Running air con from alternator - math?

[Ronnie-SKO]

A typical rooftop a/c draws about 13amps at 120 volts. This means about 130 amps at 12 volts. Even with 4 average deep cycle batteries that means about 2 hours run time if you draw down the batteries 50%. That might even be optimistic.

The battery isolators that I have used charge all batteries while the alternator is running, when not running keep house batteries and vehicle batteries separate. Used a lot of these in plow trucks. Perhaps there are different ones available now, this is just what I have used.

 

[HazMatt-SKO]

^
That seems to be a simple, straightforward way to do it. Flip-flopping between banks doesn't seem to present any benefit, more likely, it'd be one more thing liable to break...
Not talking Gene Simmons, but its best to adhere to the KISS credo.

 

[cadillackid]

I have tried running my portable 2 hose inverter A/C off of my inverter and battery banks.. I have 2 interstate group 31 AGM batteries and a go power pure sine wave inverter... my A/C unit slows down its compressor and speeds it up as needed so no surge current and its efficient as a hose-type can be..



I was surprised at how short of time I get on my batteries.. I ran 2/0 welding wire from my main bank up to the house batteries and from the house bats to the inverter.. its about a 20 foot run from mains to house. and i get very little voltage drop.. ionly about a foot from inverter to house bats.. my portable A/C pulls about 1100-1200 watts on high power very close to what ronnie has observed..



at night my runtime is greatly increased as the A/C slows itself down with no sun load ands cooler outside temps..



I even tried charging my batteries up overnight with a charger to see if i wasnt getting a full charge by alternator. and noticed very little difference...



my inverter shuts down at 10.5 volts in its deepest setting.. (it can be adjusted)..which isnt a huge deep cycle on the batteries but at 10.5 volts for all intents, the batteries are dead.. if i were to have forced my isolator on to include the 2 starter batteries too, i would no longer be able to start the bus if I let them go down to 10.5..




without anything feeding into the system like solar or a huge battery bank you ewont get much runtime...



its also a hard time for your alternator to bring the batteries back up again when you get on the road.. esp if you continue to run the A/C. and ask it to do both.. my 200 amp bosch alternator struggled when i tried to ask it to do both plus run the dash air.. and that bus is all mechanical, no EFI drawing anything..

-Christopher

 

[Ronnie-SKO]

those kits look interesting.. and if you can portion off your cab it would definitely help.. my dash air in my DEV bus is a 25000 BTU evaporator.. I went ducted instead of a unit like this as I could locate vents that are directable to blow directly on me..



your biggest expensse and difficulty may be in obtaining the compressor brackets for your engine.. these kits dont come with them..


the other thing is these condensors.. I was tryinfg to see how it mounts.. you defimitely want to mount it far away from any engine heat... fans angled down and at a good angle under the bus..


I have learned the hard way that under-body condensors arent the best... my DEV bus has it and I suck a lot of hot road and blown-back front engine heat into that coil..



so im in the process of going to a skirt-mounted condensor where it sucks the air in from the side and blows it out the bottom.. live and learn.. I liked the clean lines on the sides of my bus and didnt want to cut the skirt... the bus I have wit ha skirt mounted condensor works MUCH better..



if your bus is an RE then you wont have as much of the same issue I have and this coil could be piut underneath right up front.. although in an RE you'll need much longer hoses..
-Christopher

I was wondering where to put the condensers. This might be an option for me if my ducting of the rooftop a/c does not do what I want. Thanks.

 

[PNW_Steve]

A typical rooftop a/c draws about 13amps at 120 volts. This means about 130 amps at 12 volts. Even with 4 average deep cycle batteries that means about 2 hours run time if you draw down the batteries 50%. That might even be optimistic.

The battery isolators that I have used charge all batteries while the alternator is running, when not running keep house batteries and vehicle batteries separate. Used a lot of these in plow trucks. Perhaps there are different ones available now, this is just what I have used.

I would expect a bit higher current draw from the battery bank given that your inverter will not be 100% efficient.

What would you consider an "average" deep cycle battery? Mine are 6v 420a/h. Would that be average?

 

[Ronnie-SKO]

420 a/h each? Or all of them?

I figure about 100- 120 a/h each. Thinking of a typical marine deep cycle.

I have small optimas at only 55 a/h each, 4 of them

 

[PNW_Steve]

420 a/h each? Or all of them?

I figure about 100- 120 a/h each. Thinking of a typical marine deep cycle.

Yes. All 4 are 420 a/h. It would be problematic to use batteries of differing a/h capacity.

The batteries sold as "marine deep cycle" at the big box stores are not true "deep cycle". They are a hybrid start/deep cycle. They are cheap but not an ideal choice for deep cycle use.

Common, true, deep cycle battery examples would be GC-2, T-105, L-16 etc. They have significantly thicker plates and will give superior service/lifespan.