Hi,
I found that reading this FAQ on one of the links provided earlier, is very, very useful:
Hybridinterfaces Prius plug-ins
He mentions the Enginer kit (on the 2nd or 3rd page of the FAQ), and says that the battery life (based on the quality of the charging) will be the make or break of low(er) cost lithium batteries.
[Edit: This page in particular is INCREDIBLY helpful in understanding EV's in general, and a plug-in Prius kit in particular!
http://www.hybridinterfaces.ca/assemblies.html
All I've got to say is -- wow! ]