Deebot Ozmo 950 charging issue

[strobonautiker]

Hi folks,

I've got a faulty vacuum robot "Ozmo 950" here that refuses to charge the battery. I checked the charging station and the connections, I get all the voltages to the mainboard. I recharged the battery on a laboratory supply and it works fine.

So I traced the fault to an area of the board I assume to be a switching supply for generating the charging voltage and I found a faulty diode that does not read any forward voltage in either way. I tried to exchange it with a random rectifier diode and I got a change, that means I can hear a buzzing sound now coming from the mainboard and the voltage on one side of a mosfet has changed from the full 20 V that come out of the charging station to roughly 0.3 V below the battery voltage, in this case 14,5 V. This means there is still no charging going on right now. I guess that the forward voltage of my random diode with its ~ 0,5 V might be too high for the circuit and I might need one with a lower Vf.
Now my problem is that I don't know what spec the original diode has. I identified that manufacturer of this diode to be Good-Ark, but the marking just states KN BZ or maybe  KN B2. I went through I think all the datasheets on their website but could not find any parts with this marking code. Do you have suggestions for a beefy Schottky diode with 0,2 V Vf?

Possibly the Mosfet? could be faulty as well. It reads a diode one way over D-S but I cannot figure out its type from the marking. The manufacturer's logo is an A within an ellipse, so maybe also from Good-Ark but it differs from the diodes. The numbers read 31321 and EL9327. 

Do you have any tips on how to proceed or figure out the marking codes?

This failure seems to be quite common with this model. I even found a comment on an Ecovacs video on replacing the battery, that after around 2 years the mainboard may need replacement. But a replacement mainboard costs 135 € and is not available right now.

Cheers!