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Looking for advice on a pure sine wave power inverter
Old 12-18-2017, 08:41 PM   #1
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Looking for advice on a pure sine wave power inverter

For our upcoming SUV travel, DW needs oxygen on a constant basis. We have a portable Imogen One unit that provides 3 liters per minute of O2, but I would like to install a DC to AC pure sine wave inverter in the rear of the car to power our big oxygen concentrator. This is a model Invacare Perfecto2 V Five Liter Oxygen Concentrator With SensO2 Oxygen Sensor. This unit draws 325 watts or about 3 amps and weighs about 45 pounds.

There's a ton of inverters out there, but I know many of you are EE types and may have experience with particular models. I assume I will need one that will provide 500 or more watts and be wired directly to the car's battery.

Has anyone done anything like this? And have any recommendations for a particular brand of inverter?

Thanks!
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Old 12-18-2017, 11:15 PM   #2
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I have had good luck with a Xantrex PROWatt SW2000 that I hard wired in my RV. Xantrex offers smaller similarly looking units SW1000 and SW600.

The SW2000 draws only 6W (0.5A) when running unloaded. That's really outstanding. The smaller units would draw even less in quiescence.

Be careful you do not drain the battery and not being able to start your car. An auxiliary battery along with an isolator would be safer.

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Old 12-19-2017, 04:14 AM   #3
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+1. Xantrex makes a great product.
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Old 12-19-2017, 09:12 AM   #4
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Thanks folks. I was looking at 600 watt units since the load will only be 325 watts from the concentrator. But a 1000 watt will allow me more capacity. Next is determining wire size to run from the battery.
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Old 12-19-2017, 09:33 AM   #5
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At 1 KW you will be pulling about 100 amps out of the battery.
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Old 12-19-2017, 09:46 AM   #6
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I don't know what kind of motor is inside the oxygen concentrator, but induction motors can have a starting surge current 3x or 4x their running current. So, a 1kW inverter to run a 300W machine sounds reasonable.

The cable has to be AWG #2 at the minimum, and kept short between the battery and the inverter (~2 ft). The worse problem is that lead acid batteries have significant voltage sag when submitted to a continuous heavy load.

When I run the microwave oven in my RV which draws a bit more than 1kW, the inverter draws more than 120A from the batteries. Two marine 12V batteries in parallel can handle that for about 5 minutes when new. Two 6-V golf cart batteries in series cannot handle that at all: they have much higher internal resistance. I am using lithium batteries now.
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Old 12-19-2017, 09:55 AM   #7
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I don't know what kind of motor is inside the oxygen concentrator, but induction motors can have a starting surge current 3x or 4x their running current. So, a 1kW inverter to run a 300W machine sounds reasonable.

The cable has to be AWG #2 at the minimum, and kept short between the battery and the inverter (~2 ft). The worse problem is that lead acid batteries have significant voltage sag when submitted to a continuous heavy load.

When I run the microwave oven in my RV which draws a bit more than 1kW, the inverter draws more than 120A from the batteries. Two marine 12V batteries in parallel can handle that for about 5 minutes when new. Two 6-V golf cart batteries in series cannot handle that at all: they have much higher internal resistance. I am using lithium batteries now.

Looking at the wire size table with the Xantrex unit, #2 looks to be about right. Unfortunately, in the SUV, the battery is under the hood and the O2 unit, as big as it is, will have to be located at the rear of the vehicle. Short cable runs creates a problem. This is not an RV where its pretty easy to have two batteries and be able to mount the inverter somewhere convenient.

It looks like I will have to somehow figure out a second battery configuration (at the rear) and have the Xantrex near it.
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Old 12-19-2017, 01:36 PM   #8
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There's no need to have 2 batteries in the trunk. One, an AGM that's non-spillable, will run that inverter. AGM batteries have a much lower internal resistance than flooded batteries.

The next problem is to provide the juice from the alternator as no battery can supply the 325W load plus the inverter loss for long, unless you fill the trunk with lithium batteries.

Using AWG2 at 0.16 milliOhm/ft, assuming a run of 15 ft, you will have a resistance of 2.4 milliOhm. With losses through connectors and contact points, you will have more than 5 milliOhms easily. I am assuming that you can get a good ground through the vehicle chassis and avoid a cable for the return.

Then, the current requirement is 500W/12V = 42 Amps. With 5 milliOhms, the voltage drop is 0.2V, which is not too bad.

It's all doable.
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Old 12-19-2017, 01:53 PM   #9
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To prevent the auxiliary battery in the rear from draining the starter battery and leaving you stranded, it is simplest to put a smart battery isolator between them. What this does is to connect them only when one has a voltage higher than, say 13.4V. This can happen only if the engine is running and the alternator is charging the starter battery. When the engine stops, the voltage drops, and the isolator disengages the two batteries.

This is better and much simpler than the isolator relay usually installed in RV's. This latter old type is just a dumb relay with its coil wired to the ignition switch. You can still drain both batteries if you leave the ignition switch on while the engine is not running.

The Xantrex inverter will shut down if the battery voltage drops below 10.5V. A battery in this stage may recover if charged up right away. However, a starting battery drained this low will have trouble cranking the engine.
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Old 12-19-2017, 02:15 PM   #10
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One other thought is to also get a 110 v ac based charger with an included charge controller to maximize battery life such as this one (not a recommendation but)https://www.iotaengineering.com/12vdc.htm That way if you are near a 110 v outlet you can charge the battery while parked, as well as running the O2 concentrator from the 110 v.
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Old 12-19-2017, 02:22 PM   #11
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To prevent the auxiliary battery in the rear from draining the starter battery and leaving you stranded, it is simplest to put a smart battery isolator between them. What this does is to connect them only when one has a voltage higher than, say 13.4V. This can happen only if the engine is running and the alternator is charging the starter battery. When the engine stops, the voltage drops, and the isolator disengages the two batteries.

This is better and much simpler than the isolator relay usually installed in RV's. This latter old type is just a dumb relay with its coil wired to the ignition switch. You can still drain both batteries if you leave the ignition switch on while the engine is not running.

The Xantrex inverter will shut down if the battery voltage drops below 10.5V. A battery in this stage may recover if charged up right away. However, a starting battery drained this low will have trouble cranking the engine.
Thanks, when I said a second battery, I meant one in the rear along with the SUV starter battery, not two back there. Xantrex (the pdf manual I downloaded) shows a diagram of the two batteries and the isolator and the schematic. Looking at the SUV, I believe I will need to install the cables under the vehicle as there is no apparent easy way to get them from the firewall to the rear.

I would guess a visit to an RV shop that installs these may be a better way to see how to do this rather than installing all this stuff myself. Looks like an "after the first of the year" project.
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Old 12-19-2017, 02:26 PM   #12
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I'm no engineer, but is the normal car alternator going to be able to handle that load in addition to it's other normal loads?

When I was working police and ambulance vehicles all had special-order heavy duty alternators to handle the loads of the radio gear and all the lights.
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Old 12-19-2017, 02:27 PM   #13
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One other thought is to also get a 110 v ac based charger with an included charge controller to maximize battery life such as this one (not a recommendation but)https://www.iotaengineering.com/12vdc.htm That way if you are near a 110 v outlet you can charge the battery while parked, as well as running the O2 concentrator from the 110 v.
That is a good idea. Thanks.
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Old 12-19-2017, 02:31 PM   #14
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I'm no engineer, but is the normal car alternator going to be able to handle that load in addition to it's other normal loads?

When I was working police and ambulance vehicles all had special-order heavy duty alternators to handle the loads of the radio gear and all the lights.
Most alternators today are 90 amp or higher. The one in my VW diesel is 120 amp output. That should handle the load.

The O2 concentrator really only draws 325 watts or about 3 amps on A/C. So it's not a big continuous load. Interestingly, the 12V plug circuits in cars that you plug your cell phone charger into are rated for about 150 watts, so that wiring is not usable.

Big lights on ambulances and cop cars draw a lot of current, thus the extra large alternators, etc.
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Old 12-19-2017, 02:53 PM   #15
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I have read all kinds of stories about RV shops. So check them out somehow before you give them the job.

I am not impressed with the wiring in my class C motorhome. And it was a factory job. Not too many RV manufacturers have the knowhow and quality control of big auto makers. RV shops may not be better.
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Old 12-19-2017, 03:01 PM   #16
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I have read all kinds of stories about RV shops. So check them out somehow before you give them the job.

I am not impressed with the wiring in my class C motorhome. And it was a factory job. Not too many RV manufacturers have the knowhow and quality control of big auto makers. RV shops may not be better.
I still may do this job myself. The only difficult part is getting a nice, protected cable run from the car's battery to the rear area. Maybe I'll go to Best Buy and ask the stereo installers what they would do to get that cable run. I've used them in the past and they are OK. The rest is just hook ups and mounting the inverter.
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Old 12-19-2017, 05:49 PM   #17
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Why don't you install the inverter close to the battery up front and then you just run essentially an extension cord to the rear. I've seen some people install them in the engine compartment, just depends on the space available and ability to keep it protected from the elements.
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Old 12-19-2017, 06:52 PM   #18
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I would do the job myself. Some high-clearance SUVs have plenty of room under the chassis to run the cable. I would put two ANL 100A fuses, one on each end of the cable right at the battery posts. This prevents a short caused by cable chafing from escalating into a fire.



I would not put the inverter inside the engine compartment, definitely not this Xantrex or others like it. It has ventilation holes for cooling, and will not last long if dirt and water get inside.

Additionally, it needs cool air. Even inside my RV, this summer when I did some quick tests with my new lithium batteries, the inverter beeped to complain because I did not run the AC and the RV interior was at 110F.
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Old 12-19-2017, 06:58 PM   #19
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For the smart isolation relay, you can surf the Web for VSR (voltage sensitive relay) or ACR (automatic charging relay). Easy to wire into that cable linking the two batteries. No need to wire into the ignition switch like the old isolation relay of RVs. No worry about forgetting like with a manual disconnect switch.
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Old 12-19-2017, 07:28 PM   #20
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As I recall, a relay is superior to a solid state isolator as the solid state devices have a 0.7 volt drop. It is important to maximize the charging voltage for a secondary battery.
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