Strange Circut Board Between Charging Port and BMS

[TheBikeMan]

Hi All,
We have been fixing and building electric bike batteries for a few years now. I am currently working on one that stopped accepting a charge. Upon inspection, I discovered a tiny circuit board installed in the negative wire between the charging port and the BMS. Here's a photo.
As we have never seen this, we are uncertain as to what it is or what purpose it serves. Any ideas are welcome.

 

[TwoTired]

The plus and miinus signs at each end and what appears to be a band near the minus side of the black components suggests a diode, or rather two diodes in parallel. It would prevent reversing the polarity of the charger.