Gen and inverter sizing

[JohnEd]

I think your numbers are off.

Well, I knew they weren't spot on but I did think there were 10KBTU roof tops and 9.5 amps is what I recall them needing.  Maybe that is from the late 80's or maybe it is just an error but I agree with the 13.5 needing 13 amps to run and I thought the start-up was 26 amps and I never understood why  a 20 amp breaker was called for.  I know it is cause I looked it up.  The 26 number isn't listed.  Not that I could find, anyway.

My 13,500 BTU/hr units draw about 13 amps each steady-state in average temperatures (more in sweltering heat, and a bit less in more temperate conditions).

This I don't understand.  For my money a compressor in a sealed unit will draw a given amt regardless of "how long" it has to run.  In high temps the duty cycle goes up but the compressor runs at the same speed for each cycle.  I can see where the comp would be ingesting freon at a higher temp if the ambient went up butthe compresin ratio is fixed.   Reason?

They call for 20-amp HACR breakers and draw at least that much at start-up.  To run two of these would require a minimum 3.3 kW inverter -- 3.0 is too small.  Practically speaking, the next size up from 3 kW is 4 kW.

OK.

BTW, as I wrote in the other thread, I ran two of these A/Cs just fine on a 5 kW generator.  At 20 amps each for start up, even if they start together that's 4,800 watts.  It was a little Onan Marquis on a Fleetwood gasser.

That makes perfect sense.  You can just get by...and that has always been my question cause "what else you run" will change with each Knut.

I didn't call you on the 6.5 genny because you told us not to... for all I know, you have a 3,600-watt battery charger that runs every time the generator does (not an uncommon configuration), which would leave you just 2,900 watts for your air -- not enough, as I just described.  Now, if you want to discuss that problem, we can, although perhaps a new thread would be appropriate.


I do  and this is it.  Is the 2.900 watts  what is left after you subtract efficiency numbers for the inverter?


Thanks,

John

[Len Silva]

A couple of points John.

You may have been looking at locked rotor amps which will be considerably higher than start current.

As the ambient temperature goes up, the head pressure goes up, thus the higher current.

Most quality generators and inverters can handle a start surge much higher than their rated steady load.  The cheaper ones tend to lie bout that.

[Sean]

I think your numbers are off.

Well, I knew they weren't spot on but I did think there were 10KBTU roof tops and 9.5 amps is what I recall them needing.

Unless I misread it, you mentioned 13kBTU/h units in the other thread, not 10.  I've never seen a 10, per se, but I am pretty sure there is an 8, which is sold into the truck-camper and Scamp market (not good for cooling anything much larger).  Even there, it's an extra-cost option, meaning it costs more than the larger units.  People who need to get by on 15 amps will pay the premium.

My 13,500 BTU/hr units draw about 13 amps each steady-state in average temperatures (more in sweltering heat, and a bit less in more temperate conditions).

This I don't understand.  For my money a compressor in a sealed unit will draw a given amt regardless of "how long" it has to run.  In high temps the duty cycle goes up but the compressor runs at the same speed for each cycle.  I can see where the comp would be ingesting freon at a higher temp if the ambient went up butthe compresin ratio is fixed.   Reason?

Len already answered this, but let me reiterate.  The compressor works harder when the delta-T is higher; it's not just a matter of running longer.  So, no, the compressor does not draw the same running current under every circumstance.  When it is hotter and/or more humid outside, the compressor actually has to work against a higher head pressure.  I've seen my units draw as little as 11.5 amps and as much as 14.5 amps when running.

BTW, as I wrote in the other thread, I ran two of these A/Cs just fine on a 5 kW generator.  At 20 amps each for start up, even if they start together that's 4,800 watts.  It was a little Onan Marquis on a Fleetwood gasser.

That makes perfect sense.  You can just get by...and that has always been my question cause "what else you run" will change with each Knut.

Let me be more clear.  That 5.0 kW Marquis ran everything on the coach, not just the two air conditioners.  I never had a problem with both A/Cs running plus whatever else I was using, including the microwave.  That said, I also can't say I remember both units trying to cold-start when I was, say, nuking dinner.

I didn't call you on the 6.5 genny because you told us not to... for all I know, you have a 3,600-watt battery charger that runs every time the generator does (not an uncommon configuration), which would leave you just 2,900 watts for your air -- not enough, as I just described.  Now, if you want to discuss that problem, we can, although perhaps a new thread would be appropriate.


I do  and this is it.  Is the 2.900 watts  what is left after you subtract efficiency numbers for the inverter?

No, I was not including an inverter there.  But when the genny is running, AC is merely passed through the inverter so inverter losses are not relevant.

Here's the thing:  If you have a 6,500 watt genny, and, say, a 30-amp battery charger, then you need to make sure that charger is not drawing a full 30 amps when running two air conditioners.  That 3,600 watts uses up more than half the genny.

If your charger is part of a smart inverter, and you are using the pass-through to run all your loads, including the air conditioners, then the inverter/charger should automatically reduce charge current as the load climbs.  But if you either don't have this type of inverter/charger, or you bypass some of the load around the inverter where it can't be accounted for, then you'll need to allow for this manually.

-Sean
http://OurOdyssey.BlogSpot.com

[bevans6]

Sean said "Here's the thing:  If you have a 6,500 watt genny, and, say, a 30-amp battery charger, then you need to make sure that charger is not drawing a full 30 amps when running two air conditioners.  That 3,600 watts uses up more than half the genny."




My 12v nominal 30 amp charger is 30 amps at say 13.5 volts, for circa 400 watts.  Call it 3.3 amps at 120 volts.  Unless I am missing the point somewhere?

Brian

[JohnEd]

Thank you both for your patience and information.  Head pressure is a foreign topic to me but I think I know what you must be talking about.  With that, it makes perfect sense.

Dometic Penguin low profiles come in 11KBTU.  They weren't made back when I firs heard these numbers...I think.

I got my call back from Dick Wright a while ago.  I count him as the guy that has the last word about what "will" work....theory math or whatever.  And he uses all three of those plus observation.  The load management device that I described where the start cycle is inhibited in the B unit if the A unit is in start cycle never existed as far as he knows.  He said there was a gimmick device that switched between A and B at the ac input power on a cycle.  Dumb, no?  He also told me that my understanding that the start current was twice the run current was not correct as the actual start current may be 7 times or even higher.  Only a flash and the breakers don't respond and the genny will absorb "huge" overloads for only a fraction of a second, also.
Dick also said that running two together you would almost never see them try to start at the same time.  As you told me, Sean...and I didn't doubt you for a second.

Well, back to cleaning the carb on the genney  and putting some carbon buster in the cylinders.

Thank you all,

John

[Sean]

My 12v nominal 30 amp charger is 30 amps at say 13.5 volts, for circa 400 watts.  Call it 3.3 amps at 120 volts.  Unless I am missing the point somewhere?

Sorry, I meant 30 amps at 120 volts.  For example, my charger is 33 amps at 120 volts, for nearly 4 kW.  I have a 24-volt bus, and the charger puts out close to 140 amps at charge voltage.

A 400-watt charger would be pretty much a trickle charger.  OK if you mostly stay where there is a power hookup, giving you all night to charge batteries.  If, like us, you charge mostly from a generator when parked, you want the biggest honking charger that your batteries will take; our 920 amp-hour bank could easily be charged at 400 amps if we could produce that much current.

-Sean
http://OurOdyssey.BlogSpot.com

[Sean]

Dometic Penguin low profiles come in 11KBTU.  They weren't made back when I firs heard these numbers...I think.

Penguins are what we have, and AFAIK, they only come in 13,500 and 15,000 BTU/hr ratings (I stand corrected on this; I see now they also have an 11,000 unit).  I believe there was a Brisk-Air model in 11 or 11.5 kBTU/hr.

..  He also told me that my understanding that the start current was twice the run current was not correct as the actual start current may be 7 times or even higher.  Only a flash and the breakers don't respond ...

Yes, this is the reason why HACR breakers are used.  This type of breaker allows very high inrush current for a very short period of time, for starting motors such as those in A/C compressors.  If that high current lingers for even a fraction of a second too long, the breaker will trip.  This is what happens when you try to restart an air conditioner too soon after shutdown, as the compressor is trying to start against too high a head.  The amount of current a motor will draw when the shaft can't turn, such as when the compressor head pressure is too high, is called the Locked Rotor Amps or LRA.

-Sean
http://OurOdyssey.BlogSpot.com

[bevans6]

I was, frankly, astounded at that high rate of charging on a bus - but then thought "what if I make that little DC power plant thing, that's gonna have that 50DN alternator and have 6500 watts of charging potential..."  so why would a mere 4000 watts of charging make me surprised...   But I was surprised, that is a fast charge, but I think you have AGM's, which do accept a high rate of charge quite handily.  But way cool  none the less.

What do you calculate your charging time is for your batteries if you are driving?  Typical usage, typical drive?   Can you keep things together just driving, or do you really need the pedestal or generator time?

Brian

[Sean]

I was, frankly, astounded at that high rate of charging ... but I think you have AGM's, which do accept a high rate of charge quite handily.

Yes, we have AGMs, which can safely be charged at C₂₀/2, or, in plain English, half as many amps as there are amp-hours (at the 20-hour rate) in the bank.  We have a 920 amp-hour bank, so we could charge at 470 amps, theoretically, without harming the batteries.  Some in the business will say that AGMs can charge at as much as C₂₀/1, and there are a handful of folks who would be conservative and say not to exceed C₂₀/4.  For us that would still be 235 amps, which is more than either the charger (140 amps) or even the alternator can provide (after accounting for chassis loads).

By contrast, flooded cells should not be charged faster than C₂₀/5.

What do you calculate your charging time is for your batteries if you are driving?  Typical usage, typical drive?   Can you keep things together just driving, or do you really need the pedestal or generator time?

The best way to say this is that, without air conditioning, if we drove for two hours or more every day, we'd never need to plug in or run the genny.  And, in fact, when we travel, we never do.  Air conditioning, of course, changes everything.  If we run two air conditioners while driving (normal, when we need it), instead of 200+ amps being available for charge, there are only 50-60 amps available.  It would take five hours of driving in that case to make up for the ~175 amp-hours we used over night.

Of course, if it's hot enough to need two A/Cs for the whole drive, then we're probably going to either be looking for a power outlet or having to run the genny for 30% of the time we are parked anyway.  That changes the whole calculus.  This is the reason we try to stick to temperate weather as much as possible.  Right now, we're heading toward the gulf, and now that we're south of St. Louis we are hopping receptacle to receptacle.  All we need to be comfortable while parked is a single 15-amp outlet.

I have an ammeter on the cross-tie, so I can tell exactly how much current flows from the 50DN into the house system.  An ammeter on the house batteries tells me how much of that actually goes to charging.  When the cross-tie first closes, the current almost always shoots up to over 200 amps right away, then quickly retreats from that number down into the range of 60-180 amps.  If the batteries are almost fully depleted, it would be at the high end of that range, and only 20% depleted would be the low end.  As we drive we can see the needle slowly dropping as the SOC goes up.

-Sean
http://OurOdyssey.BlogSpot.com

[LEH44]

 I looked at Gas/Fuel usage per hr ratings when I was generator shopping for our bus. I was supprized at the high rate of fuel use of the gas units. Started looking at the diesel units and found they were much better. Although a little pricey on the front end the diesel is the way to go. Onan/Cummins 12500 uses .62 gal per hr at full load and will run anything you have. We charge with gen. running down the road and if the need for dry camping we have a 4 bank of house batterys and two 3000 watt inverters.