PHEV add on battery, Max voltage? what can Prius NIMH tolerate?

Awesome website, been reading for a year finding answers to all sorts of technical questions, you guys have some incredible knowledge.

Seems the Plug In Supply kit shuts off when the add on pack drops to 235v or so, which makes sense because we don't want the ICE burning extra gas to recharge it.
The PIS batteries have 277v(edit: 256v actually) at full charge, does that reduce the Prius factory NIMH battery's lifespan? How about 300v or 330v?

Starting at 318v (with DCIR raising to ~330v) would give me a desirable ~80% Depth Of Discharge in my (custom non-PIS) lithium pack, instead of the 45% DOD I would get with 277v-235v.
Yes I will focus on increasing range via additional parallel cells... once we figure out the optimal series cell count

(edit: Don't use a voltage this high, it would cause wear and tear on your Prius factory NIMH battery, see my post below)

 

Here's the answer to my own question, in case future readers want to find out: Robb said the charger for his Plug In Supply LiFePo4 batteries goes up to just 256v for a full charge.

For a while, I was starting with 283v in my custom battery pack (made from Tesla Model S battery modules) but this voltage seemed to cause extra stress to my factory Prius battery (110 degrees F and batt fan spun up enough to be heard over tire noise.) It's clear that the higher the voltage, the more wear and tear on the factory NIMH, because it forces faster charging when the PIS contactors engage.

Prius regen voltage is 260v so battery voltages above that lose out on regen. I now charge to just 256v so I can soak up regen power.