I'm now working on my senior design as well, and I have a question if you guys happen to see this! I saw that replacing the 4.2 volt chip with the 8.4 volt chip allowed you to charge two LiPo cells. In my case, I need to charge a 2S 7.4 V LiPo battery pack for use in an RC car (using solar power). I have seen two potential solutions to the multi-cell issue on adafruit: either do the chip replacement as you did, or make a simple balance charger and charge the LiPo cells in parallel.
The advantage of the chip replacement is that it seems like I could still charge the battery and simultaneously run the car (maybe I'm wrong), but with the simple balance charger there is a switch and the car is in either run mode or charge mode. So my questions are:
(1) Is it difficult to replace the 4.2 volt chip with the 8.4 volt chip?
(2) Did you run into any problems because of uneven charging of the LiPo cells?
(3) How could you tell when the LiPo battery was fully charged or when the battery was low? Was overcharging of the cells prevented?
Sorry, but I'm in Mechanical Engineering and don't know much about electronics. It seems like the safer route is to go with the balance charger, but it would be really nice to run our car and charge the battery at the same time!