The last time i used this charger it didn’t shut off (so called Smart) and the cells were almost too hot to touch.
i suppose the high temperature is harmful to the cell lifetime.
Opened it up and found a loose solder joint where the trace was lifted from the board. This was on one of the arms that makes the + contact to one of the 4 cells. There is a 14-pin IC but the marking code has been ground or etched off. i can see it has current sense resistors but no temperature sensing device. More reverse engineering needed.
What is a normal temperature increase that might be expected during charging of AA NiMH cells?
Warm > noticeably warm.
Too hot to hold is TOO hot and damaging.
Very old or poor batteries with high internal resistance may not terminate properly in a good charger.
You SURE you even want that charger? Did a bit of searching and does not sound like a great piece of kit, charger or batteries.
You can get decent basic multi-chemistry chargers inexpensively. You can get good ones for not a whole lot more.
none of the aldi nimh products are worth anything.
cells get 500mohm IR after only 30 cycles.
now IKEA is a completely different story. even their new cheap stenkol charger is good. terminates a bit cooler than my BC700 as it uses 0dV.