15 years in one case. But then my BM2 lithium battery is about 10 years old and as good as new. So its expected if the current levels (C rate) and upper and lower voltages are carefully controlled. They are very careful about using only around 70% of the battery capacity avoiding being full or empty if humanly possible. Sometimes it isnt due to mars seasons and dust build up. But mostly they can. And so extending the life of the cells. And just like the PL8 everything is very controlled and balanced in the same way. Albeit built into their on board computers and controlled and monitored on earth by radio signal. A bit like our BT modded pl8's! So yes they dont have a PL8 obviously, but do have its equivelent as a built in computer controlled system able to control the charge rate as they balance. Just as most EV like tesla do.
But with laptop style lithium ion cells that your phone and laptop, teslas. and mars rovers use you can determine state of charge by voltage quite easily. And so a simple BMS can control charge much more easily than it can with lifepo4 cells. The reason we are avoiding those is because each one is a very angry firework. And the dissabled cannot get off and run away. Or push it outside rapidly so it doesent set your house on fire.