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 10 years, depending on the carmaker and the location.

Battery toxicity is a concern, although today's 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.
There's no definitive word on replacement costs because they are almost never replaced. We have seen anecdotal reports of total battery replacements costing about $3,000.