Electricalanomaly

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vcollazo
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Electricalanomaly

#1 Post by vcollazo » Wed Jul 20, 2016 8:33 am

I have a 1968 PA31-310 Navajo with an aftermarket AC unit that uses what I believe is about a 40 amp motor to drive the compressor. The battery is a 17amp hr wet Gill that's 2.8 yrs old. The alternators are 50 amp. Last friday it was so hot I had to run the AC continuously at cruise altitude of 6500ft. I noticed that my ammeter was showing a constant 20-25 amp charge to the battery which was unusual and the voltage was 27.8V which was normal. When I turned off the AC in cruise the ammeter indicated zero, but as soon as the AC went back on I got the 20-25 charge indication. The airplane started normally twice that day. On Sunday when I went to fly my daughter's family home, the battery was dead. The grand kids swore they had not touched the switch for the aft cabin light which is on the hot bus. I thought of getting an APU to start but remembered there is a note in the AFM to not attempt takeoff with a DEAD BATTERY BECAUSE THE ALTERNATORS NEED A MIN OF 3V TO EXCITE THE FIELD. I suspect that I have a bad battery since the plane often goes 2-3 weeks between flights. I'm getting a new battery tomorrow, but I don't know what to make of the ammeter's berhavior. Any educated thoughts on the matter would be greatly appreciated.
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dirtdr
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Re: Electricalanomaly

#2 Post by dirtdr » Wed Jul 20, 2016 8:43 am

it sounds like your alternator is behaving correctly. with the A/C off, you shouldnt have zero charge, but it may be close to zero if you have minimal lights, radios, etc on. if the battery is weak or dead, you should still see your 28v being generated from your alternators. If your battery has no staying power, it might hold enough charge between flights to start the engine, but not for an extended period. The thing to check would be the battery voltage immediately after shutdown.

I say get that new battery in and it should be ok
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Heliian
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Re: Electricalanomaly

#3 Post by Heliian » Wed Jul 20, 2016 10:45 am

vcollazo wrote: wet Gill that's 2.8 yrs old.
This would be the most likely cause. It sounds like it just can't handle the draw anymore.

We used to get about 12mths on one in the heat. 2.8 yrs is great!
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culver10
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Re: Electricalanomaly

#4 Post by culver10 » Wed Jul 20, 2016 12:46 pm

You can buy a digital voltmeter that plugs into the cig/aux plug to see what your voltage is doing. I even saw one that had voltmeter and 2 USB charging ports the other day. Knowing your voltage is extremely important in diagnosing electrical issues. http://www.ebay.ca/itm/2-Ports-12V-24V- ... SwwpdW86yS
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TeePeeCreeper
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Re: Electricalanomaly

#5 Post by TeePeeCreeper » Wed Jul 20, 2016 5:59 pm

culver10 wrote:You can buy a digital voltmeter that plugs into the cig/aux plug to see what your voltage is doing. I even saw one that had voltmeter and 2 USB charging ports the other day. Knowing your voltage is extremely important in diagnosing electrical issues. http://www.ebay.ca/itm/2-Ports-12V-24V- ... SwwpdW86yS
Good point culver10!

Alternatively, most Garmin units wired to draw current from the A/C can also give the user a voltage read out...
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CID
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Re: Electricalanomaly

#6 Post by CID » Sun Jul 24, 2016 8:22 pm

it may be helpful to discuss the difference between a "charge" indicator and a "load" indicator. First, figure out what you have.

A charge indicator simply lets you know if the battery is charging. After a start off batteries, the indicator should show a significant charge that decreases over a few minutes to small amount and then over more time to nothing more than a light charge during normal operation.

If you see a discharge indication when your alt/gen is on line, you likely have an alt/gen problem. If you see a significant continuous charge during flight, you likely have a bad battery or a serious load problem.

Load meters simply show the electrical load of the alt/gen. After a start off batteries it should show a significant load that decreases over a few minutes to the normal running load of the equipment on the corresponding bus(es).

If you see the load meter go to zero under normal operation, there is a problem. Either the alt/gen went offline or some protection kicked in. If it remains high after a battery start, you may have a problem with the battery or a serious load problem.

With either system, it's important to note the corresponding voltages to address the problem. Don't take the local know-it-all's advise on what the battery voltage should be unless your manual doesn't provide you with a figure. The OEM manuals often have different voltage indications specified depending on where the measurement is taken and other factors. Some recommend increasing the generator voltages in cold weather. Others warn against taking a voltage measurement from certain spots in the system due to inaccuracies that may be caused by loads. An extreme example? Don't take a generator voltage measurement from an electric hydraulic pump motor terminal especially when it's running. You will not get a useful reading.
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