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Old 03-28-2008, 12:01 AM   #1
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Powering off inverter while in motion

I have my "house" batteries -- three 12v AGM's -- connected in parallel to my two starter batteries with a cutoff switch. If I'm driving, and have the switch set to where the batteries are being charged by the alternator, is it bad to have the inverter on and using stuff... like some fans for circulating air? Is there any reason doing this would be bad for any of the equipment?

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Old 03-28-2008, 12:35 AM   #2
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Re: Powering off inverter while in motion

I have no personal experience or expertise, and I am not sure this helps with your question, but I came across a "combiner" that is supposed to make it safe to charge flooded and AGM batteries together from an alternator. See it here:

http://www.yandina.com/mixedbattery.htm

I've not tried that product - just found it via google.

hth - Tony
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Old 03-28-2008, 01:25 PM   #3
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Re: Powering off inverter while in motion

Hmm. I didn't realize there would be a difference for the alternator to charge the AGM's. I haven't tried it yet. Does anyone know if there will be a problem with your typical bus alternator charging the starter batteries and when the switch is closed, also the AGM deep cycles? I don't want to damage these $150 batteries.

Thanks,
Hans
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Old 03-28-2008, 09:26 PM   #4
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Re: Powering off inverter while in motion

Using the inverters with the alternator on won't cause damage for the most part. The alternator is going to try and supply all the current for the inverter's demands which may prove to be tougher on it over the long term, but I wouldn't fret. Meanwhile the batteries are going to suppress surge loads from hitting the alternator.

The AGM's might be a bit of a concern because of their charging algorithm. They really don't like to see more than about 13.5 volts for charging. Now this is a bit contradictory to what Optima might tell you about their batteries, but remember that those are automotive batteries in the first place and they are in the business of selling you new ones the day after the warranty expires. So what would I do...

It might be a bit of a project to tackle, but I would consider installing a second alternator devoted solely to powering the inverters and charging the house batteries. This will give you a back up and take the strain off the crucial engine alternator. It will also give you another benefit in that you can use one with an external regulator which will allow you to run a lower voltage such as 13.7 or so at the alternator. Factor in insulated voltage drops and you're right at about 13.5 for the batteries. The cool thing is that an alternator producing 100 amps at 13.5 volts is working less than one producing 100 amps at 14.4 volts because the wattage is different. Your inverter isn't going to know the difference, but the heat generation within the alternator will mean you will see a difference in life expectancy.
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Old 03-28-2008, 10:30 PM   #5
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Re: Powering off inverter while in motion

I don't know about agm's, but i agree that running your inverter from an alternator is a good idea. Adding a 2nd alternator just to run the "house" system including the inverter is pure genius!

Quote:
Originally Posted by the_experience03
The cool thing is that an alternator producing 100 amps at 13.5 volts is working less than one producing 100 amps at 14.4 volts because the wattage is different. Your inverter isn't going to know the difference, but the heat generation within the alternator will mean you will see a difference in life expectancy.

hmmmm.

lower voltage at the inverter will make more heat inside the inverter running a given load and will make it able to run fewer watts @ 120 volts.

example:

1,200 watts @ 14.4 volts = 83.3 amps.
1,200 watts @ 13.5 volts = 88.8 amps.

An alternator would make more heat with the lower voltage

lets look at it a different way real quick. Current = Heat which = Amps

the 1,200 watt inverter from the above example is rated at 83.3 amps @ 14.4 volts

83.3 amps @ 13.5 volts = 1124 watts. That means the inverter max load is 76 watts less with the lower voltage alternator.

if the lower voltage alternator is best for your batteries then i'd use it, just making the point that lower voltage = more amperage, and amperage = heat.

I"ll agree that the bearings and other moving parts might work less hard if the alternator is producing fewer watts....but when it comes to the electrical portion of the alternator ie: the windings, diodes, voltage regulator etc, they are only concerned with current which is measured in amps. Volts or watts are unimportant to how much heat is generated. The electrical guts of an alternator making 100 amps at 12 volts are working no harder than the same alternator making 100 amps at 120 volts.

here's an excerpt from the following link:

http://islandcastaway.com/stuff/wind...%20Secrets.htm

Quote:
Suppose, for example a windmill, waterwheel, or treadmill provides one horsepower of mechanical power to our 60 amp alternator. At 12 volts and 60 amps we get 720 watts out -- almost one horsepower.

Now suppose that more energy is available because of high winds or higher water head. The mill or wheel can now provide two horsepower, but because we cannot exceed 60 amps without overheating wiring or popping diodes. We only provide the maximum 720 watts at 12 volts. The additional horsepower is available, but not convertible.

Most storage banks are built from many batteries in parallel to provide 12 volts with at least 200 amp-hour capacity. Suppose that for those periods of high wind or water, that the batteries are connected in total 36 volt battery pack and that the alternator is regulated by a special 36 volt regulator. Suppose, too, that we run the current all the way up-to 60 amps output. Now we are converting 36 volts X 60 amps, or 2160 watts -- almost 3 horsepower. If the voltage could be run up to 120 volts, total watts at 60 amps would be 7200, ten times that available at 12 volts from the very same alternator.

At first impression you might think that the alternator could never handle it, but it can. Voltage is limited by the thickness of insulation on the windings and breakdown (PIV--peak inverse volts) voltage of the-diodes. Current through the windings and diodes produces heat. 'As long as the manufacturer's rated maximum 'current is not exceeded, the windings and diodes will not overheat and melt. If you can provide the mechanical power at an excess of 5000 shaft RPM, you can extract the 7200 watts without electrical damage. REMEMBER: The waste heat generated in both the diodes and windings is proportional to the current being produced whether it be at 12 or 120 volts.
Sorry to hijack the post.....
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Old 03-28-2008, 11:30 PM   #6
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Re: Powering off inverter while in motion

Algorithm????? Yikes!!
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Old 03-30-2008, 01:34 AM   #7
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Re: Powering off inverter while in motion

I would agree with you on the heat thing in the inverter, Jason, but I would argue that it would probably be limited to just the input side of the inverter. I don't know this to be fact, but I would venture to guess the input side of the inverter is voltage regulated using MOSFETs, much like you might find in an audio amplifier. In that sense it probably wouldn't much care if it saw 13.5 volts or 14.4 volts. The transistors certainly are going to run a little hotter or cooler depending on the voltage, but everything else would be otherwise unaffected. It would be interesting to get the input of an electronics guy on this.
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Old 03-31-2008, 11:55 AM   #8
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Re: Powering off inverter while in motion

Jesper,

Thanks for the info. That gives me some peace! Right now I use the bus day-by-day so I don't need to charge while driving. But, when I do eventually have that need, I'll have more confidence in it.

Thanks,
Hans
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Old 07-27-2008, 01:51 AM   #9
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Re: Powering off inverter while in motion

If I might revieve this topic with an add on question.
What is the normal output of an alternator in Volts? I know on my pick ups its normaly around 14+ somewhere. However my Thomas at high idle only puts out 12.95V
Is my Alternator going bad or is this common at high idle, will output increase if I'm on the road?
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Old 07-27-2008, 04:36 AM   #10
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Re: Powering off inverter while in motion

Even without my inverters on my power "surges" at low idle. The defroster fans and heater fans run noticeably slower and my lights are dimmer besides doing the "surging." If I kick it up to 1100-1200 RPM (high idle) everything smooths out and it works fine.

Where are you reading your voltage from? 13.5 or there abouts seems reasonable at low idle. Your 12.95 could indicate 13.5 with either an insulated voltage drop issue or a ground side voltage drop issue. Have you checked all your connections? I would check not only the power cable, but also the grounds and the wiring to the gauge itself...unless of course you're using a meter at the battery.

That said, 12.95 is low voltage, but is higher than a lead acid battery's fully charged standing voltage so you should be getting some charging. What are you getting at higher RPM's? Also, how old is the battery? Internal resistance, best measured with a conductance style battery tester, will cause low voltages like you describe. The battery is still functioning, but is incredibly inefficient due to internal resistance from age. The solution is to replace it.
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