Aircon power from 240v Inverter

Submitted: Thursday, Jan 09, 2014 at 18:05
ThreadID: 105742 Views:9523 Replies:8 FollowUps:7
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I'm continually being told that (despite planning to have a 330Ah battery bank and 1500W inverter) caravan Aircon can only run from 240V power or a generator. Can anyone explain to me why a cooling cycle and the Aircon tech data saying "Max rated current 5.4A" the power inverter will "fry"?? I'm considering a 10A/15A RCD adapter to run the caravans 240V system from the battery bank. I am aware the fridge and H/W will need to run from gas when the A/C is on, but this thought bubble is for extreme circumstances only. Can it work?
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Reply By: Alloy c/t - Thursday, Jan 09, 2014 at 18:26

Thursday, Jan 09, 2014 at 18:26
The problem is the aircon compressor start up power requirement which is much much more than ' Max rated current 5.4A ' ,,,,,,, look in the archives and you will see that even the 'ducks guts ' [ supposed to be 2kva but really is only 1600w ] 2kva honda genny has trouble starting most C/van air conditioners ,,,, also 330 ah battery at 12v converted to 240v = no where big enough to run air con for more than an hour or so .
AnswerID: 524168

Follow Up By: Brian 01 - Thursday, Jan 09, 2014 at 19:13

Thursday, Jan 09, 2014 at 19:13
The problem is a factor called the locked rotor current.
This is the maximum starting current that the motor can draw under load conditions and is normally around 6 times the listed running current shown on the name plate.
So, at 5.4 amps running, (some of that will be the fan/s, which will normally start before the compressor on cooling, and after it on heating) you can expect around 33 amps starting current, and this is on the 240v side, a little less for an inverter type AC.
So your instantaneous power demand at startup on the output of the inverter is about 8 kVa (close enough to 6500 watts at 0.8 power factor).
On the input side, this equates to somewhere around 550 amps.
Now, whilst this is only for a short time on initial startup, and could possibly be handled by a 2000w continuous rating inverter with a large connected input capacity, or a decent generator, once the AC has been running and the compressor turned off on thermostat, the next start will be under load for longer due to compressor head pressures, and so is likely to trip the overloads in the inverter or generator.

Due to the high current draw, severe voltage depression on the 12v side is very likely with a small battery capacity such as yours, and this is likely to cause similar undervoltage on the output side, which will cause the motor to draw its maximum amps and yet still be underpowered for startup.
There is a very real chance of over-driving the inverter electronics with subsequent failure.

Even without the above constraints, your 5.4 amps on the 240v side equates to around 110 amps on the 12v side.
Allowing for Mr Peukert, this will give you around 75 minutes running time till your batteries are at 50% Soc.

You would also be running your 1500w inverter almost flat out, which, although rated continuous, they do not like to do.
Locked rotor current is normally listed on the motor name plate as either a figure in amps or kVa, or a letter corresponding to a fixed list of values.
Your inverter would likely handle the hot water or the 3 way fridge more readily as there are no transient events.

You then have to recharge those batteries, and if doing so with solar, the catch is that the hot day (when you are most likely to use the air conditioner) is less conducive to good solar input than a cooler day.
FollowupID: 805758

Follow Up By: Member - Steve Fourbyfour - Thursday, Jan 09, 2014 at 19:26

Thursday, Jan 09, 2014 at 19:26
Strewth Brian01, I wanted details but you type too quick ;-)

The locked rotor current is 20A, the three batteries run off the car and a 150W (kW??) solar panel.

The 110A above would be when it is cooling, not constantly after the van reaches temp?? Not sure what the "head pressure" does to the starting current.

Treat me kindly :-)
FollowupID: 805760

Follow Up By: Brian 01 - Thursday, Jan 09, 2014 at 20:25

Thursday, Jan 09, 2014 at 20:25
The 110 amps is only whilst the compressor is running, but these very small units that they put in vans, coupled with the relatively high heat energy intake of the van itself (thin walls, lack of insulation, lots of windows, lots of body heat for small area, cold air loss through door vent etc.) means that the compressor will have a duty cycle somewhere north of 80%.
So.. if you multiply 110 x 0,8 you get 88 Ah for an hour's running.
That gives you about 2 hours down to 50% Soc, and that's not allowing for any other loads, but it will be less time than that due to both the starting current draws and Peukerts effect.
Increased head pressure, once an air-conditioner has been running makes it harder for the compressor to overcome inertia, and so increases the high load starting time, this can be more prevalent when duty cycling is high due to lack of time for pressures to equalise.
Roof top air conditioners also have a greater propensity to high head pressure due to the generally higher temperatures of their condenser units
FollowupID: 805764

Follow Up By: patsproule - Friday, Jan 10, 2014 at 06:32

Friday, Jan 10, 2014 at 06:32
Guys, I did a lot of research on this when I last built a TV OB truck intended to run on portable generators when required. A good Inverter air conditioner has a very small in-rush / start up current or almost negligible locked rotor current. Some literally run a large stepper motor and ramp the compressor speed up from dead stop to whatever is needed. Others run a variable frequency drive to a more conventional motor to control speed. We ran a 24kw cooling capacity dual head Mitsu unit off a 6.5KVA Honda inverter generator and never saw north of 3.2KW current draw with no discern-able spikes whatsoever. It was clean enough that I was also able to run some of the equipment power off this gennie rather than fully load our second one. And as for inverter gennies not liking inverter power supplies connected to them... We never had an issue and this thing had about 120 power supplies connected from small to quite large.
FollowupID: 805784

Follow Up By: Brian 01 - Friday, Jan 10, 2014 at 09:56

Friday, Jan 10, 2014 at 09:56
I may be corrected here, but to my knowledge RV units in general, and Dometic and Aircommand ones in particular, are not inverter type. I have never seen any of their advertisements stating that point, which I would expect them to do if it were true.
Would be happy to know differently.
The heavy thumping when the compressor starts and stops has been one of the biggest complaints with these units, and still seems to be the case with the newer ones.
FollowupID: 805788

Reply By: Member - John and Val - Thursday, Jan 09, 2014 at 18:33

Thursday, Jan 09, 2014 at 18:33

The problem isn't restricted to the inverter.

Your aircon may be rated as "5.4A maximum" , but that is at 240 volts. To get the same maximum energy from a 12 volt source (ie 1/20th of the mains voltage), you will need to supply NOT 5.4 amps, but 20 times that ! ( ie well over 100 amps) from your batteries. Plus another probably 20% to handle inefficiencies in the voltage conversion. Your batteries will not like delivering 130-150 amps and will be totally exhausted (flat) inside a couple of hours. With this sort of treatment they will have a very short life.

There are other issues with the inverter itself, in that the a/c is almost certainly an "inverter" type, which, like the inverter you are planning to power it with, is a digitally controlled switch mode device. These two power devices can interact to their mutual disadvantage, possibly demise.

Sorry to throw cold water on your plan, but it really isn't practicable to run such a load from battery power. While I loathe (other people's!) generators, I think this may be a better way to go if you must run the a/c when away from mains power.


J and V
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AnswerID: 524169

Reply By: patsproule - Thursday, Jan 09, 2014 at 18:34

Thursday, Jan 09, 2014 at 18:34
Volts x Amps = Watts.

240 V x 5.4 A = 1,296 Watts.

Ok, that's under your 1500W inverter limit and it should run, the only issue might be the in-rush current when the compressor starts. If it is an inverter AC unit then it might be OK. But an older style will likely draw an overload for the short period it starts up and upset your inverter.

Now, battery life....

1500 Watts @ 12 volts = 125 amps current draw. But no inverter is 100% efficient. So lets call it 140 amps at 12 volts (and that's probably quite conservative). On a hot day at near 100% duty cycle on the AC your fully charged 330AH battery bank will last about 2.5 hours. Run your battery bank that hard repeatedly and you will be replacing the batteries regularly I'd suggest. ANd there is also the complication of recharging 330AH in a reasonable time frame.

Mate, I wouldn't do it. I'd be buying a gennie.

AnswerID: 524170

Follow Up By: Member - Steve Fourbyfour - Thursday, Jan 09, 2014 at 18:44

Thursday, Jan 09, 2014 at 18:44
Thanks Pat, and John - it's a new Ibis roof mounted so not sure what startup it draws - looks like a gennie might be the way to go for the "odd" occasion I might need it. Small van (17') so would only take 10min to cool - I was assuming a reasonable (say 26deg) setting in a small van would not draw too much.
FollowupID: 805755

Reply By: Member - John and Val - Thursday, Jan 09, 2014 at 19:05

Thursday, Jan 09, 2014 at 19:05

Your comment re frig and h/s just arrived in consciousness! "I am aware the fridge and H/W will need to run from gas when the A/C is on..."

Hmmm.... Hope you weren't planning on running a 3 way fridge or a hot water heater from your batteries, even without the a/c ????? Neither is quite as outrageously greedy as the a/c, but certainly good candidates for gas, never for battery power! (While the vehicle engine is running, it should generate enough power to keep a 3way fridge cold, but running such a fridge from batteries is a very big ask . And hot water - NO!))


J and V
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- Albert Einstein

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AnswerID: 524172

Reply By: The Bantam - Thursday, Jan 09, 2014 at 22:10

Thursday, Jan 09, 2014 at 22:10
Expecting to run aircon off any sort of battery pack you want to carry or pay for is just rediculous.

If you realy want some sort of off grid aircon for ya van look at this.

This is what lots of trucks use to chill sleeper cabs, so that they are tolerable to sleep in in the australian summer.

NOTE, that a sleeper cab is smaller than a caravan.

Now the ice pack unit linked..has a 6 horespower diesel motor...that should give you some idea of the the amountn of energy you need.

AnswerID: 524178

Reply By: Member - Toyocrusa (NSW) - Friday, Jan 10, 2014 at 06:36

Friday, Jan 10, 2014 at 06:36
There are people doing it on the Caravaners Forum. Only in short run times though, with different a battery source to the norm and I think, a larger inverter. "Dapope" is one poster on there that does. He has at least 350 Usable AH's at 13v though. Cheers Bob
AnswerID: 524183

Reply By: braggy - Friday, Jan 10, 2014 at 06:55

Friday, Jan 10, 2014 at 06:55
Have a look at the Kimberley Kruiser site they are running split system A/C off Lithium batteries.

cheers Ken
AnswerID: 524186

Follow Up By: Brian 01 - Friday, Jan 10, 2014 at 07:46

Friday, Jan 10, 2014 at 07:46
Yes they are, but LFP batteries do not suffer severe voltage depression under heavy load as do LA types. This is the biggest problem with starting a motor on an inverter.
FollowupID: 805786

Reply By: Member - KeithB - Saturday, Jan 11, 2014 at 20:50

Saturday, Jan 11, 2014 at 20:50
Also look at the 12 Volt aircons that that use in truck prime movers and sleepers. They run all night off a bank of lead acid batteries. They are very expensive, but look quite efficient for small areas. They key is to have an oversized condenser to keep the pressures, and therefore the current draw, down.
Try this mob
Very expensive, but looks like good gear for 12 volt operation. Yau can mix and match components.
AnswerID: 524305

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