Adding LA batteries and inverter to run AC

[Phoebe3]

OK, how dumb is this idea?

I have two 105 AH Lead Acid batteries currently. I also have a 13,500 BTU AC that requires 120 V.

Using online calculators, the AC works out to 2000 Watts and at 12V, 167 AH.

Can I put in a 3000 W inverter and add two more 105 AH LA batteries (for a total of 420 AH) to get an hour of battery-powered AC?

I'm thinking about the times I have to leave my free-to-good-home cat in the van while I'm in the grocery store or getting lunch, etc. While this has not yet been an issue, I'd like to know if it might work. Would the current just be too great for the inverter to handle it?

[BBQ]

Originally Posted by Phoebe3

OK, how dumb is this idea?

I have two 105 AH Lead Acid batteries currently. I also have a 13,500 BTU AC that requires 120 V.

Using online calculators, the AC works out to 2000 Watts and at 12V, 167 AH.

Can I put in a 3000 W inverter and add two more 105 AH LA batteries (for a total of 420 AH) to get an hour of battery-powered AC?

I'm thinking about the times I have to leave my free-to-good-home cat in the van while I'm in the grocery store or getting lunch, etc. While this has not yet been an issue, I'd like to know if it might work. Would the current just be too great for the inverter to handle it?

Well... with LA batteries, only 50% is usable.
(some people would draw lower, it is up to you).

ie you have approx 200AH of available power.

Add the inverter inefficiency (approx 15% penalty).

The batteries will power your AC for approx an hour.

Then you have to start the Onan/engine.

[avanti]

Originally Posted by Phoebe3

OK, how dumb is this idea?

I have two 105 AH Lead Acid batteries currently. I also have a 13,500 BTU AC that requires 120 V.

Using online calculators, the AC works out to 2000 Watts and at 12V, 167 AH.

Can I put in a 3000 W inverter and add two more 105 AH LA batteries (for a total of 420 AH) to get an hour of battery-powered AC?

I'm thinking about the times I have to leave my free-to-good-home cat in the van while I'm in the grocery store or getting lunch, etc. While this has not yet been an issue, I'd like to know if it might work. Would the current just be too great for the inverter to handle it?

That would work fine--I did something very similar. In fact, your calculations seem conservative. A typical 13.5K A/C unit draws more like 1500 watts, not 2000. Also, you are not likely to see a 100% duty cycle.

Here are some actual data from our rig:
We have 440Ah of AGM and a high-quality (Outback) 2400 watt inverter. By actual measurement, the A/C draws 102A @12VDC, measured by a shunt at the battery. We typically get about 2 hours of AC on a hot day before the battery gets to 50% SOC.

Just make sure you get a high quality inverter.

[booster]

Also be aware that you will need to replace those 200+ah of battery capacity. That is where the big alternators come into play. Other than that is a long drive needed or shore power to get recharged.

[markopolo]

Originally Posted by avanti

...................

Just make sure you get a high quality inverter.

+1

I've observed a similar amp draw to what Avanti reported. On a 440Ah AGM bank (in a Class A) the 13,500 BTU A/C draws just over 100Ah.

Look up the actual specs of your particular A/C. Some are way more efficient than others. The A/C in a Travel Trailer I had would draw closer to 150Ah for example.

I'd select a physically very heavy inverter for that type use. That sounds simplistic but the heavy transformer based units are more likely to have all the internal parts rated for heavy duty inductive loads. Some light weight units are rated 2000 or 3000 watts but that rating is for non-inductive loads with power factors of .95 or above.

[Phoebe3]

Thanks for your responses. I'm researching the AC requirements and potential inverters along with battery locations (i.e., where will they fit), etc.

Do you find the inverter gets really hot? Do you do anything about it? Mine would sit inside of a bench seat next to the converter, Truma and BMS.

[avanti]

Originally Posted by Phoebe3

Do you find the inverter gets really hot? Do you do anything about it? Mine would sit inside of a bench seat next to the converter, Truma and BMS.

If you follow Marko's advice and get a physically massive transformer-based inverter, I don't think you will have much of a heat issue. They are basically large heat sinks that shed heat pretty well. They DO have fans that can get a bit noisy under heavy load--some more so than others.

[markopolo]

The 64lb Samlex EVO all in one inverter / charger unit I installed in the Class A gets warm even when just passing maybe 350W through to the absorption fridge and maintaining 5 batteries while plugged in. The fan comes on fairly often so I'm going to improve the ventilation to the cabinet. I often leave the door to the cabinet open now and I'd like to keep it closed. It is in a cabinet approx 6X the size of the Samlex. The main vent is approx 7" x 11" but covered by a loosely woven fabric that seems to not let the heat escape. A second vent is a 3" diameter hole in the rear of the cabinet. I'm going to redo the main vent decorative covering using a much more open perforated plastic screen panel to see that will allow more air to pass and lessen the amount of times the unit's fan comes on.

This unit is always on so the interior of the cabinet never gets a chance to get cool. Even low wattage operational losses from the unit on a continuous basis can make a fairly enclosed space feel quite warm.

If you can have two vents, one lower and one higher then there's a good chance chance of a helpful convection current occurring pulling ambient air into the cabinet through the lower vent when exhausting warmer air into the coach through the upper vent.

[booster]

Our Magnum MS2000 is 42#, and is in an area of somewhat impaired ventilation in the rear storage area behind the wheelwell and totally panel around with vent holes. It runs the AC easily, but I haven't thought to check the temps of the unit (can be done from the remote to see temps of important internals) as we don't ever use it to run AC. Our inverter runs at high power are mostly microwave so quite short.

It does get fairly warm on charge, however, when running at a continuous 100 amps in warm conditions. We have never seen it get to the temps where they start to taper the output, but it does certainly warm up the van and especially the storage area.

Marko's suggestion about weight to handle inductive loads is a good one, as are the recommendations about paying attention to heat, as there certainly will be a lot heat generated, which the AC then needs to remove.

As was mentioned earlier, you will also need to know how you are going to replace all the used power, which can be more difficult to do than the adding of the batteries and inverter. It will likely be an issue of taking a long period of time to recharge, or investment in higher capacity charging components.

[JohnnyFry]

You should provide plenty of ventilation. If you are pulling 2,000 watts and the Inverter is 90% efficient (probably typical) you will be dissipating 200 watts of heat. Visualize 2 100 watt bulbs back there. It amounts to 682 btu/hour.

[booster]

As was just mentioned, the efficiency can give an indication of how much heat you will get, so wise advise.

It is also a good idea to get the efficiency curve of the inverter you are looking at, as they can vary a lot as the load increases. Low end inverters can get very bad at full load, both for efficiency and waveform. Looking at the efficiency curve once you know the approx watts you will be using will tell you a lot.