APT 750C display inaccurate voltage reading with 6V overvoltage

[Andrewol]

My APT display was inaccurate from the start, but when was new it showed 1V overvoltage only with 72V battery. After 1-2 years of use overvoltage reading went to 6V. I noticed one strange behaviour of the display: when battery is disconnected from the controller overnight or more the display at start shows 1V overvoltage for some seconds, then it starts to raise the voltage slowly. Besides, my BMS and controller app are showing not the same but relatively correct voltage.
Do you know in what way the battery voltage is measured in the display? Is there a way to adjust voltage reading by replacing some SMD parts?

Thanks for your help.

 

[amberwolf]

It probably uses a resistor/zener as a voltage reference to the MCU. It might be just a resistor voltage divider. Either one is vulnerable to solder problems causing resistance not intended as part of the circuit, and to changes in resistance of the parts that causes voltage changes.

It might also use the RTC battery as a reference, so as that drops in voltage the other readings based on it change by however their code is written to use the reference.