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 Apr 26 '19 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 Apr 27 '19 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 May 15 '19 at 18:42

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 Apr 27 '19 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 May 15 '19 at 18:13

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