FYI - Real-life A/C on inverter power consumption numbers

[Debo]

Hi all,

I'm finishing up the installation of my Magnum inverter (4K watt hybrid) and wanted to share a few numbers with you guys in case anyone is interested. My system has two 12 volt chassis batteries (for 24 volts) and four 6 volt golf cart batteries (for 24 volts.) The batteries are tied together by a 200 amp continuous duty solenoid that's triggered by the blower delay circuit on my MCI MC9. When the bus is running down the road the A/C units will, for all intents and purposes, be running off the alternator. The inter-tie can also be made manually by the switch momentarily, or turned off to isolate the batteries. There's a 200 amp Class T fuse in the inter-tie, and it uses 2/0 cable. All of the cables from the house batteries to the inverter are 4/0, and I was able to do it with about 4.5 feet of cable (one way.) There's a Class T fuse rated 300 amps in that line as well as a Blue Sea Systems switch rated at 500 amps. The only other thing is a 500 amp shunt in the inverter negative line to feed information to the Trimetric battery monitor. My bus has the 50DN alternator (24 volts.)

I ran my two 15K BTU Carrier A/C units yesterday to test the whole thing out and here's what I found. Each unit running on high with the compressor running draws 74 amps for a total of 148 amps at 82 degrees. That's 3552 watts. That's easily under what the inverter can handle, and the whole thing worked beautifully. I'm not saying do it like this, but this is what I'm doing and it seems to work in my situation so far. Hotter days will draw more amps, but I know that and can keep an eye on things to keep everything where it needs to be. So far, so good.

[lvmci]

Hi Debo, must be quiet with no generator running, what happens when the batteries run low and voltage drops? Lvmci...

[bevans6]

Your usage is right in line with what I saw, with a very similar setup - mine was 65 amps on low for one AC unit.  Mine is manual switch not automatic and one AC not two.  You only run the AC from the inverter when the engine is running, so you don't see any issues with voltage drop, in practice.  Four golf cart batteries (which is what I have too) are only good for about 30 minutes of AC with the engine off.  Mine are 230 AH at 24 volts.

Brian

[Audiomaker]

So is the goal to be able to run the A/C during quiet times or?

I've read about people wanting to do that.  It seems like hard use on a pretty large bank of batt's to me, but I suppose if it's dangerously hot and one cannot run the generator, then it would be worth it here and there.

I think a more practical non-standard use for the A/C-inverter formula is being able to run the A/C's while driving without starting the generator.

Being able to cool the coach while underway using the alternator's power seems like a practical goal.

Harder for me with a 12v system as a 200amp alternator is only putting out 2400w.   I suppose it's ok to roll down the highway with the genset running....just feels weird to me putting hours on two engines at once.

As usual, my head goes back to the APU (auxiliary power unit) where there is an A/C compressor directly driven by a small (smaller than most generators) engine.   It surprises me that more RV gen's don't have belt driven compressors but instead rely on a fuel to A/C to DC to battery to AC conversion....and then power yet another electric motor drive compressor from there.  A lot of middlemen.

Edit:  My coffee has kicked in and I see that underway powering of your electric A/C units is what you are talking about... sorry.

I'll probably do the same thing... it's just so darned inefficient in the end.

[Debo]

Hi Debo, must be quiet with no generator running, what happens when the batteries run low and voltage drops? Lvmci...

Sorry - forgot to add that. There's a 6K diesel genny for times when I'm not rolling down the road, or plugged into shore power. It's never my intention to run the A/C's purely from the batteries, although it can be done for short bursts. My inverter is capable of load sharing (shore power + battery) so a 6K genny will run them - even with the start-up load because the inverter can draw from the batteries for the extra juice. In practice, I don't plan on running everything wide open all the time, but we all know how "planning" goes, right?

[Debo]

Your usage is right in line with what I saw, with a very similar setup - mine was 65 amps on low for one AC unit.  Mine is manual switch not automatic and one AC not two.  You only run the AC from the inverter when the engine is running, so you don't see any issues with voltage drop, in practice.  Four golf cart batteries (which is what I have too) are only good for about 30 minutes of AC with the engine off.  Mine are 230 AH at 24 volts.

Brian

Yep - exactly the same setup Brian. Working good so far, and I'll shake it all out before I do any real traveling with it.

[Oonrahnjay]

So is the goal to be able to run the A/C during quiet times or?

I've read about people wanting to do that.  It seems like hard use on a pretty large bank of batt's to me, but I suppose if it's dangerously hot and one cannot run the generator, then it would be worth it here and there.   ...  

    You'll notice that Brian has said that his system will only run about 30 minutes and I'm guessing that that will pull the batteries down really far.  While in theory, you could have a huge battery bank (that's spelled "$$$$$$$$$$") but you'd only be measuring in minutes, not like overnight.  It really isn't practical to run A/C off of batteries for any extended period of time.  A good thing about Debo's system is that he has many options for powering his system.  He can run the engine and power off the engine alternator.  Or he can stop and fire up the generator and use that as the power supply to the inverter (in this case, the inverter will probably be in "pass through" mode and the 120V from the generator will be going through to the A/C units.  Another alternative would be to plug into "shore power" through a main cable; this would also be "pass through".  Both the generator and shore power would run the A/C units and provide excess power to recharge batteries and run other loads on the bus.  (The alternator on the bus would provide some extra power too but it doesn't look like there's a lot when both A/C units are running.)

    About the only thing that you'd be able to do with batteries is to be able to keep the A/C going for a few minutes after the engine was shutdown and before one of the other power sources would come on-line.

    But the idea of charging up your batteries and running the A/C all afternoon or overnight with no other power input?  No, not practical for most people.

Bruce H   NC   USA

[Debo]

I think a more practical non-standard use for the A/C-inverter formula is being able to run the A/C's while driving without starting the generator.

Being able to cool the coach while underway using the alternator's power seems like a practical goal.

Yeah, running underway with the A/C on is the real reason for all of this. I don't use a lot of electricity at rest, and my battery bank should be sufficient for my needs. I'm an old sailor and used Nigel Calder's "Boat Owners Mechanical & Electrical Manual" to figure up all of my needs and size everything accordingly. I'd otherwise never need a 4K watt inverter, but having that A/C on running down the road with no generator running is nice.

[bevans6]

The thing is, it can be designed to be extremely flexible.  I do seem to recall that Sean the Boat Guy had around 12-odd batteries and could run one AC most the the night.  But his setup was an extreme example for a bus.  My long term plan is to build a DC generator and use it to power the inverter, making double use of the inverter and a DC generator can be designed to be very quiet, since it has no frequency maintenance requirement it can run at quite low speeds (same as any inverter-generator).  But I immediately saw one big benefit to my setup on my last trip.  The start batteries had been acting a little weird, but since I had them on a trickle charger they were up to their normal sitting voltage.  They cranked the engine very slowly when I went to start, but they started it.  I let them charge a bit, hit the switch to bridge in the house bank, turned on the AC and hit the road.  Three hours later I got parked at my truck stop of choice, unbridged the starts and started watching TV.  A little while later it became apparent I had parked in a high traffic area for the truckers instead of what I thought was a low traffic area so I went to start the bus to move to a less in-the-way spot, and the starts were dead.  I just threw the switch to bridge in the house bank and carried on.  Next night I measured and one start battery only had 10 volts, so I bought some group 31's and just left the start bank disconnected, ran just off the house bank for the next day until I could yank out the old 8D's and install my new Gp 31's.  My set up has the flexibility to run the bus side off the starts only, the house bank only, or both combined, and run the inverter off the starts/house/alternator combined, or just the house bank alone.  I can charge the house or the combined bank from the inverter.  Being the honest lad that I am, I never designed in the ability to run the bus just off the house bank and I had to test it to be sure it worked (I couldn't remember exactly what connections the combiner switch made) but at the end of the day it worked great and probably got me out of a pickle...

Brian

[Debo]

    A good thing about Debo's system is that he has many options for powering his system.  He can run the engine and power off the engine alternator.  Or he can stop and fire up the generator and use that as the power supply to the inverter (in this case, the inverter will probably be in "pass through" mode and the 120V from the generator will be going through to the A/C units.  Another alternative would be to plug into "shore power" through a main cable; this would also be "pass through".  Both the generator and shore power would run the A/C units and provide excess power to recharge batteries and run other loads on the bus.  (The alternator on the bus would provide some extra power too but it doesn't look like there's a lot when both A/C units are running.)

    But the idea of charging up your batteries and running the A/C all afternoon or overnight with no other power input?  No, not practical for most people.

Bruce H   NC   USA

Well articulated Bruce, and exactly my thoughts.

Like I said, it's not for everybody, but I just wanted to be able to provide some real-world information to people tossing the idea around in their head. I think it's a very flexible system, and it fits my needs.

[bevans6]

One odd thing, relevant to running AC units in a system with a Magnum 4000 inverter.  I have a Yamaha 3000SEB generator that runs my AC unit flawlessly.   I also have a 15 amp plug in my shop that I sometimes use to run the AC, and it runs the AC flawlessly as well if the day is sorta cool.  I couldn't get the AC to start on either with the Magnum in the circuit.  Even with the Magnum turned off, both charger and inverter, from the remote panel.  What would happen is I would start the AC compressor, the Magnum would interrupt the circuit for a second or two, it would start to invert, it would realize that it was supposed to be turned off, and then after sitting for another second or two it would let the AC compressor start on the generator or the remote feed.  What was happening was I had the Magnum's low voltage cut-out set at 100 volts, which seemed reasonable to me, but the start surge on both was enough to drop below 100 volts for an instant.  I reset the low voltage cut-out to 90 volts and all was normal again.  Dang overly smart inverter!  Mine is not the Hybrid version - the Hybrid version would probably bolster the supply in some way.

Brian

[belfert]

Running A/Cs under way are going to burn up extra fuel whether you run them from the alternator, or from the generator.  I figure you're taking away from the main engine's power when putting that much load on the alternator so I just run the generator.  If you're mostly a flatlander it won't matter so much.

[Iceni John]

Isn't this a reason to consider mini-split A/Cs, especially the inverter versions  -  they use much less power than typical RV roof-warts, so one could run them off batteries for a reasonable time, or off alternators smaller than 50DNs.   Yes, I know they cost more initially, but for folk with different needs than most they could be useful.

Food for thought?
John

[Audiomaker]

Isn't this a reason to consider mini-split A/Cs, especially the inverter versions  -  they use much less power than typical RV roof-warts, so one could run them off batteries for a reasonable time, or off alternators smaller than 50DNs.   Yes, I know they cost more initially, but for folk with different needs than most they could be useful.

Food for thought?
John

My own personal thought on this is that if one wants more air cooling, then the best way is to improve on the mechanically driven system rather than trying to do the wattage fight.

Has anyone looked at the A/C issue from the perspective of what can be done to, or added to the mechanically driven system?

[bevans6]

You have to do the math at some point.  At $450 that I paid for a new 15K btu Briskaire, not sure how many mini-splits you can buy.  In my area, about a quarter of one, then you have to install it, get it charged, all of that.  Numbers do not work for me.  As far as the mechanically driven system, it might work great for you forever, but mine needed around $2500 to recommission, and an estimated $1,500 a year to maintain, and finding someone to do the work is literally impossible.  Numbers do not work for me.  As far as running the generator, I need to pull 2.4 hp from my main engine.  That is around .75% of it's capacity, and the fuel usage is literally in the noise, as is the impact on performance.  It takes around 20 times that to simply make my engine spin at 1800 rpm, and a 2 mph difference in headwind overcomes that drag cost handily.  The cost to run an AC from the main engine is essentially free once the infrastructure is in place.  The cost of a typical 6 KW generator that can run OTR is at least 3 times the cost of my total infrastructure.  Once again the numbers do not work for me.  I do have a gas generator that I carry, the Yamaha, but I need that for emergency backup at home anyway.

Everyone has their reasons for doing what they do, but I am Scottish, and I spent the least amount of money possible to do what I am doing.  Pennies scream when I pinch them!  It really is a good solution. 

Brian