Great question! You can find a lot of answers to your hybrid related questions here:
As for whether or not you need to replace the battery, here is some info I found:
How often do hybrid batteries need replacing? Is replacement
expensive and disposal an environmental problem?
The hybrid battery packs are designed to last for the lifetime of the
vehicle, somewhere between 150,000 and 200,000 miles, probably a whole
lot longer. The warranty covers the batteries for between eight and ten
years, depending on the car maker.
Hybrids use NiMH batteries, not the environmentally problematic
rechargeable nickel cadmium. "Nickel metal hydride batteries are
benign. They can be fully recycled," says Ron Cogan, editor of the
Green Car Journal. Toyota and Honda say that they will recycle dead
batteries and that disposal will pose no toxic hazards. Toyota puts a
phone number on each battery, and they pay a $200 "bounty" for each
battery to help ensure that it will be properly recycled.
There's no definitive word on replacement costs because they are
almost never replaced. According to Toyota, since the Prius first went
on sale in 2000, they have not replaced a single battery for wear and
I hope that helps! Good luck!