I don't get many reports of hybrid batteries needing to be replaced. Can't even remember the last one. But what happens is the electric range on start-up gradually decreases. So, from a new range of maybe a mile or so, it goes down to maybe 50 metres. However, the hybrid function of re-charge/discharge continues to work so you still get reasonable economy. Just not as much as you got when the car was new. An ageing hybrid used only for short distances can go down from an mpg of over 60 to around 40.