nominal voltage very high in a module

Depending on your state, Your car migh still be in the warranty window, don’t wipe the code and take the car to a Toyota dealership and get a free replacement.

 

When the p0A80 is triggered, it's normally due to one or two modules failing low. It kind of fools the car into thinking the overall SOC is low and the battery is typically placed into a charge condition, which will result in a "real" high level of "state of charge" on the good modules. The modules will typically be over 8v, as they are typically above the normal 40-60ish% range maintained by the car. A bad module will often drop in voltage relatively quickly of you allow it to sit idle. Often, but not always.

 

Voltage for a NiMH battery is 1.2 - 1.3v for about 20 - 80% charge (or 7.2 - 7.8v), but could get to 1.4v (8.4v) when fully charged, so over 8v is normal. As TMR-JWAP says, the car tries to keep the battery at 40-60%, so it normally stays at about 8v, but can go much higher, for example, at the bottom of a hill.

Just let them sit for a couple of days, and the failing modules should be pretty easy to find as they would self discharge to a much lower voltage than the other modules. You can also test them with an RC hobby charger/discharges to get a more exact capacity value. A new module is 6.5Ah.

 

I'm not sure where you got that figure ... nominal voltage for a six-cell NiMH module is lower than that, 7.2 volts specifically. But also, nominal voltage is just that—a number that is named for talking about things—and as CatNinja explained, it's not weird at all for the actual voltage in a charged battery to be above that.