If you check the prius and insight forms you will see projects where folks have make their own phev system using lifepo4 cells from China or even old prius packs. THey use off the shelf bms systems or design their own. Use those mean well power supplies for chargers and use either a direct connection to the pack voltage or buck converters to step down.
Keeping the pack in balance was just one of many issues with the enginer kit. You also got issues with the charger and converter. If the converter didnt burn up under use you would have a cell drop too low due to sag or draw of the converter, trip the bms and turn off the whole setup.
