Running an AC

[canuckrv]

I have a 1997 roadtrek with the original AC with 2 100 amp hr LiFePo4 batteries and a 3000-watt pure sine wave inverter. To see what would happen I tried the AC on battery power. the batteries were about 45% charged. The AC started and ran fine, but as I watched the battery monitor, the amps kept going up from about 35 to around 48 when I switched it off after about 3 mins. I tried to switch it back on and the compressor only hummed. I switched to shore power and it was fine.
Now I have some questions why were the amps increasing? How much Battery would you need to run an AC? if I was to replace the AC (they're window units in a Roadtrek) what would I look for?

 

[John From Detroit]

Amps were imcreasing for one of two reasons.
1: If the condenser coils.
The way an A/C (or heat pump for that matter works) is to circulate a Refrigerant First it compresses and due to some laws of physics when you do that it gets HOT (Boyle's law if you want to look it up) The job of the CONDENSER is to transfer the heat to the outside air.. Then the now liquid gas is sent to the Evaporator where it's sprayed in and gets cold (Same law) and absorbs heat.
IF THE CONDENSER is not clean.. For example it's cottonwood season here and that clogs things up faster than I can type.... and I type fast... The condenser can not pass enough air to properly cool.. So the heat starts to build.. and as the heat builds (Boyle's law again) the system pressure increases.... And the compressor has to work harder... The type of motor used in the compressor thus draws more and more current .
Now if you are reading BATTERY amps.. The inverter draws not volts and amps but WATTS.
To make math easy we will make a few invalid assumptions.
First: the A/C draws about 1000 watts (Likely needs more)
Second: 90% conversion effiency (This may be accurate or not)
Thiird: Battery voltage 10 volts (You may plug in the proper voltage)
So the inverter needs to deliver 1,000 watts. This means it will draw about 1111 watts
Or 11.11 amps from a 10 volt battery
Though LIFePO4 do not loose voltage as fast as Lead Acid. they do lose voltage s they discharge (Just a lot slower) so now at 9 Volts. Goes up to 123 amps.
As I said. with the exception of the conversion effiency (90% is "in range") all "Givens" are for example only.. the math however is precise.

 

[JayArr]

The short version is that you will get about an hour of AC from those two batteries. 45A actually seems pretty low to me, either your AC units are small, efficient or your battery monitor is out of calibration.

Ball park figures: you need one, 100AH battery for every hour you want to run the AC.

If you replace your AC units with something that uses less power you will get more. You just need to get good at doing the math so you can figure out what will work. John explained it one way, I'll do it again, you need to be able to do this in your head so practice.

Here are the numbers for a typical rooftop AC unit. Find the current spec for you units and you can adjust for them.

A 100AH battery will deliver 100A for one hour.

a 1500W Air Conditioner will draw 1500W / 120VAC = 12.5A (AC)

When converting 12VDC to 120VAC the inverter will use 125A DC to make 12.5A AC. It's a basic factor of 10.

Additionally the inverter is only 90% efficient so you lose 10% of your battery power in heat given off during the conversion.

125A DC / .9 = 138.8A DC

So you'll be drawing 138A out of your batteries 100/138.8 = 0.72 hours. (each)

A 3000W pure sine wave inverter is a good choice to run AC, it's about twice the power needed which may keep the inverter cool running.

 

[Lou Schneider]

if I was to replace the AC (they're window units in a Roadtrek) what would I look for?

You have to find the BTU rating of the window unit air conditioner in your Roadtrek, then shop around for one with the same or higher capacity. Many current air conditioners are Energy Star certified, these will have the highest efficiency. Comparing the SEER (Seasonal Energy Efficiency Rating) can also help you find the most efficient unit. Most likely you'll find the most efficent ones use significantly less power than your present unit. I have a Frigidare 5000 BTU Energy Star air conditioner that keeps my shed cool while using about 400 watts. It starts and runs on a Sportsman 800 watt inverter generator.

A Kill-A-Watt meter will measure your A/C's power consumption, just plug the A/C into it.

An interesting possibility is to replace your window A/C with one designed to be installed in a double hung window like this Midea. Available in 8,000, 10,000 and 12,000 BTU versions. These allow the window to be lowered into a U channel in the middle, providing sound isolation by keeping the noisy compressor outside and only having the air fan inside. Similar to a mini split but in a single package. Build an insulated wall panel to fill the space where the window pane would go.

Despite the claim of the design being "unprecedented" I had a similar unit in my mobile home back in the 1980s. I don't remember the make, I bought it used for $25 and then adapted it to fit in a regular sliding window.

Midea 8,000 BTU U-Shaped Inverter Window Air Conditioner WiFi, 9X Quieter, Over 35% Energy Savings ENERGY STAR MOST EFFICIENT MAW08V1QWT

 

[Gary RV_Wizard]

The AC started and ran fine, but as I watched the battery monitor, the amps kept going up from about 35 to around 48 when I switched it off after about 3 mins. I tried to switch it back on and the compressor only hummed. I switched to shore power and it was fine.

Were those AC amps or DC amps from the battery? 35 to 48 is too high for AC and too low for DC unless that is a very small a/c unit (3000 btu?). The simplest explanation for rising amps is that the output voltage was dropping below 120v, so the a/c amp demand increased to keep up the power (watts) draw. LiFePO4 batteries usually keep the voltage up quite well, but you started out at only 45% charge.

 

[canuckrv]

I would assume DC amps as I was reading the battery monitor not sure on BTU's as I can only see the front.