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Author Topic: Question for the electrical mathematicians?  (Read 1622 times)
bigtim44
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« on: August 31, 2006, 07:16:09 PM »

Here's the question how long should 1 Carrier roof air run on the the inverter starting with fully charged batteries.
Here's the system
6 new trojan t105 batteries 200A/hrs, set up in 3 pairs  12v/pair
Xantrex RV2012 inverter 2000w
Carrier roof air 12.8 amp draw when cooling.

Reason I'm asking this it seems the AC is depleting the batteries too quickly , or do I need more battery capacity
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Basalt Colorado
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Len Silva
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« Reply #1 on: August 31, 2006, 07:30:51 PM »

Just very basic without allowing for efficiencies, losses, starting current, etc.

12.8 Amps x 120 volts is 1536 watts

1536 watts / 12 volts is 128 amps

You have 600 amp hours of battery divided by 128 is 4.68 hours.

Not letting the battery get below 50% is 2.3 hours.

It takes a LOT of battery to run A/C, in the thousand amp hour range.

Len
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gumpy
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« Reply #2 on: August 31, 2006, 07:33:51 PM »

You can't run your A/C off your batteries. Not for very long, anyway. I'd expect no more than maybe 1 1/2 - 2 hours max and then you'll need to recharge.  With that large of draw, conventional math will not predict accurately. The higher the draw, the faster they will run down.

On the way home from Colorado this summer, we stopped for lunch along the way. I turned the basement air on low, one compressor, running off the inverter. In about 30 minutes, it had drawn my eight T105s (24V) down to 90%.  

craig
« Last Edit: September 01, 2006, 06:45:54 AM by gumpy » Logged

Craig Shepard
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http://bus.gumpydog.com - "Some Assembly Required"
Moof
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« Reply #3 on: August 31, 2006, 07:38:50 PM »

Not meaning to pick on Len, but 6 batteries at 200 amps is 1200 amps.  Still only about 4.5 hours in theory if you don't run your batteries below 50%. 

You also need to calculate the efficiency of the inverter.  Some of the good ones brag 85 % and better.  I think in most cases you should expect to lose at least 30% through the inverter.

Your A/C time just got shorter.  Now figure in what Craig just said.  You'll need a trailer to carry your batteries.
« Last Edit: August 31, 2006, 07:40:22 PM by Moof » Logged
bigtim44
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« Reply #4 on: August 31, 2006, 07:45:51 PM »

SO.... if I run a 24v inverter the current draw is less??    With a 24v inverter could I plumb a heavy positve  lead from the engine alternator to the batteries ? and run the ac going down the road that way.
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Basalt Colorado
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« Reply #5 on: August 31, 2006, 07:50:23 PM »

BigTim,

One more thing to add to your equation....

The hotter it is outside the more amps the A/C will draw, Adds up to less operating time than you first thought.

Then, your recovery time will set you back to where you started from after you have recharged your batt bank.

Nick-
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Devin & Amy
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« Reply #6 on: August 31, 2006, 07:55:08 PM »

Tim,

I can.

 I run an old 24v inverter when I'm going down the road. if i stop the motor then I will run my Batts down in about 1.5+/-.

You can't really run an A/C off of batteries, unless yoe have a large fancy inverter. At least I can't.
Devin
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bigtim44
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« Reply #7 on: August 31, 2006, 09:42:25 PM »

Thanks for the info chaps,
 Devin and craig(gumpy) is your bus alternator connected  to your battery bank via an isolator box?
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Basalt Colorado
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ceieio
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« Reply #8 on: August 31, 2006, 09:48:42 PM »

SO.... if I run a 24v inverter the current draw is less??    With a 24v inverter could I plumb a heavy positve  lead from the engine alternator to the batteries ? and run the ac going down the road that way.

Yes - watts are voltage times current (W = V * I) [I is current] so 2400 watts (for example) at 12 volts is 200 amps, while 2400 watts at 24v is 100 amps.  Double the voltage, half the current for any given wattage.

I run my air off my inverter (occasionally) while going down the (open) road.  It doesn't take long for the inverter to trip offline due to low volts when the RPM's are not there to supply the big amps.  Also since I run a 8v71N, I can feel the load on the engine when I hit the hills and especially at altitude.  When I run in the hills and/or at altitude, I run the genny, and often on the open road as well.

Hope this helps,
Craig - MC7 Oregon
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gumpy
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« Reply #9 on: September 01, 2006, 04:53:58 AM »

Not meaning to pick on Len, but 6 batteries at 200 amps is 1200 amps.  Still only about 4.5 hours in theory if you don't run your batteries below 50%. 

You also need to calculate the efficiency of the inverter.  Some of the good ones brag 85 % and better.  I think in most cases you should expect to lose at least 30% through the inverter.

Your A/C time just got shorter.  Now figure in what Craig just said.  You'll need a trailer to carry your batteries.

Not meaning to pick on Moof, but T105s are 6 volt batteries, 220 AH each. He has 6 of them configured at 12v (paired), so he has a about 660 amp hours, as Len stated.  Just clarifying because it wasn't obvious from any of the postings.
 
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Craig Shepard
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gumpy
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« Reply #10 on: September 01, 2006, 04:58:31 AM »

Thanks for the info chaps,
 Devin and craig(gumpy) is your bus alternator connected  to your battery bank via an isolator box?


Tim,

I connect my house and coach batteries together with a relay. I used the 200 amp relay found in the A/C junction box within the front baggage bay of an MC9. Works great. (Grainger's sells similar relays)

I charge my house batteries while driving, and can, if necessary, run the house A/C while driving (I still have coach A/C). When I'm parked and plugged in, I keep the relay closed and float the coach batteries along with the house bank. I use a Trace SW4024 inverter.

Been working well for me for a few years.

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Craig Shepard
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Len Silva
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« Reply #11 on: September 01, 2006, 06:01:49 AM »

In addition to all the other notes, battery amp-hours are calculated at a 20 hour rate.
A 220 AH battery will deliver 11 amps for 20 hours.  Any higher load than that and the rating drops dramatically.  You could not pull 220 amps for one hour.

Len
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Moof
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« Reply #12 on: September 01, 2006, 08:20:06 AM »


Quote

Not meaning to pick on Moof, but T105s are 6 volt batteries, 220 AH each. He has 6 of them configured at 12v (paired), so he has a about 660 amp hours, as Len stated.  Just clarifying because it wasn't obvious from any of the postings.
 
Quote

Thanks Gumpy, and sorry Len, I didn't read close enough.  I think I did get the part about needing a trailer right.
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pvcces
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« Reply #13 on: September 01, 2006, 06:09:32 PM »

And just one more note to add to the mix: the T-105 is about the worst battery for fast discharges that you can buy. The T-145 is among the best, if not the best. When you consider that they are the same size and weight and the same amp hour rating, there had to be some reason for their existence.

The key lies in the Peukert Exponent; check the specs to compare.

Tom Caffrey
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Ketchikan, Alaska
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