I don't think it's an issue with the PHEV as it very rarely fully discharges (normally around 15%ish reserve). The actual capacity of the battery tends to be less than the usable capacity so it may well never actually fully charge and the BMS looks after the battery optimisation. Renault are pretty good at this when compared to other manufacturers who often misquote figures to make their cars look better than others and go for range rather than battery life. Either way, the warranty cover the battery for 8 years. When we had a Zoe we used to fully charge it all the time and the battery was still at 98% capacity after 5 years.