time on the hook and that would not only solve the A/C problem but it would also be a very good generator to have around the house when it's not on the boat. Thanks

 

[WaltCh]

I have been thinking about and playing with my R21 AC for a while now. (New boat to me last year.) I agree with both previous posts. I'm not sure what they were thinking when they put in those two batteries. Your run time is compatible with batteries in decent condition. One thing I did that improved the situation considerably was to install a 40 amp Pro Mariner charger in place of the factory 20 amp unit. It is exactly the same physical footprint of the 20 amp one, so it's an easy replacement. I got mine used on e-bay for a reasonable price. That helps, but still won't quite keep up with the AC running continuously. It runs for a couple of hours, then shuts down to cool off or something, and starts up again. When Dometic says the AC draws 30 amps I think they are not counting the seawater pump, which draws another 8 amps or so. What I think would really solve the problem would be a 50 or 60 amp charger. These are the same width as the smaller capacity units, but about two inches longer (higher, when installed vertically in the boat.) I wasn't sure I could make it fit.

My comments apply to my kind of installation, which is a 110-volt unit run by an inverter, which steals some efficiency. I have removed the system, (more or less end of real heat in New England) and am going to try to wire up an arrangement to run it either from the inverter when under way, or directly from 110 volt shore power when docked. In your case, regardless of whether you have a true 12-volt system, or my kind, I think the bigger battery charge would solve your problem.

Good solution to the problem - install a bigger charger! I am going to follow that suggestion, I like to sleep on the boat, but I like to have dependable A/C all night, so thanks for the info and the really good idea.

 

[WaltCh]

The 12 volt systems draw substantial power, amps, from the two batteries. These A/C units were not designed to be run without another power source such as shore power, generator or high amp alternator.

Yeah, it looks like a much larger battery charger is going to have to be added. Thanks