It's a Panasonic BQ-390 AA charger, which used to be renowned as the best "smartest" AA charger.
The addon is an ESP32C3 with a 96x64 color OLED display, and an ADS1115 ADC.
The flashing dots indicate which cell is currently charging, as the charger only gives 0.5A to one cell at a time. The delay in voltage display is due to smoothing and filtering out stray voltage readings.
It can be powered either from the 5v regulator coming from the AA charger itself, or external USB power using a switch - the regulator in the charger isn't strong enough for the ESP32C3 to connect to wifi, so if I want to track charging status over wifi, I have to give it the extra USB power.
I got a lot of help from this ancient Japanese blog post I found of someone else who did a similar thing to the same charger:
http://act-ele.c.ooco.jp/jisaku/BQ390/bq390.htm