I've had a hybrid car since 2006. The battery system is indeed expensive to replace. Eight years was about how long my car went before the battery system (it is comprised of 100 lithium ion battery cells) was no longer holding a charge for very long and was relying almost solely on the gasoline engine for acceleration. The manufacturer (Honda) replaced the battery cells without charge though since they had determined that the software that controlled the battery system was not configured properly and caused the batteries to be maintained in a sub-optimal way, which cut the battery life short. Because the batteries were under recall due to this, they replaced them all at no charge. So while I got lucky there, it does also suggest that the batteries (in this hybrid at least) are intended to last longer than 8 years. Perhaps with a pure electric cars, they are not projected to last as long because there is so much more charging and discharging of the battery.