Full charge or not, the car can control the descent by using the engine as an air pump in effect, almost like a diesel truck jake brake, once the capability of that is reach, then mechanical brakes are used. It all happens automatically with no input needed from the driver, neat stuff. The way I understand it, when the regen bar graph reaches the end of the display, then and only then do the brakes come into play. So the game is to keep the indicator slightly away from the end, maxxing out the regen, while not careening out of control, while also looking out for random deer, skunk, turkeys, and cattle. The only thing I dislike about all this, is someone behind me will think my brakes are being used the whole way, not being hybrid savvy, as the brake lights are activated when in regen though not using the mech brakes. Going down the mountain here with your brake lights on the entire way is a sure sign you are a tourist or worse, from town.
It looks like a 1.5 hr charge is the sweet spot to get me to the start of the down slope, and that at the bottom of it, with a full charge to easily handle the remaining more or less level ground into town. The PluginPrius is way smarter then I am, so I'll let it's onboard system worry about voltage per cell, main traction battery life span is not much of a concern in practice, according to the Prius forum I frequent. The car is pretty brilliantly engineered, and it gives me a further appreciation for the WW2 vets in winning against Japan, they are damn clever people. That reminds me, I was at the new local Hyundai dealer the other day on a crane related issue, I meant to ask someone there if they follow the news at all, I mean of course the threat of a total nuke war with NoKo can't bode well for the car company. Maybe they are made in Kentucky or Ohio, don't know.