Harbor Freight Inverter Problem

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.

Af71

New member
Joined
Feb 28, 2021
Posts
1
Location
Georgia
I installed a 4000 watt inverter to power my wifes oxygen machine so I would not have to use the generator all the time. I also bought and replaced the emergency battery with a marine battery and wired the inverter to it. I also upgraded to a 200 amp alternator. My problem when I connect the O2 machine I get a battery low alarm. Suggestions
 

Henry J Fate

Well-known member
Joined
Jun 14, 2018
Posts
1,239
What device is signaling a "Low Battery"?

Have you wired the inverter with the correct wire size and length?

Was/is the battery fully charged?

A common mistake is wiring the inverter in such a way that it sends 120 volts to the converter charger which in turn sends charging current to the battery. This charging loop will deplete your battery fairly quickly. Make sure that is not happening.

The machine should not need much more than 1500 watts so it's safe to assume you have a wiring problem, low battery or possibly a faulty inverter.
 
Last edited:

Lou Schneider

Site Team
Joined
Mar 14, 2005
Posts
10,767
What size is the marine battery and how much power does the oxygen concentrator use? A single automobile battery size marine/RV battery doesn't have enough capacity to supply current to a large load without the voltage dropping to less than the inverter's cutoff voltage. Think about how the headlights dim when you crank the engine from a similar size battery.
 
Last edited:

DonTom

Well-known member
Joined
Apr 21, 2005
Posts
3,178
Location
Auburn, CA or Reno, NV or Cold Springs Valley, NV
I installed a 4000 watt inverter to power my wifes oxygen machine so I would not have to use the generator all the time. I also bought and replaced the emergency battery with a marine battery and wired the inverter to it. I also upgraded to a 200 amp alternator. My problem when I connect the O2 machine I get a battery low alarm. Suggestions
Sounds like too much load for your battery, but need more facts. Mainly the DC voltage going into the inverter with and without the load. And measure the same on the battery and compare the two.

Without some numbers, there is too much guesswork involved. But that is where to start.

Also, how much current does the machine use? It should say in amps (or perhaps in watts) near where the AC cord goes into the unit.

-Don- Auburn, CA
 

Gizmo100

Well-known member
Joined
Sep 28, 2018
Posts
2,921
The startup of a O2 concentrator is drawing your battery down below the minimum volts for the inverter.
And now the bad news...
The concentrator will not run all night on a single battery. Depending on your unit it will draw about 300 watts while running.

 

John From Detroit

Well-known member
Joined
Apr 12, 2005
Posts
24,505
Location
Davison Michigan
4,000 is a lot of inverter.. Ok some inverter install info
Though it's not a good idea to put the inverter and the batteries in the same compartment/air space (Inverters do not do well if breathing battery gas) you do wan't them as close as possible. For a 4,000 watt inverter you need to have very large cables. I'm not doing the research but I'd guess about 0000 gauge That's 4/0 Ga (one zero per thousand watts) this is for a 3-4 foot run... You also want a whole lot of battery.... Xantrex suggests 200 Amp hours per kilowatt so thats 4 pair of GC-2 Golf car batteries. You can likely get by with half that however.

Next to last GC-2 golf car batteies are DEEP CYCLE and do not mind going down to half full... MARINE/deep cycle are starting batteries that pay lip service to deep cycle. they shold be at least 75% full at all times. (or more).

Last item: IT is expensive but if the inverter is also your charger and programmable you might consider one of the many Lithium types Battle Born or Bionneo (if i spelled the latter right) Might set you back 1500 (300 amp hour) or 3000 but they do have a few advantages over (Heavy as) lead acid. One being weight.

You mignt also consider a 2000 watt inverter.

On edit, One thing that Xantrex told me (voice call)
For as much of the run as you can tape the positive and negative DC leads together (side by side) to reduce the inductive loss at power on.
 

Ex-Calif

Well-known member
Joined
May 15, 2020
Posts
1,264
A single normal battery may have up to 130 amp hours in it. About 1/2 of those are usable until voltage drops. So let's say you have 70 amps stored.

A 4,000 watt inverter in theory can supply 330 amps. About 10 minutes of use on that battery. Let's say the CPAP is 1500 watts. That's 125 amps or about 30 minutes with that battery.

The other thing is that discharging a battery causes heat in the battery. Discharging a battery really fast (high load) creates a lot of heat. Heat causes resistance in the battery. When resistance goes up and amps stays the same, voltage has to drop.

That's why you are getting a low voltage alarm.

Bottom line is that you probably can't run a CPAP machine overnight without a ton of battery storage. To run a 125 amp machine for 8 hours you'd need like 2000 amps or about 15 batteries.
 

DonTom

Well-known member
Joined
Apr 21, 2005
Posts
3,178
Location
Auburn, CA or Reno, NV or Cold Springs Valley, NV
For a 4,000 watt inverter you need to have very large cables. I

You mignt also consider a 2000 watt inverter.
A 4KW inverter will only draw the large current when it has near the 4KW load. It will draw about the same as a 2KW converter with a 2KW load. So as long as you don't exceed the 2 KW, you can use the same size cables with the 4KW inverter as the 2KW.

The larger inverters usually draw a little more current when they are used with VERY light loads, such as when on with no load at all. But when used under heavier loads, a few hundred watts or above, the difference is negligible between a 4KW and 2 KW inverter.

-Don- Auburn, CA
 

DonTom

Well-known member
Joined
Apr 21, 2005
Posts
3,178
Location
Auburn, CA or Reno, NV or Cold Springs Valley, NV
One more issue that could make a difference. All HFT inverters, are Modified Square Wave (AKA Modified Sinewave, which is really just a sales pitch). Some stuff will not run properly with MSW.

I don't think HFT sells a single true sinewave inverter.

-Don- Auburn, CA
 

Gary RV_Wizard

Site Team
Joined
Feb 2, 2005
Posts
72,670
Location
At our Silver Springs FL home
It's not a CPAP - it's an oxygen concentrator. That involves a compressor as well as a purifier unit, so the power load is fairly high at compressor start-up. A single "marine type" battery could be as little as 85 amp-hours, so odds are the concentrator power demand immediately exceeds the ability of the battery to deliver DC amps to the inverter. That causes a voltage drop and the inverter shuts down to avoid damage.

As Don says, we need power consumption numbers for the oxygen machine and the battery capacity (AH) or case size (e.g. Group 24) to make an educated guess, but I'm confident about the basic nature of the problem.
 

John From Detroit

Well-known member
Joined
Apr 12, 2005
Posts
24,505
Location
Davison Michigan
A 4KW inverter will only draw the large current when it has near the 4KW load.
If I were designing an inverter I'd stick a farad or two of capacitor in it to smooth provide the instant current needed when load changes big time.. (To over come inductive effect in the wires) and that sucks max current during power on self test.
 

Lou Schneider

Site Team
Joined
Mar 14, 2005
Posts
10,767
If I were designing an inverter I'd stick a farad or two of capacitor in it to smooth provide the instant current needed when load changes big time.. (To over come inductive effect in the wires) and that sucks max current during power on self test.
I was tempted to do that before I got the lithium batteries but they made starting surge problems moot. Supercapacitors are pretty inexpensive and pack a big wallop of short term power. Just be prepared for a huge arc anytime you connect power or short the leads after you disconnect them
 

Ex-Calif

Well-known member
Joined
May 15, 2020
Posts
1,264
Is anyone here running a CPAP machine on batteries and if so what is your setup?
 

Larry N.

Well-known member
Joined
May 26, 2010
Posts
7,546
Location
Westminster, Colorado
My wife's CPAP says it uses 24 VDC at 3.75 amps (though she uses the AC adapter). We plug it in to 120 VAC in our Ventana, even when boondocking for a week at Quartzsite in January, and it's never been enough of a draw to change our routine of running the generator morning and evening to recharge batteries and cook breakfast and supper. So it runs all night with no problem for us.
 

Ex-Calif

Well-known member
Joined
May 15, 2020
Posts
1,264
My wife's CPAP says it uses 24 VDC at 3.75 amps (though she uses the AC adapter). We plug it in to 120 VAC in our Ventana, even when boondocking for a week at Quartzsite in January, and it's never been enough of a draw to change our routine of running the generator morning and evening to recharge batteries and cook breakfast and supper. So it runs all night with no problem for us.
That's kinda what I expected. Even 60 amps (90W X 8) over night is doable with a very modest set up.
 
Top Bottom