In my warranty book for the 2006 HCH it says;
Integrated Motor Assist System (IMA)
Battery Assembly (included all internal components) 10/150

it also stipulates Federal & PZEV, also states, Ca. Ct, Ma, ME, Ny, Vt.

Why buy extended warranty?
To cover the Trans, and ac components I guess.
I mean 10 years? 150,000 miles is a long time folks.
Are you going to own that car that long?

I firmly believe the battery and components will last a very long time.
I am planning to keep my car 4 to 5 years.