question

me1 avatar image
me1 asked

run 13,500 air conditioner from batteries?

Is it reasonable to think I can run a 13,500 btu air conditioner from a 12/3000/120 and 6 battleborn lithium in parallel @ 12v? (and of course nothing else at the same time)

After a couple of years acquiring stuff, I have just finished setting up my whole infrastructure in my RV (yah me!). As mentioned I have (6) 12v lithium in parallel, 5' of 4/0 between the battery output and the inverter. I tested the air conditioner and the fan started, but as soon as the compressor started it shut down. I looked at the alarms and saw I had previously set with the VE.Config, the DC low shutoff at 11.5 per a blog on the battleborn website. I lowered that to 11.0 and was able to get the compressor to come on for about 30 seconds before it hit the 11v threshold.

My batteries are fully charged and show 14.6v on the cerbo gx screen (pictured below) before I start the air conditioner. When the condenser turns on the AC loads show 1100'ish to 1300'ish watts, the battery bank shows -1300'ishW and dips to 13.1v and up to -123 amps. Within a short time something else happens that then draws more and it shows -126 amps and the inverter shuts down.

I read on the battleborn specs that their batteries...

"allows for 100 Amps continuous, 200 Amp surge for 30 seconds, and 1/2 second surge for loads over 200 Amp"


Before the air conditioner is turned on:

1661209901769.png



This is about the highest surge I've seen during compressor startup:

1661210001570.png


This is when it shuts down:

1661210154437.png


According to the numbers from the bottom left blue battery icon (the shunt) I have not surpassed these values, but could there be a huge drop on the 5 feet of 4/0 to the inverter?

If so what is the solution? I'm not sure I can fit another 400amp fuse and 500amp shutoff switch to duplex my cables in my battery bank box. I thought 4/0 would handle this very well over 5 feet.

--

solar->breaker->charge controller->breaker->batteries
the battery out, pos and neg are on opposite ends of the bank to draw from the whole bank
neg->500amp shunt->lynx->inverter
pos->400amp fuse->500amp cutoff switch->lynx->inverter

Thank you


batteryaircondition
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9 Answers
snoobler avatar image
snoobler answered ·

What size interconnects between batteries?

You you TRIPLE CHECKED that all connections are clean, properly torqued (cable terminals, fuse terminals, breaker terminals, inverter terminals, etc.), and any crimps are of high quality?

Have you enabled DVCC and turned on SVS and SCS to share your shunt voltage/current with all GX attached components? STS too if you have a temperature sensor.

Run a lighter constant load - say 50-70A. With a separate voltmeter, measure voltage at the battery terminals and at the inverter terminals. 5' of 4/0 should only drop 0.03V, but there are other resistances in the system, so a little more is expected. If you see a notable drop, start measuring voltage across all components in the circuit, and you should find the culprit.

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me1 avatar image
me1 answered ·

Those are some great suggestions. I have checked all of the connections to make sure they are torqued and the crimps are high quality.

The cable size between batteries is 2 awg. I might wonder about that, but the shunt is reading their output and it doesn't drop as much as the inverter thinks, so I'm optimistic about them.

I have not enabled dvcc nor any of the other settings. I will do those and run a lighter load to test them.

Thank you

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me1 avatar image me1 commented ·

Major faux pas found and fixed! I had the last battery inner-connect cable butted up against the cable going to the outbound fuse (both on one side of the battery post) and there was a gap between the two lugs. The gap was caused by the heat shrink tubing on that cable to the fuse and wasn't getting a good connection. I separated the two lugs (now one on each side of the battery post) and trimmed off the heat shrink that was causing a bad connection. I found this by using your suggestion of testing the voltages and I tested each battery and the string of batteries until then I tested the string to the fuse and bingo! All of the battery tests were 13.45v and the battery to the fuse was 13.39v (-.06v). At first I thought it was the fuse holder, but then I saw the gap. After the gap was fixed I again tested the battery inner connected voltage was still at 13.45v and the voltage at the inverter was 13.44v without any real load -- that is much better. I could still test with a 50-70amp load but...

I previously had returned the inverter configuration to it's original settings and so with the gap fixed, I tested the air conditioner again. It ran for 10 min before I manually stopped it. I had to turn on some dc lights and open the slide out, and the cerbo gx display showed 13.3v before I started the AC and during the test went down to a low of 13.0v and stayed there during the 10 minute ac run and it drew the batteries down to 62% but it worked as I thought it would.

I will probably need to invest in larger battery connect cables to see if that also might help. But I don't know if there is anything else that I can do to make a difference or if this is the best I can hope for.


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snoobler avatar image snoobler me1 commented ·

Ah.. posted later down the thread before I saw this. Glad to hear it. Impressed it worked as well as it did. All too often operational issues can be traced to a poor or improper connection.

Since you can get the A/C to run for 10+ minutes, no need to use 50-70A - do the voltage drop test with the A/C. Record:

  1. Shunt amps
  2. Shunt voltage
  3. Inverter voltage (enable inverter battery monitor, monitor with VRM)
  4. battery terminal voltage w/voltmeter
  5. inverter terminal voltage w/voltmeter

2 should equal 4

3 should equal 5


Do you have the shunt Peukert value at 1.05 for LFP batteries?


62% after 10 minutes? What was it when you started?


With no other loads, you should be able to run the A/C for about 3.5-4 hours.


Wouldn't touch the interconnects until you measure the voltage drop.


Did you check all the Lynx fuse terminals for clean and tight?

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me1 avatar image me1 snoobler commented ·
I'm a bit confused by the details but here is what I know. First I did not have the peukert value set, it was defaulted to 1.25 and I changed it to 1.05. I also updated the shunt ah setting from default of 200 to 600. Then I used the solar charge controller to charge the system and the voltage started at 14.6 when I turned off the charge controller.

Then I started the AC

1) shunt amps per the victron connect app said: 105, but continued to climb as the wattage continued increasing and went to 150 when the compressor turned on

2) shunt voltage via the app said: 13.3v and to 13.0v when the compressor turned on

3) inverter voltage. I did not turn this on yet via the veconfig

4) battery terminals with voltmeter: from opposite ends of the battery string they were: 13.04v but checking the cables just before they left the battery box (output of the shutoff on the positive and output of the shunt on negative it was 12.76). Quite a drop

5) inverter terminal voltage with a voltmeter: 12.55

The question about 62% after 10 min... I don't remember what it was when I started, but that might have been wrong due to the AH only set for 200 amp.

The Lynx terminals are all tight. They also had a different voltage than the inverter terminals and were 12.59v


Under heavy load:
battery = 13.04v
cables after shutoff and shunt = 12.76v
4 feet of cable to lynx = 12.59v
1 ft cable to inverter = 12.55


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snoobler avatar image snoobler me1 commented ·

Okay. We can work with this. We have to assume that all measurement devices are correlated.


Your shunt settings would cause the SoC to drop substantially faster than normal. 200Ah vs. 600Ah means 3X as fast. With Peukert 1.25, 150A is 0.75C meaning the 200Ah battery is likely only going to put out ~70% of the 200Ah programmed, so even faster drain in SoC.


With the corrected shunt parameters, your SoC % change should be far more accurate.


It is important that you enable the battery monitor on the inverter and display it on VRM. Need to confirm the inverter is seeing a similar voltage as the meter when loaded and the shunt when unloaded.


From main GX screen:

Settings > System Setup > Battery measurements


Set the inverter to visible:


1661460977671.png


Will appear on VRM at the bottom:


1661460987783.png


Assuming 150A


0.36V drop in battery resting @ 100% SoC (13.4V) to battery loaded. That's about 3mΩ - can't complain. The 2awg is a significant bit of this.

0.26V drop "cables after shutoff and shunt" - this may be excessive. Are you using a high-quality shut off? What's the cable gauge? Worth measuring the voltage across the cut-off.

0.17V drop across 4 feet of cable is about 3X what the wire should be alone, but the connections add to that as well.

0.04v drop across 1 foot of 4/0 cable, same as above.


Overall, nothing is setting off alarm bells, but there could be some tweaking as I'm eyeing the shut off first and interconnects second.


I think 90% of the problem was the connection issue you discovered and resolved.


Right now, you have a 0.81V drop @ 150A. That's 150A * .81V = 122W of wasted power. You should be alble to notice that as heat. let it run for 20 minutes and see if any part of the circuit feels warm.


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me1 avatar image me1 snoobler commented ·

I enabled the vrm inverter voltage setting and I let the solar charge the system back up and then as usual I have disconnected the solar so it wasn't throwing off the other measurements. Then I took a screen shot of vrm with the batteries at 100%.

1661470812108.png


I turned on the air conditioner and a few minutes later took a screen shot of it again.

1661470830611.png


After 5-8 min I noticed that the inverter wasn't exactly in sync but pretty close and the volts came back in agreement in another few seconds. But looking at the amps they are different. Multiplus Details box is showing 141.50 and the Discharging box is showing 148.40.


1661471070166.png


Finally after 15 min it shows:

1661471302859.png


Compared to the first one I did for 10 min, the voltage is further down (now 12.98v and before stayed at 13.0v). But the SOC % is higher as per setting the 600AH.


John

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me1 avatar image me1 me1 commented ·

After about 45 min.

1661473123602.png


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snoobler avatar image snoobler me1 commented ·
Is DVCC enabled? Do you have a VE.Smart network? Did you enable the battery monitor in VEConfig?


It's very suspicious that all your voltages are essentially the same. This suggests the shunt values are being passed to all components.


Don't worry about the amps disparity. There is lag and a little bit of noise. There's also the DC-system. Your shunt current should pretty much equal your inverter current plus DC loads current, and they look pretty close.



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me1 avatar image me1 snoobler commented ·

I will obviously need to upgrade my battery interconnect cables to something bigger or a bus bar as suggested by another. But after the last 45 min test I did feel all of the cables and I think I might have another one to replace. I was happy that the cable to the lynx and from lynx to inverter were not very hot. A couple of the interconnect cables and the positive from the battery to the fuse were very hot. So those will get some attention soon.

DVCC - is on with SVS, STS and SCS enabled.

Smart network - no, don't you need multiple of the same to use smart network (ie 2 of the same charge controllers).

Battery monitor in VEConfig - no. I have just checked it "Enabled" and set:
State of Charge when Bulk finished 95.0%
Battery capacity: 600 Ah
Charge efficiency 0.99
tell me if these should be something else

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snoobler avatar image snoobler me1 commented ·
I thought I stressed the importance of disabling DVCC to see what the inverter measured at its terminals. This is still probably worth doing to confirm it's similar to your ~12.55V reading. With DVCC/SVS enabled, everything is going to report shunt voltage. You want to see what the inverter measure to make sure something's not out of whack (it should be close to the voltmeter measurement)


The inverter battery monitor is fine. I wanted it enabled to make sure you could get the data out. It may not be needed, and once you've sorted the problem and chased down all the gremlins, it should be disabled.

Anything that's hot is either under spec or poorly connected.


The hot + from the battery to the fuse is very concerning. That heat may cause the fuse to prematurely blow. It may also be generated by the fuse. If you're using any cheap no-name fuses/breakers from Amazon, toss 'em and splurge on Blue Sea, littlefuse or equivalent.

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me1 avatar image me1 snoobler commented ·
I thought I had disabled it, I was pretty sure. But when I went in to check, it was enabled. So I'll have to disable it, do the checks again.


I'll have to take a few days off before I can double back to this, but thank you for so much help.


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snoobler avatar image snoobler me1 commented ·
Sounds good. I think you are on the right track and have the tools needed to get your stuff sorted.
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snoobler avatar image snoobler me1 commented ·

Also, concerning VE.Smart, you can have a smartshunt/BMV-712 in a VE.Smart network to share voltage, temperature and current.

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snoobler avatar image
snoobler answered ·

The drop to 13.0V feels excessive to me. Recommend you conduct the testing before you enable DVCC.

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me1 avatar image me1 commented ·
I'm not sure what I can use to pull 50-70 amp draw so tonight after work I'll be checking how much the microwave draws though the inverter from the batteries. I might have to add something like a hair dryer to increase the dc amps. But I already enabled DVCC yesterday so I'll disable it before doing these next tests.
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snoobler avatar image snoobler me1 commented ·

1000W microwave will burn 1600W from inverter. 700W will burn 1100W from inverter. A hair dryer on medium might get you near the 50-70A level.


If you can run the microwave without issues, something else is up.


50-70A isn't mandatory. Just looking for something that won't shut down on you and allow plenty of time to take measurements.


Also, record shunt voltage and inverter battery monitor voltage as well when you take your manual voltage readings.



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jimduchek avatar image
jimduchek answered ·

I run a 13500 A/C in my RV no-problem on a 24/3000/2x120. A few comments on things you've said:

"allows for 100 Amps continuous, 200 Amp surge for 30 seconds, and 1/2 second surge for loads over 200 Amp" -- if this is the spec for a single battery (which sounds about right), and you have 6x in parallel, then you can do 600A continuous, 1200A surge for 30 sec, etc. Your batteries themselves can handle the load just fine.

"The cable size between batteries is 2 awg" "5' of 4/0 between the battery output and the inverter." 2 AWG seems _way_ too small for battery interconnect (unless you mean 2/0). Yes, they're shorter, but still running a _lot_ of juice. Honestly I'd go at least 1/0 here. Look into very fine-stranded cable here too as it's incredibly flexible vs standard "welding" cable (though it has more voltage drop).

Questions:

How are your batteries connected? For an example of a 1s2p (1 series, 2 parallel, i.e. 2 batteries total) setup -- please excuse my _awful_ scratched-together paint skills -- obviously you've got 6, but are you connected like the left, or the right (with green going to the load/inverter)?

untitled.png

Are you using the external voltage sense on the inverter?




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me1 avatar image
me1 answered ·

Hi jimduchek, thanks for the response. Yes it is 2awg not 2/0, so if 2awg for battery interrconnect is way to small then that might be the majority of my problem. I originally based those sizes because the max wire size into/from the charge controller is 2awg. Perhaps I need to increase those quite a bit, but because the 4/0 is like your green wires in your right hand drawing I am pulling from all of them in parallel.

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snoobler avatar image snoobler commented ·
Please conduct the voltage drop testing first. You may also want to enable the inverter battery monitor and see what the inverter is measuring on VRM (make sure DVCC is disabled). It's not common, but it could be measuring a notably lower voltage.
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Kevin Windrem avatar image
Kevin Windrem answered ·

With 6 batteries in parallel you do NOT want to daisy chain from one battery to the next. You need to use a busbar or common stud with equal length cables from each battery to the busbar/stud:


1661302034226.png

This way you divide total current equally between batteries and the interconnections can in fact be smaller. To match the 4/0 wires you are using from battery to Multi, each battery to busbar/stud needs 1/6 the area. So #2 from each battery should be sufficient.

If you wire like the top drawing, currents in each link between batteries add with each battery so the end link much more current and higher voltage drop than the first one. You may also exceed the ampacity of the jumpers. Not so in either of the lower drawings.

The current will also not be balanced between batteries so one battery may be tasked with supplying more than it's BMS permits and shut down. This would cause a cascade in which all batteries will eventually shut down.

You should also look at voltage drops across other components in the system. At these currents EVERYTHING is a resistor and there will be voltage drop across it. Check the voltage drop across fuses, switches, each cable, etc.

For high current tests in my RV I use the water heater's electrical element. It's about 1500 watts and a pure resistor of constant load. About as perfect a load as you can get. Note that some hair driers use diodes for their low power settings and inverters and other power sources don't really like that asymmetric load. You'll hear nasty noises coming from your inverter's transformer. Should be OK but best to use something else for system testing.


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me1 avatar image me1 commented ·
I get your point, but putting in 2 bus bars might not fit, unless I can make it attach directly to the battery terminals without the wires. What kind of dimensions should the bus bars have?
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Kevin Windrem avatar image Kevin Windrem me1 commented ·
I would not attach busbars directly to the battery terminals because of the stress created on the terminal especially while the RV is moving down the road.

The size of the bar should be enough to avoid significant voltage drop and will depend on the length. I drew a long bar with short interconnect cables in my middle diagram. Because of it's length, it will need to be about the same cross-sectional area as your 4/0 wire. You can use a shorter bus bar -- just long enough for the terminals you need, then run equal length wires from there to the battery terminals. In some applications, a single stud or even the bolt on your shunt can act as a "busbar" but with 6 wires converging on the central point (and at least one leaving) that probably isn't practical. 3 wires on one stud is about max.

Busbars will typically have a maximum current rating. That will be based on the heat generated. Remember, everything is a resistor.

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Trevor Bird avatar image Trevor Bird commented ·
I don’t understand how the top method could have different current drawn from different batteries. The load impedance of each battery is the same.
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Kevin Windrem avatar image Kevin Windrem Trevor Bird commented ·
Every connection is a resistor! Redraw the diagram with resistors in each connection and you'll see how some batteries will supply more current than others.

Yes, battery impedance factors in as well.

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snoobler avatar image
snoobler answered ·

I would normally agree with the busbar recommendation, but given that you're current draw is barely over the 1C rating for a single battery, and if you have a bank in which all batteries are operating within limits, you just accept that some of the will last a little longer than others.

Additionally, beefy interconnects simulate bus bars with very short connections to the battery terminals.

Lastly, even with bus bar connections, I would follow Victron's recommendation to attach the main leads to the opposite ends of the bus bars per Victron Wiring Unlimited.

In other words, upgrade to 1/0 or 2/0 interconnects, and you effectively have a busbar configuration.

But before you do any of that, I would conduct the testing necessary to confirm the presence of voltage drop.


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Kevin Windrem avatar image
Kevin Windrem answered ·

You can see there is quite a bit of imbalance in the batteries (13.04 vs 12.76). That's 0.28 volts of your drop right there. At minimum, you are taking a significant amount of current from the battery at the exit end. I'd focus there. In a heavily load situation this could trip the first battery's BMS which would then cascade to the rest of the bank. Additionally, the first battery in the string will discharge faster then the far end. The good thing in your system is your 6 batteries aren't heavily loaded. The 3000 VA Multi max current draw would be around 250 amps out of an available 600. Peaks should also not be a concern since the batteries have 30 seconds of double current. But look closely at system voltage drops. You are measuring at substantially less than the 250 and WAY less than the 500 peak currents. Peak currents are usually what causes a low voltage shutdown.

The first change I would recommend is to increase the jumper size. 4/0 is recommended but even 2/0 will help substantially. #2 is 0.5 ohms per 1000 foot, 2/0 is .15 ohms per 1000 foot (3 times better). Just replacing the jumpers wit 2/0 should take the 0.28 volt drop (first batter to last battery) down to 0.08 volts.

Next, take positive from one end of the bank and negative from the opposite end. This will come close to cutting the overall voltage drop in half.


A further enhancement would be to take power from BOTH ends of the bank. That is two positive wires and two negative wires. These can be half the size of your 4/0 monster cables -- down to 2/0 would be similar in overall voltage drop. Just keep both positives and both negatives equal length.

An infrared temperature meter helps locate hot spots. Run the system for 10-20 minutes on high load to heat everything up then scan the wiring, connections, fuses, switches, ... This should provide focus for additional improvements. The cable crimps can be a source of voltage drop if not done correctly. Check these carefully with the temperature meter or by hand.


As an illustration of various wiring topologies, I ran a couple of Circuit Lab simulations:

The first shows positive and negative taken from the same end of the battery bank. Notice the currents in the series resistors decrease drastically as you move away from the battery:

screen-shot-2022-08-25-at-44857-pm.png

The second is if you take positive and negative from opposite ends of the bank:

screen-shot-2022-08-25-at-44857-pm.pngWhile not exactly balanced in this simulation, you can see that there is far less variation in individual battery currents. Obviously, reducing any resistance will improve balance.




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me1 avatar image me1 commented ·
Hi and thanks for the information. As you might have read, I'm in an rv with a battery box and there isn't much room to make everything the same length. But I agree that I'll need to use bigger battery connects.

Thanks


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snoobler avatar image snoobler commented ·

If you would re-read the original post, the OP clearly indicate that he is connected at opposite ends of the bank.


solar->breaker->charge controller->breaker->batteries
the battery out, pos and neg are on opposite ends of the bank to draw from the whole bank
neg->500amp shunt->lynx->inverter
pos->400amp fuse->500amp cutoff switch->lynx->inverter

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Trevor Bird avatar image Trevor Bird commented ·
@Kevin Windrem I can't see any difference in these simulations. They both seem the same to me.


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scott-guenther avatar image
scott-guenther answered ·

I haven't seen any mention of a soft-start on the AC. My big AC (15000BTU) only pulls 1700 watts. easily handlable by a 3000VA inverter. It's only ~150amps from my 12v system. however, compressor starting is a tough business. my inverter and shunt are not fast enough to see the surge load startup. with my clamp meter, I can see a peak of 571 amps drawn on compressor startup. that's 6800 watts and should trip the overload protection on a 3000VA. adding a soft-start to the AC will stop the capacitor from rapidly charging after a compressor start.


to make things worse compressors get harder to start when they are hot and after 10 minutes of use this is probably what you are seeing. it clicks on, drains the cap, and that cap asked for all it can and trips the over current protection.


with a soft-start on both my AC's I can run both the 15000 and 13000btu AC on a MultiPlus-II 2x 120V at around 2300 watts on average. can't do it long as I only have 1000AH usable storage but with my 320amp alternator, I can run them while cruising down the road which was actually my goal as before I had to run my generator while driving.


soft starts aren't cheap ($250 per ac) but they work and they make your ACs quieter, no more clunk when they turn on and off.

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me1 avatar image me1 commented ·
Sorry I didn't mention it but I do have a soft-start. I did that last year so that my inverter generator could kick it over.
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Kevin Windrem avatar image Kevin Windrem commented ·
I'm a big fan of soft starts also. I have on on my RV air conditioner and one on my home heat pump. The latter makes it possible to run the first stage (compressor) of the heat pump on my 7000 watt inverter generator I use for backup power when the grid goes down.


Starting any motor creates a current surge that's way too short to see on most meters. You need an oscilloscope to capture the real peak current. It's on the order of a few cycles in duration and could be 30-60 amps for an RV air conditioner and 80 amps or more for a home unit. The soft start spreads the inrush ENERGY out over a longer period of time. So the peak current is much reduced. The down side in spreading things out is the amount of peak torque is also reduced, so there is a limit. You need enough torque to build up momentum in the motor. The MicroAir soft start units actually monitor the compressor start and provides just enough peak current to start reliably. Soft start units also protect against power brownouts or brief power interruptions that can stall the compressor.

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