Looking for advice on a pure sine wave power inverter

[aja8888]

The long beefy cable that runs under the car if shorted to the chassis would cause a fire or the batteries to explode. That's why I would put an ANL fuse at each end of that cable right before it gets attached to each battery.

I guess a manual disconnect will not hurt and comes in handy at times if you want to remove the auxiliary battery and worry about the positive cable end hanging loose and shorting against the car body.

Yes, I understand the protection needs. This is going to be more expensive than I thought, but if it works right, it will be worth it. I suppose I could also put the long battery cable under the car into a sheath of sorts or PVC pipe for protection.

 

[meierlde]

The long beefy cable that runs under the car if shorted to the chassis would cause a fire or the batteries to explode. That's why I would put an ANL fuse at each end of that cable right before it gets attached to each battery.

I guess a manual disconnect will not hurt and comes in handy at times if you want to remove the auxiliary battery and worry about the positive cable end hanging loose and shorting against the car body.

Actually if you go over to the solar (or the marine) side you find they have 12 v dc circuit breakers of various sizes, so you could pick one that could do double duty, both to interrupt over current situations and as a disconnect. It will likley need to be over 40 amps to just run the concentrator from the alternator. Here you get involved with how many amp hours capacity (amperes delivered times time) you need which depends on how often you want to run the engine. Note they say for max life a deep cycle battery (which is what you want here not a car starting battery) should only be discharged 50% to maximize battery life.

 

[travelover]

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...........

This could turn out to be problem - you might want to make sure the inverter / battery system you buy can provide the starting surge before you get it all plumbed in. In RVs, numerous people have tried to run small AC units or refrigerators off inverters and been surprised at the difficulty of providing that starting kick consistently. Not to be negative, it is just surprisingly challenging.

 

[NW-Bound]

The typical RV roof AC is an awful load for an inverter to start, even a 2kW one. I don't think a refrigerator with a 300W motor presents the same problem.

Many users do not understand the huge current drawn by a large inverter, the voltage drop due to the wires between the inverter and the battery, and finally how much a lead-acid battery voltage sags under such large currents.

The 1kW Xantrex has a surge capability of 2kW, hence I thought it would be able to handle a 325W motor.

About the auxiliary battery, its function is more to provide power close to the inverter to handle the starting current. To put a battery inside the car, one needs an AGM battery which does not spill acid and discharge hydrogen/oxygen. And AGM battery is head-and-shoulder above deep-cycle marine or golf-cart batteries in terms of handling large currents.

A common misconception is that a 100Ah battery will last 1 hour under a 100A load. No, it's more like a few minutes.

The 100Ah capacity is measured at a 5A load for 20 hours. When the load is multiplied up 20x to 100A, the duration does not reduce by the same factor of 20x to become 1 hour. Under 100A load, the battery capacity becomes more like 10Ah, and the duration is 6 min (1/10 hr).

This effect was observed on lead-acid batteries in 1897 by Peukert. Look up Peukert's law to learn more.

AGM batteries are much better than flooded lead-acid types. The best is lithium battery which is closest to the ideal battery, but its cost can be prohibitive. There are several 12V-compatible LiFePO4 (lithium iron phosphate) batteries on the market now. Look up the price of a 12V 100Ah lithium battery and see for yourself.

I recently tested these lithium batteries last summer, and a 100Ah battery ran a window AC drawing around 500W from a Xantrex 2kW inverter. The current drawn on the battery was 45A at 13V. The AC ran for 2 hours.

PS. Forgot to add, the 2kW Xantrex was cruising under that load. Its fan did not even turn on. The case was barely warm. Increase the load to 1kW+, and it started to get warm and turned its fan on.

 

[travelover]

The typical RV roof AC is an awful load for an inverter to start, even a 2kW one. I don't think a refrigerator with a 300W motor presents the same problem.............

I was referring to window AC units - like 5000 btu. You may be right, but it is hard to measure the initial surge current, as it is so fleeting. I only mention it because it I've seen others struggle with inverter projects where the inverter/battery combination just can't deliver the startup current. If I was doing it, I'd bench test it thoroughly before I went to all the work of installing it in a vehicle.

 

[NW-Bound]

Yes, one may play it safe and put in more oomph to be safe.

In the case of our friend aja, I am quite certain that a AWG2 wire running front to back, an AGM auxiliary battery in the back, and a Xantrex inverter will work. Now, I don't know if he should upgrade to a 2kW to be safe, or a 1kW job will do.

 

[folivier]

For more advice you might try joining and posting this in the Escapees forum: Forums - Escapees Discussion Forum

 

[ERD50]

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. ...

Questions:

1) Why not use the portable unit? It looks like they design portable units to be more energy efficient, and a simple cigarette lighter outlet (or whatever they call them these days) may do it (depending maybe on the next question). For a unit designed to only ever run on AC, 325 watts is no problem, and pushing efficiency probably cost more than the saving in electricity, so they just don't bother.

2) How long will she use it in the car? You might get away with a battery connected to a cigarette lighter outlet to supplement and charge the battery when not in use (especially for a more efficient portable unit).

Your 325 W unit will draw ~ 30A at the battery, so 10 A from the cig outlet means you only need 20A from the battery. Hmmm, but as was mentioned, AH rating of batteries are for 10-20 hour discharge rates, so that a big honking battery for a few hours.

But the portable ones can provide 4L for 4 hours on the included battery, (didn't find an Ahr rating on those yet), so supplementing that and charging in the car should take you pretty far.

-ERD50

 

[aja8888]

Questions:

1) Why not use the portable unit? It looks like they design portable units to be more energy efficient, and a simple cigarette lighter outlet (or whatever they call them these days) may do it (depending maybe on the next question). For a unit designed to only ever run on AC, 325 watts is no problem, and pushing efficiency probably cost more than the saving in electricity, so they just don't bother.

2) How long will she use it in the car? You might get away with a battery connected to a cigarette lighter outlet to supplement and charge the battery when not in use (especially for a more efficient portable unit).

Your 325 W unit will draw ~ 30A at the battery, so 10 A from the cig outlet means you only need 20A from the battery. Hmmm, but as was mentioned, AH rating of batteries are for 10-20 hour discharge rates, so that a big honking battery for a few hours.

But the portable ones can provide 4L for 4 hours on the included battery, (didn't find an Ahr rating on those yet), so supplementing that and charging in the car should take you pretty far.

-ERD50

1. We have a portable battery operated unit that is good for travel, but I don't want to RELY on that staying running for a 4,000+ mile driving trip. We have 6 back up O2 bottles, but they only last a couple of hours each. They can only be exchanged/refilled by the supplier who is near home, which we won't be.

I don't want the portable unit to be broken down in West Texas or some other desolate area with no way to provide DW with 3 liters per minute of 90% O2. It's not that I don't trust the portable unit, but they are pretty fragile and known to stop working unexpectedly. As an engineer, I like back up systems to minimize risk. And having a unit that is designed for continuous use is my preference for a #1 source.

2. I would guess we will be in the car for over 60 continuous hours driving.

The 12V source wiring in the car is rated at 150 watts so the inverter is needed for the larger unit.

 

[aja8888]

For more advice you might try joining and posting this in the Escapees forum: Forums - Escapees Discussion Forum

Thanks for the link. Looks like a great source of information.

 

[aja8888]

Yes, one may play it safe and put in more oomph to be safe.

In the case of our friend aja, I am quite certain that a AWG2 wire running front to back, an AGM auxiliary battery in the back, and a Xantrex inverter will work. Now, I don't know if he should upgrade to a 2kW to be safe, or a 1kW job will do.

NW-Bound, thanks for all the great information you have presented here.

While I am a pretty good engineer (IE & ME), I really look forward to experienced knowledge in the electrical areas such as you have provided. My parts list has grown (haha) and will be gathering up the battery and other components and seeing about getting this going this week. I'm going with the 1,000 watt inverter instead of the 600 watt version.

 

[NW-Bound]

You are welcome.

Even as an EE, there are a lot of things about batteries, particularly the lead-acid type, that did not work as I thought. It is only after a few years of RV'ing that I know the pitfalls. These are not secrets, really, but it was hard for me to disabuse myself of my own prior misconceptions. And often, what other people report as their real-life experience is not appropriate to my own application.

 

[travelover]

.........Even as an EE, there are a lot of things about batteries, particularly the lead-acid type, that did not work as I thought.. It is only after a few years of RV'ing that I know the pitfalls. These are not secrets, really, but it was hard for me to disabuse myself of my own prior misconceptions.................

I've had long running RV forum battles with EEs on why trailer batteries don't charge well from the tow vehicle. I don't want to get into it here, but it is apparently harder to model a battery in a simple circuit than it at first seems.

 

[NW-Bound]

... trailer batteries don't charge well from the tow vehicle...

I believe it. Mostly just voltage drop across the long wiring runs, particularly across the tetheting cable.

It's nothing that a simple voltmeter will not tell you. Measure the voltage at the tow vehicle battery with the engine running. Then, measure the voltage at the trailer battery. One probably sees a big voltage drop of a volt or higher. That keeps the trailer battery from getting fully charged unless the drive is many hours long.

Flooded lead-acid batteries also need time to "soak up" the juice. They call it the "absorption phase" which will take several hours. Give the battery a puny voltage, and it will never get fully charged, no matter how long it sits there. And the absorption voltage is higher at low temperatures. A honest-to-goodness RV battery charger has a temperature probe that is mounted at a battery post to measure its internal temperature. The charger then applies a preprogrammed compensation to raise the output voltage appropriately.

Rolls, a big name in lead-acid batteries, says to use 15V to charge its 12V batteries at 77F. At 32F use 15.75V. What car alternators put out such high voltages? Now, the tow car has its battery under the hood, toasty warm and too hot in fact. The trailer battery on the other hand sits out in the cold on the trailer tongue. That's another reason for it to be poorly charged.

In terms of ability to accept charge, AGM batteries are much better. Lithium batteries soak up the juice like a sponge. Just wonderful.

Another story about the effects of long wires. I tow a car behind my motorhome, and wired the tail and signal lights from the motorhome to that of the car. The voltage drop was so bad that the toad's brake and signal lights were dim. It made it dangerous because of the poor visibility.

So, I used relays that are controlled by the MH light wiring in a circuit to boost up the drive to the toad. It is as bright as when the toad engine is running.

 

[travelover]

I believe it. Mostly just voltage drop across the long wiring runs, particularly across the tethering cable.

It's nothing that a simple voltmeter will not tell you...........

Bingo. But look at the tow vehicle's OEM wire gauges for the charge wire to the 7 way connector. Many are 12 gauge. Add in similar wire gauge in the trailer with the battery in the back, plus a 12 volt refrigerator running while towing pulling 10 to 20 amps and you've got a dead trailer battery at arrival.

EE's claim the tow vehicle's alternator is too small. Uh, no.

 

[NW-Bound]

I was describing how one could be sure to get a trailer battery fully charged, unused. If you are drawing power from it while driving, tough luck!

There are plenty of EEs who do not own a voltmeter. Same as in other fields, a degree does not mean much if one does not practice what he learns in school. I learned Ohm's law when in 9th grade. Actually, they taught Ohm's law in 9th grade, but I was reading 9th grade physics book when I was in 6th grade.

Now, it were me, how would I be sure of getting the battery charged without running a coiled AWG2 wire (if they make such a thing) over the trailer hitch?

Ah, it's nothing that modern electronics cannot solve. A boost DC/DC converter on the tow vehicle side to pump the voltage up to, say 24V. Brought over to the trailer side, who cares if the voltage is now, say 22V? A buck DC/DC converter to bring that voltage down to a regulated 13.8V, and that trailer battery is happy as a clam.

 

[travelover]

I was describing how one could be sure to get a trailer battery fully charged, unused. If you are drawing power from it while driving, tough luck!

Actually it is very common to have 12 volt refrigerators in camper trailers for this very purpose. In the past, it meant very heavy wiring to make it work properly.

..........Ah, it's nothing that modern electronics cannot solve. A boost DC/DC converter on the tow vehicle side to pump the voltage up to, say 24V. Brought over to the trailer side, who cares if the voltage is now, say 22V? A buck DC/DC converter to bring that voltage down to a regulated 13.8V, and that trailer battery is happy as a clam.

Exactly and one can buy this very device off the shelf. It is designed to keep the lift gate battery charged on long semi trailers.

 

[meierlde]

How long would you want the oxygen concentrator to work without the engine running? Note that a 100 ah battery would to maximize battery life with a 30 amp draw at 12 v run about 1.5 hours drawing 50 ah or so from the battery, which is the recommended 50% discharge limit to maximize battery life. Since these batteries would cost $330 you could consider a 1000 w inverter generator from Walmart for about $200 as an alternative for when the engine is stopped and your not near 110 v service.
I have been playing with the idea of building a UPS for the fridge since modern fridges run about 120 wh/hour it is now doable. (look at inverter chargers)

 

[aja8888]

How long would you want the oxygen concentrator to work without the engine running? Note that a 100 ah battery would to maximize battery life with a 30 amp draw at 12 v run about 1.5 hours drawing 50 ah or so from the battery, which is the recommended 50% discharge limit to maximize battery life. Since these batteries would cost $330 you could consider a 1000 w inverter generator from Walmart for about $200 as an alternative for when the engine is stopped and your not near 110 v service.
I have been playing with the idea of building a UPS for the fridge since modern fridges run about 120 wh/hour it is now doable. (look at inverter chargers)

The plan is to not run it with the engine off. We have the battery powered Imogen One O2 concentrator for that purpose. We also have 6 O2 cannisters for additional backup.

As I stated earlier, the main purpose for running the large (45 pound) unit in the vehicle was for a steady O2 source without using the backup unit. Also, we will use the large unit in the hotel and at the destination.

 

[NW-Bound]

... Exactly and one can buy this very device off the shelf. It is designed to keep the lift gate battery charged on long semi trailers.

Hah! I guess I can shelf the plan to manufacture and to sell this thing.

This 2-in-1 commercial boost job is more convenient to install than a separate boost, then buck DC/DC converters. However, it is not as efficient in terms of wiring loss compared to two converters, one at each end.

But, but, but, can I beat their price of $250.80? I bet I can. There's a new IC controller to make it easy to build a buck/boost converter. I recently discovered that they had new ICs to implement this new topology recently when I was busy enjoying retirement. It uses a single inductor in the middle of 4 transistors similarly like an H-switch. Very cool.

The plan is to not run it with the engine off. We have the battery powered Imogen One O2 concentrator for that purpose. We also have 6 O2 cannisters for additional backup...

A good AGM battery will have no problem running the inverter and its load for 1 hour when you stop for gas or at a rest stop, so that you do not have to run the car engine all the time.

 

[aja8888]

A good AGM battery will have no problem running the inverter and its load for 1 hour when you stop for gas or at a rest stop, so that you do not have to run the car engine all the time.

Thanks, I assumed it could run for a while given the size of the battery and small load. Very handy setup with the extra battery.

 

[meierlde]

Thanks for the more info it does eliminate some of the issues. Given the battery powered unit, then the battery setup might not be required.

 

[aja8888]

Thanks for the more info it does eliminate some of the issues. Given the battery powered unit, then the battery setup might not be required.

....As long as I can trust a battery powered portable unit designed for occasional use. Not happening. One must realize that DW NEEDS 3 L/min of 90% O2 all the time to function somewhat normally. If that source stops, her blood/oxygen levels start to drop very rapidly and we could have some tense times in front of us.

We can't go off on a 4,000 mile jaunt with a small, portable battery powered unit and 6 small tanks of O2, each with ~2 hours of capacity. The large continuous O2 unit is primary. Everything else is backup. For short hops around town, we have the battery powered unit and carry a tank also.

 

[meierlde]

....As long as I can trust a battery powered portable unit designed for occasional use. Not happening. One must realize that DW NEEDS 3 L/min of 90% O2 all the time to function somewhat normally. If that source stops, her blood/oxygen levels start to drop very rapidly and we could have some tense times in front of us.

We can't go off on a 4,000 mile jaunt with a small, portable battery powered unit and 6 small tanks of O2, each with ~2 hours of capacity. The large continuous O2 unit is primary. Everything else is backup. For short hops around town, we have the battery powered unit and carry a tank also.

I was just considering the issue of needing the battery, not the issue of the inverter. I.E. in particular when engine is turned off for refueling and the like (i.e. 5 min or so stops). The battery might be needed if for example you buy lunch at a store and stop at a roadside table to eat it (near the car).

 

[aja8888]

I was just considering the issue of needing the battery, not the issue of the inverter. I.E. in particular when engine is turned off for refueling and the like (i.e. 5 min or so stops). The battery might be needed if for example you buy lunch at a store and stop at a roadside table to eat it (near the car).

I misinterpreted what you were saying. Got it now.