Negative Amps in a 172

Murdoughnut

Murdoughnut

Well sized member
I took my club owned 172 up for the first time in awhile the other day. During the run-up, I noticed the amps were reading in the negative direction. I figured they'd jump to positive once the engine was run to full power, so I went ahead and took off. Several minutes in, the amps were still pegged at -15. Volts were still reading 23.

I decided to bring it back to the pattern just in case alternator failure was the culprit. After a circuit or two in the pattern, I noticed that the amps jumped back up to +5.

Because I'm not very good with electrical stuff - negative amps means the battery is pulling more from the alternator than it's being supplied with - correct? Should I have disregarded it due to the volts still reading normal?

Thanks!
 
That depends on where in the system the ammeter is, but yes, it typically means that current is flowing OUT of the battery.

As for the voltage, 23 is NOT normal for a running airplane. If the alternator is working right, it should be 28 volts on the system.
 
Roger said:
That depends on where in the system the ammeter is, but yes, it typically means that current is flowing OUT of the battery.

As for the voltage, 23 is NOT normal for a running airplane. If the alternator is working right, it should be 28 volts on the system.
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It may have been 28 - my bad. It was on the upper end of the scale. 28 sounds right now that I think about it.

Thanks! Glad to hear my caution wasn't unwarranted
 
If the alternator was dead, you'd be seeing 24 volts (battery's voltage), or more likely about 23 volts because nothing is perfect.
 
This used to happen alot on a 152 I flew. The "low voltage" light would illuminate and the ammeter would show a discharge. Cycling the alternator switch brought it back online.

The only time it didn't work was when the alternator belt broke. :p
 
It's very possible that the alternator is dying. Sounds to me like one of the bridge diode's is going and you are getting a crappy ripple AC current and voltage instead of DC. If I'm right, no guarantees, that battery is getting bad power sent to it and will effect its longevity. Avionics usually have enough internal capacitance to deal with the fluctuations, depends how old the stack is though.

I'd recommend having a mechanic look at the alternator while it's running. We had a simular thing on our FBO 172 years back (back when I instructed), it took a few times but they finally discovered the problem after a few electrical failures. :panic::o
 
If you were several minutes into a flight and the alternator wasn't working at all, you would likely have less than 23 volts. 23 volts sounds normal for a 172 battery that isn't under much of a load. Like others said, you may have an "intermittent" charging problem. When the alternator doesn't keep up with the electrical load, the battery picks up the rest. When that happens you see a "negative" draw. It could be slight if you exceed the limits of the alternator, even when it's working. In some airplanes, this is possible if you turn on everything in the electrical system.

So basically, no, you shouldn't have disregarded it because of the battery voltage being normal. A good battery will show nearly full voltage for a very short period of time if the alternator kicks off while there's a normal load for a 172, in flight. But if the alternator comes back either partially or full, the batt voltage may climb and mislead you to believing everything is ok. There's just not enough information available to the pilot to disregard either instrument. If either batt voltage or amps go abnormal, it's best to land and look into it.
 
falconvalley said:
If you were several minutes into a flight and the alternator wasn't working at all, you would likely have less than 23 volts. 23 volts sounds normal for a 172 battery that isn't under much of a load. Like others said, you may have an "intermittent" charging problem. When the alternator doesn't keep up with the electrical load, the battery picks up the rest. When that happens you see a "negative" draw. It could be slight if you exceed the limits of the alternator, even when it's working. In some airplanes, this is possible if you turn on everything in the electrical system.

So basically, no, you shouldn't have disregarded it because of the battery voltage being normal. A good battery will show nearly full voltage for a very short period of time if the alternator kicks off while there's a normal load for a 172, in flight. But if the alternator comes back either partially or full, the batt voltage may climb and mislead you to believing everything is ok. There's just not enough information available to the pilot to disregard either instrument. If either batt voltage or amps go abnormal, it's best to land and look into it.
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Lead acid batteries have very little voltage difference between full charge and empty charge. You can't rely on the battery voltage as an indicator of anything.
 
Ralgha said:
Lead acid batteries have very little voltage difference between full charge and empty charge. You can't rely on the battery voltage as an indicator of anything.
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It is true that lead acid batteries supply good volts to no charge, but the indicator is basically "batt voltage". That's what it indicates. It's not really usefull to the pilot beyond that.
 
Thanks for the input guys - I really appreciate it. One of the other owners experienced the same problem today, though no loss of power on a 1.5 hour flight. We're having the mechanic look at it on Thursday - hopefully just the gauge. The digital backup was reading "0" the whole time. I couldn't remember if it had been hooked up originally or not, though. Guess it had been and wasn't working.
 
Murdoughnut said:
Because I'm not very good with electrical stuff - negative amps means the battery is pulling more from the alternator than it's being supplied with - correct?
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Battery being discharged at a rate faster than the alternator is capable of replenishing. I could be wrong.
 
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