dead horse
One final idea..
There is a connection to the battery from the charger wire and the charge controller sense voltage line is possibly connected to this connection.
If this connection has developed some resistance over time, it could affect the charge controller but nothing else. For example, at 60 amps, you would only need .016 ohms to give 1 volt rise at the location where the charge controller sensing was connected. The charge controller sees the battery voltage plus the extra "parasitic" voltage and undercharges the battery.
If you have a seperate starter battery, you might not see this issue and if you measured the voltage to the charge controller with no current flowing (or low current), you might not also see the drop.
The solution to this is to have the charge controller have its own seperate sense voltage connection to the battery post somehow. Since this connection now has no current flowing through it, small impedances do not cause any problem. And if your present connection does have a small impedance and you seperate the charge controller sense voltage, the small impedance should not matter.
The small impedance developing in the battery connector might explain the month to month degradation?
One final idea..
There is a connection to the battery from the charger wire and the charge controller sense voltage line is possibly connected to this connection.
If this connection has developed some resistance over time, it could affect the charge controller but nothing else. For example, at 60 amps, you would only need .016 ohms to give 1 volt rise at the location where the charge controller sensing was connected. The charge controller sees the battery voltage plus the extra "parasitic" voltage and undercharges the battery.
If you have a seperate starter battery, you might not see this issue and if you measured the voltage to the charge controller with no current flowing (or low current), you might not also see the drop.
The solution to this is to have the charge controller have its own seperate sense voltage connection to the battery post somehow. Since this connection now has no current flowing through it, small impedances do not cause any problem. And if your present connection does have a small impedance and you seperate the charge controller sense voltage, the small impedance should not matter.
The small impedance developing in the battery connector might explain the month to month degradation?
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