I have the same issue on 2 diffrend kind ob boards.
INormally I have to set the multiplier around 2.20 to get the right voltage.
On 2 diffrend board I have to set around 4.50.
And the result is very instable. Not usable.
I changed the Attiny from the defective boards to new ones.
They work. The Attiny is not the problem.
It seems somthing on the boards is defect.
I would like to safe them. And I assume one of the SMD parts is the trigger.
Based on the chat title you are using modules v4.4.
If so pin the cell voltage is actually measured on pin 10 (PA3).
Pin 13 is used as an accurate external reference voltage for all ADC be it cell voltage, int/ext temp.
Unfortunately to conserve power the ‘enable’ pin is only on as needed and then quickly disabled. The enable pin provides quasi Vcc to D1 (voltage reference) and R3/R4 voltage divider which measures the actual cell voltage via ADC3 (pin 10).
This short term enabling makes simple voltage measurements hard - do you have a CRO ?
That said, i once had the exact same issue (of cat chasing its tail) with some of my modules as i accidently left the voltage reference diode D1 off the modules.
Finally i can’t count the number of times i’ve pulled my hair out trying to make a change and it doesn’t stick - this is due to a stale cookie used for session tracking implemented by Stuart - all you have to do is refresh your browser. I now ensure i do a Page reload ( F5) whenever things don’t appear to be going to plan - you may find it helps.