I'm going to bump this thread because I have a very similar question. I just bought a 2012 Insight and I'm new to this forum, and so far I have read through only a portion of all of the previous threads and posts on battery capacity and such (there are a lot of them
To me, this topic breaks down in to two scenarios:
1. the PHEV scenario, where the goal is to increase MPG (and accel) under all driving conditions by essentially using stored energy provided by charging an extra battery at home from the grid. This is great for people who want increased MPG under all conditions, but it is a fairly expensive option because in addition to an extra battery, there is is an entirely separate bms, power converters, etc., and the system must be charged from the grid each night.
2. A simpler scenario, where a user, such as myself, is happy with the nominal 50 MPG or so that is obtained under "normal" driving conditions, but who lives in a hilly area where the IMA battery constantly "full cycles" and runs out of assist on even moderately long hills, resulting in reduced fuel economy. The battery then fully recharges before the end of the next downhill stretch, wasting a lot of regen potential. In this case, a simpler, and hopefully cheaper, solution would be to just add pure battery capacity and not a full PHEV system.
Basically the idea would be to just parallel extra battery capacity, and still use the IMA's existing charging, discharging, and bms capability. The idea isn't to increase the nominal MGP, as with a PHEV system, but just to enable the IMA to assist the ICE over the larger ups and downs encountered in hilly areas.
Again, I haven't read the entire back history of all of the threads yet, but has anyone heard or seen any more on this in the year since the original poster started this thread? I'd be very interested in this. From my limited reading, it seems like with some work, the actual terminals of the HV battery are available and it might be possible to parallel an external battery, but I am probably greatly oversimplifying the difficulty of actually doing it