Greenworks markets a range of power tools such as leaf blower, snow blower, string trimmer, etc. They mostly use a common 80v battery and charger system. I happen to have a couple items and each came with a battery and a charger. They're all interchangeable. The extra battery is handy when you have a big job, like leaves in the fall or snow in the winter. A spent battery can be almost fully charged during the time you're using the other one. After two or three batteries, I'm ready for a recharge myself.

Anyway, one of the chargers is having a problem: Charging starts as usual (green light, fan starts) but soon stops and the red warning light flashes. This typically indicates a problem with the battery, such as overheating. But I know both my batteries are fine. Everything works if I avoid this one charger.

So I took the charger apart and dug in, only to find it's far more complex than I imagined. About the only thing I know to check is that the battery is in fact connected to the PCB. The symptoms I have could result from the battery's thermistor not being seen by the charger, or maybe even a poor connection to the power poles.

Anything else to consider? I'll get some photos as I dig in. Here are some pictures of an identical unit getting a fan replacement:
https://www.ifixit.com/Guide/Greenworks+Pro+80V+Battery+Charger+Cooling+Fan+Replacement/159384

Here's a video where the battery pack is the problem, same symptoms at ~2:45 but a different cause than mine:

 

Here are photos of my unit: (no giggles!)