Open hardware trickle charger

I would like to set up several “smart trickle chargers” for different batteries and voltages around the house. I hate replacing batteries all the time so I plan to wire these in myself and void all the warranties. The batteries will be connected separately. Some metrics I would like to get back to HAOS would be 1) voltage, 2) estimated charge on the battery. Fancy products exist for doing this at 12v for cars and 120v for UPS systems; this would be the same thing but wired in perallell with rechargeable AAs directly into the device.

Worst case I will be building this from a ras-pi, but it would save me a lot of hassle to just buy them.

PS: I do know about overvolting, charge cycles, battery types, circuit design, and the associated dangers. Further warnings here won’t stop me.