Cell or HV battery needs replacing?

A few months ago I got the "check hybrid system" message, with yellow check engine, yellow ((!)) and yellow ! in the triangle. Took it to the dealership and they told me to change the HV battery.
I didn't have the money for that so I took it home, opened the battery, and checked each cell voltage. Thay were fine, both charged and discharged. When I reconencted the 12v, the mEssages disapeared until a few days ago when I got it 3 times in a week.
Is there a proper way to check the cells health that I am missing, or should I get another HV battery, used most likely, as the new one costs a bit too much.

 

Hello! I bought it in April, with 220 000 km, and when it first happened it was in the ussual hilly terrain I live in, 2 people inside, at arround 225000km, in the summer. Now (4 months and 6000km later) in cold weather it started to get more frequent.
I checked the voltages with a multimeter with the battery full, drained it with the A/C then checked again, both while the car was off. Then I got a small digitall voltmeter for each cell and watched them while they were draining, in ready mode with the A/C. They kept simmilar voltages 7.5-8.3v. I was carefull not to short anything.
I'm not sure about the age of the 12V battery, but it's an OEM Toyota agm battery. I think its since the car was new.

 

12.65v on idle, after a cold night. 12.2v with the headlights on. Although the hud/mfd/speedometer woult take a minute to turn on in some mornings, not sure if it's connected.
Do you know how to determine the age of the 12v on an OEM toyota battery?