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Old 03-23-2011, 08:41 PM   #1
Join Date: Feb 2011
Posts: 17
Year: 1997
Coachwork: Amtran
Chassis: International
Engine: T444E/Allison pusher
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inverter- alternator size

Hello everyone;
I am attempting to power my bus with an inverter while ridin down the road. how big of an inverter and alternator combination should i go with and what are the down falls. right now i have a 2500 watt pure sinewave (5000 peak) and what i believe is a 160 amp alternator. it seems to work ???? but for how long i dont know. when i run the tv and powered ant. refer (high efficiency residential) and the ac (13500) all runs fine for about 15 minutes. my amp draw on the 120volt ac side is 19.8 amps and on the 12 volt dc side feeding the inverter i draw 44 to 46 amps. the alternator gauge on the dash never goes to the discharge side, (stays in the green)but it still shuts down after 15 minutes even only pulling 2400 watts. I only have the two chassis batteries right now. thinking of adding a couple of deep cycle batteries next. oh and the bus is a 97 amtran with international t444e engine. appreciate the input thanks
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Old 03-23-2011, 11:34 PM   #2
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Re: inverter- alternator size

Since you are staying under the continuous duty 20 amp rating on the AC side (2500 watts/125 volts) let's look at something else. Your DC and AC current readings don't jive. Watts equals amps times volts. To find the 12-volt DC draw required, multiply the AC amps by about 11 to include inverter efficiency losses. When the AC is drawing 19.8 amps, the inverter is drawing about 218 amps. When the DC is drawing 44 to 46 amps, the AC could only put out about 4 amps.

My guess is that you did not make the readings at the same moment, and the refrigerator or air conditioner cycled off between the AC and DC readings.

So, not seeing your system, I have two guesses about the shutdown:

1. You are drawing more current than the alternator can replace, and a low-voltage disconnect in the inverter is shutting down to save the batteries, or
2. There is not enough cooling where the inverter is mounted, and a temperature sensor is shutting down the inverter to protect the solid-state devices.

Either of these could also be made worse by DC wiring that is too small, too long, or poorly connected.

[geek speak warning]
Lets say you are running 2500 watts of AC loads and have 90% inverter efficiency. While charging at 14 volts, you will draw 2500/14 or 178.6 amps to power the AC, allowing for the losses in the inverter figure that the total DC draw would be 198.4 amps. This is equal to a resistor that is 0.07 (7/100) of an ohm. If I add 0.03 ohm of added loss in the cabling, the voltage at the inverter is now only 70% of the battery voltage, or in this case 9.8 volts. But many inverters will try harder, and attempt to draw 283.4 amps at 9.8 volts to continue supplying the requested 2500 watts AC. This causes the voltage to drop even more, and the current goes up more if it can. Meanwhile, the "lost" voltage is actually heating high resistance connections, and could start a fire.

So, the lower voltage due to an "almost good enough" DC wiring draws ~ 280 amps instead of ~ 200 amps, increasing heating of the solid-state devices, and causing either an early low voltage or a thermal shut-down.
[/geek speak warning]

So the best practice is:

1. Mount high-current devices like inverters very near the batteries, and keep the wires short.
2. Use the heaviest wire possible, without being too big to fit the connectors securely, or cause mechanical stress on them.
3. Make sure all connections are clean and tight. "Good enough" isn't good enough with high current.
4. Make sure the inverter can get a reasonable amount of air circulation, and any fins or fan ports are not blocked.

You may want to wire a voltmeter to your batteries, and not look at a charge/discharge ammeter only. See if the battery voltage is coming up when charging, (13.2 to 15 volts) or instead just breaking even (12-13 volts). You also may want to compare the voltage at the inverter with the voltage at the battery under full AC load to see if there are serious (more than ~ 0.3 volt) voltage drops.

Good luck!
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.
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Old 03-24-2011, 04:30 PM   #3
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Re: inverter- alternator size

I would add high amperage fuses to the DC+ connection. When you have cables capable of carrying that kind of load, a short is more like a welding arc than like simple 12VDC sparks. Look at your cable routing closely, and install chafing gear to anything that even looks like it will rub/abrade.
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Old 04-13-2011, 08:42 PM   #4
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Re: inverter- alternator size

thanks guys for the great answers, I now have a trip under the bus's belt so to speak. first let me try and explain or answer some of your concerns you both had. first, the length of cable from batts to inverter. neg. side is a 24" long 5/16" (fine stranded) and the pos. side is 30" long and 3/8" in dia. (same materials)this is the copper dia. not including the insulation. I dont know the awg or gauge off the top of my head but when i bought them i know i got what the inverter manual recommended. for the 24" length so i stepped up the possitive side since it was 6" longer. i can tell you these are not heavy enough, they get pretty hot, pretty quik. I will be changing to some welding cable lead that is double what i am using now.
next concern you had was the bulkhed penetrations, no worries i own a welding and fabrication business and am very familiar what this type of amperage can do. at the penetration i made metal wraps 2'' long for each cable and siliconed them to the cable then siliconed them through the holes using some uhmw gromets i made on the lathe, rock solid with no chance of movement. gonna suck getting them out to change cable size though.
next issue is airflow , no worries there wide open under a bed frame , not an enclosed space . I mounted it on an aluminum 1/8" plate which I perferated with 1/2" holes and formed to make a space of 2" below the unit 6" above and all around or greater. all cooling fans are working.
it still wont run the air conditioner only with all other circuits turned off.
the one thing I did not account for was the efficiency rating, but it still should be in the range. I will keep tinkering
one thing i am concerned with is when i run the inverter it blows the fuse in my flat screen, so i must be getting a surge when it kicks out cause you dont know till you plug back into shore power that the tv dont work anymore. thanks again guys
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Old 04-13-2011, 09:09 PM   #5
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Re: inverter- alternator size

redbear, i see what your saying on the numbers not jivin, i'll have to recheck things and repost . my guess is i dont have enough alternator output. I did get 3 more deep cycle baterries that will also be going in when i up the size of the cables. thanks again
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Old 04-18-2011, 01:19 PM   #6
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Re: inverter- alternator size

With big inverters that i've used in the past (2k watt) i prefer to use very short 00 (double aught) welding cable with heavy duty copper connectors. I generally mount an inverter inside the bus directly above the batteries (which are mounted outside the bus under the floor)
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Old 04-19-2011, 11:01 AM   #7
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Re: inverter- alternator size

Originally Posted by gseufzer
... it still wont run the air conditioner only with all other circuits turned off...
I don't think you will be able to run an Air unit over the road unless you have a generator or a huge amount of batteries. I know our 13.5k BTU Coleman Roof top Air drags down our 2.8k POS Onan generator with the surge startup. For over the road Air, you might want to look into an after market automotive air unit and reserve the AC air for when you are hooked up to shore power.
This post is my opinion. It is not intended to influence anyone's judgment nor do I advocate anyone do what I propose.
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