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In my cessna 172S, when engine running at any RPM, the current at the main battery is below zero, indicating that the alternator is not charging the battery or the battery discharges, but the standby battery indicates a current above zero and it is charged by the alternator.

How can this happen?

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    $\begingroup$ I see TAS 0kt, TIMER 00:03:29 and a slightly low OIL TEMP, so I imagine that this is soon after engine start. Is that correct? Which battery (main or standby) is the starter hooked up to? You can Edit your question to add clarifying information requested in comments. $\endgroup$
    – user
    Commented Apr 26, 2019 at 9:33
  • $\begingroup$ I take a picture when I running up the engine for maintenance, main battery that hook up to the starter $\endgroup$
    – Achmad Syahrul Angga Pratama
    Commented Apr 27, 2019 at 9:06
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    $\begingroup$ 28 is pretty high. If your machine is smart it's wise to discharge the battery a little occasionally. $\endgroup$
    – user3528438
    Commented May 15, 2019 at 18:42

2 Answers 2

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Our mechanic had a similar issue with a Piper Cherokee where the battery would only charge intermittently. It turned out there was a bad connection at the Master Switch which ran from the Alternator/voltage regulator to the Master Switch. Most of these types of electrical issues tend to be bad wiring. I would check your wiring thoroughly.

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  • $\begingroup$ I has been perform continuity test following the wiring diagram, but the result of continuity test is ok, and this problem still exist $\endgroup$
    – Achmad Syahrul Angga Pratama
    Commented Apr 27, 2019 at 9:08
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    $\begingroup$ @AchmadSyahrulAnggaPratama Wiring can be bad in ways that still allow a simple continuity test to pass. In fact, I'd say that's probably the worst kind of bad wiring, because it would result in a voltage drop which gets dissipated as heat, and/or intermittent function. In a worst case scenario, it can even lead to arcing, which in principle can start a fire. $\endgroup$
    – user
    Commented May 15, 2019 at 18:13
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One of two things: Either there is a fault somewhere between the alternator terminals and the DC aircraft electrical system and busses or you are placing a greater load on the A/C electrical system than the alternator can handle.

For electrical issues, I would have the airplane inspected by a qualified mechanic and check the alternator, the DC rectifier as well as all of the connections between these systems and the electrical busses. For load issues, take the airplane out on the ramp where you can safely do a run-up and follow the run-up checklist for your airplane. With the engine running at between 1700-1800 RPM, switch on all of your electrical components - all of the lights, the pitot heat, the boost pump, the 12V lighter, etc and monitor the ammeter to verify it remains positive throughout the runup. If it starts drawing current off of the battery, you may need to have the alternator inspected and replaced.

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