Low voltage

[Chilito]

Hi everyone. On my C150 I have a USB voltage indicator in my cigarette lighter. After my last few starts, the amp indicator shows discharging, and the cigarette indicator indicator chimes low, indicating about 11.7 or so.

During taxi, or sometimes during run up, it swings back to charging and normal voltage again for the rest of the flight. Voltage regulator? Alternator? Engine always starts normally on the first crank. Best way to isolate the issue? Thanks.

 

[Dan Thomas]

Hi everyone. On my C150 I have a USB voltage indicator in my cigarette lighter. After my last few starts, the amp indicator shows discharging, and the cigarette indicator indicator chimes low, indicating about 11.7 or so.

During taxi, or sometimes during run up, it swings back to charging and normal voltage again for the rest of the flight. Voltage regulator? Alternator? Engine always starts normally on the first crank. Best way to isolate the issue? Thanks.

Check all the connections and wires at the alternator and regulator. Broken or loose connections at the alternator are the most common. Make sure the regulator isn't loose on the firewall. If that all looks good, the alternator is first to check. When was the last time that alternator was off for an internal inspection? They should be done every 500 hours. Brushes could be shot and conducting intermittently. They'll do that. Disconnect the field wire at the alternator and put an ohhmeter on it and ground and see what the resistance is. Should be 3 to 5 ohms. Disconnect the spark plugs (for safety) and move the propeller some to see if the resistance jumps around. It shouldn't.

 

[Wheels]

Sounds like running the engine at idle isn’t enough for the alternator to provide the rated voltage. Most alternators are rated for a voltage @ a given rpm. As long as your voltage is good when the engine is above 1000rpm then you don’t have a problem.

 

[Doug Reid]

First...if all of a sudden your shiny new usb voltage device is indicating an issue you were never aware of..verify you really have a voltage problem before condemning your electrical system. I used a cigarette lighter plug voltmeter that showed voltage problems...the voltmeter was the problem...Chinese junk.

 

[chemgeek]

The first thing to check is your open circuit voltage on the battery. If you are 12.8-12.9V likely everything is well with your charging system.

Assuming your cheap voltage meter is correct, your bus voltage should be 13.8-14.2 V or so when up to speed and the battery is charging. At idle rpm your alternator may not output sufficient voltage.

FYI 11.7 V would indicate a fully depleted battery, so that sounds a bit suspect.

 

[Chilito]

Thanks for all of the good responses. I will pull the battery and do an open circuit test. It could be something I've left out of the equation, which may prove to be a quick fix: My old Gill. Maybe, maybe not.

The cigarette volt meter (yes, it was cheap) shows approx 12.0V when I pull the master switch on - engine not running. This alone would indicate the battery is at 25% life, correct? When I begin to pull lights on for pre-flight, it drops further. Yesterday, it dropped to 11.5V - which would indicate 0% on the battery, and nothing would be powered, let alone start the engine, which it still did. I do not trust the digital meter. Looking back through the logs, however, I also can't find any new battery installation in the past five years or more - I've only owned it since last summer.

Is it possible this discharging could be caused by an almost depleted battery, which could account for why it doesn't swing back to 'charging' until during run up with sufficient RPMs to re-charge?

 

[Dan Thomas]

If one reads the original post, one sees that the ammeter went along with the voltmeter, indicating that the alternator or its system wasn't working reliably.

Next, the alternator can and will generate lots of power at idle. That's one reason alternators supplanted generators more than 50 years ago.

Electrical problems are vastly misunderstood and misdiagnosed by almost every pilot or owner and by many mechanics. I learned that a long time ago and spent a lot of time fixing stuff messed up or missed altogether by other people. Fixing charging problems by repairing a simple broken wire instead of replacing the alternator like many guys would. Or replacing the alternator brushes instead of the entire alternator. Lots cheaper. Of finding that the overvolt sensor's ground wire was loose and fixing that instead of replacing the alternator or regulator or both. Or finding and replacing worn-out master or starter contactors instead of the starter or battery that were perfectly fine.

One needs to understand both the systems and electrical theory, and that understanding is in really short supply. It takes study. A surgeon can't just go and cut someone open and start taking stuff out without knowing exactly what the problem is and where to look and what to do. A mechanic should have the same approach.