Botvac won't charge battery

I have both a Botvac D3 and D4. Recently, the D3 stopped working (dead battery). When I put it on the charger, it glowed Red/Orange (fading in and out). The app indicated that it was charging, but it never did. I assumed the battery was dead, so I bought a new one. After two cleaning runs, the behavior was the same. I put the battery in the D4 and things worked fine. So, I've determined that there is something wrong with the D3. I took it apart, cleaned everything and did a visual examination of the circuit board and wiring, but didn't see anything amiss. Has anyone else experienced this behavior?

Thanks,

Shawn