The Chevy Volt HV pack is made of multiple modules, as is usual in the EV industry. Some of those modules are conveniently 12S…
Here is a 2.3kWh module for sale:
Pros: Cheap, high quality LG chem cells, 12S with connector for balancing.
Cons: Would have to figure how to package it to the OpenPPG. May not be a good form factor. On the heavy side (likely due to packaging the liquid cooling system…).
This has a specific energy of around 140Wh/kg.
The 20 amp hour 6S Hobbyking Multistar packs are at 185Wh/kg.
Every bit counts here, but something to consider.