Smart Adaptive 12/24V Battery Charger have high output voltage

[hdduc]

Hello,
I have a smart adaptive 12-24V battery charger, (6 stages charger), it have high output, overcharged error on stage 5.
Stage 1: startup -> ok
Stage 2: constant current charge -> ok
Stage 3: constant voltage charge -> ok
Stage 4: pulse voltage charge 14.4V- 15.5V pulse -->ok
Stage 5: pulse voltage charge (continue) 15.6V - 16.8V pulse -> the battery is overcharged.
Stage 6: output reach 16.8V -> Relay cut off ->
The output is overcharged at stage 5.
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Works: the circuit have 15.8V zener + 9013 NPN feedback (TL431 replacement), the reference voltage is 0.99V. I tried to remove 15.8V zener, then replaced by 15.3V zener+ 2K ohm to 18K ohm in series, to increase reference voltage to 1.24V. But the microcontroller controlled the output feedback impedance (i think so), and automatically boosted the output to the same level at every stage. And the output at stage 5 is still getting high at 15.6V to 16.8V overcharged.
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Do i also need to replace any resistor on the control board as well?
Does anyone have any ideas how to fix this?
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Thank you for your big supports.
~ David