Inverter size for AC

[jmblake]

Hi guys.
My question is I have a Zantrex Model RV3012M Inverter with a bank of 4 golf cart batteries and a 135 amp out put alternator to charge going down the road is this big enough to run one of my 15,000btu Dometic roof airs going down the road. If I can ever get my over the road air going again this would just be as a backup if it quits again. it wouldn't be that hard for me to hook up to my inverter fuse box. Thanks Jason

[bevans6]

The inverter itself looks to be a good fit for this, it's large enough to run the air no problem.  the 135 amp alternator is not large enough to supply the current, which will be more around 200 amps gross.  The battery bank will have to supply the rest, call it 80 odd amps after losses and conversion factors.  So the question becomes what is the state of charge of your battery bank before you start your trip, how hard will you run the AC unit, how big is the battery bank?  I would probably upgrade the allternator to 225 amps or a little more, and I would probably take a close look at the wiring to make sure it was large enough.  Also, I would investigate if the alternator was actually rated for continuous output at peak load, or if that was a selling feature only, and it would melt into a puddle after an hour at 135 amps.

No calculator handy, so numbers are mental math, which isn't my strong point!

Brian

[Nick Badame Refrig/ACC]

Hi Jason,

Just to add to Brian's good reply, you also have to concider traffic jams and idoling at intersections. Will your alt. also put out

225 amps at idol? You really need a very large batt bank to overcome this if not.

check your pm
Nick-

[RTS/Daytona]

The run watts on that A/C (average outside temp - more on a very hot day) is 1500 watts

lets say 100 amp  x 13volts = hmmm only 1300 watts (before the 5-10% inverter loss)

(135 amp rating is most likely - peak amps / short period / wide open & UNREGULATED )

perhaps a simple solution - another cheap scrap yard alternator - 2 altermators work fine charging batteries (no conflict)

drive it off your pto - or some other spare pully - ??

hope that helps

[Sean]

... I have a Zantrex Model RV3012M Inverter

This is a modified sine wave (MSW) inverter, a very poor choice to run air conditioners.  You really want a true sine wave model.

If you run an a/c on this type of inverter, it will use an additional 10-20% more energy, which will be wasted as heat.  Moreover, the a/c compressor will work much harder, and you will shorten its life considerably.

.. a 135 amp out put alternator to charge going down the road ...

A 15,000 BTU/hr air conditioner will draw approximately 15 amps steady-state in normal conditions (it will vary up and down based on the inside and outside temperatures).  At 12 volts, that translates to 150 amps, and then you need to add another 10-20% for inverter losses (close to 10% for sine wave, 20% or more for MSW), so you are looking at 165-190 amps at 12 volts to run your air conditioner.

Your 135-amp alternator will not be able to keep up, even if it is constantly running at full capacity.  More realistically, however, it can not even put out the maximum 135 amps full time, because it will not always be running at the optimal rpm.

Your battery bank would have to make up the difference, and, of course, that means you will have no battery charging while on the road.  If you start your trip with a fully charged bank, assuming approximately 400 amp-hours (at 12 volts) capacity, you will be able to run the air conditioner at a 50-amp deficit for four hours before you have discharged the batteries to their maximum recommended DoD of 50%.  That assumes that you have nothing else running, such as fridge, lights, etc., to also be depleting the batteries.

HTH,

-Sean
http://ourodyssey.blogspot.com

[jmblake]

Thanks for the replies it was just a thought that probably won't go any further than that right now, hopefully I can get the over the road air fixed that's the way I really want to go. Thanks again Jason.

[gus]

Jason,

Just run the rooftop directly from your genset, I do this all the time.

Simple and efficient. Since I use my genset very little otherwise, this keeps it exercised.

[jmblake]

Gus,
I don't have a genset yet, We very seldom get anywhere we don't have full hookups. But that is on my wish list just not on top yet. Thanks Jason