The main challenge to adding a PHEV pack to the original Prius was figuring out a way to trick the Battery controller into using more battery capacity. The Prius battery controller uses a current sensor to measure amps in and out of the pack, and calculates SOC primarily based on current flow integrated over time to get Ah in and out. This is commonly called coulomb counting, and is common in NimH and Li battery systems where voltage is a poor indicator of SOC. It knows how many the Ah the battery can hold, so be counting them as they go in and out, it knows what SOC should be.
If you injected current from the second battery on one side of the current sensor, the battery controller would "count it" as current coming out of the primary pack, and when X Ah had been counted as coming out of the primary pack it would assume min SOC had been reached and start recharging. It assumed a fixed battery pack size, and didn't care that you had added more battery capacity. It would still only use the amount of Ah it expected the original battery to be able to supply.
If you injected current on the other side of the current sensor, the battery controller wouldn't "see" it, and would only count the Ah actually going out of the original pack. So far so good. Unfortunately the HV controller also measures current at the inverter / motor end, and when it saw significantly more amps arriving at that end than the battery said it was putting out, it shut down the HV system assuming a fault had occurred. Not so good.
As a result there were two common approaches to PHEV conversions.
-The budget Enginer approach was to use a dc:dc converter to inject a controlled amount of current on the motor / inverter side of the battery current sensor, keeping it low enough to keep from triggering the fault/shutdown. When not being consumed by the motor / inverter the battery controller would "count" the supplemental current as regen topping up the original battery. The downside to this approach was limited instantaneous power, meaning it couldn't always supply enough current to keep the SOC of the original pack up.
- The more expensive kits generally tapped into the battery side of the current sensor, and required a custom add on controller to either replace the original battery controller, or spoof the right CAN traffic to get the HV controller to use the extra battery capacity.
I would assume the Leaf uses a similar coulomb counting scheme for managing battery SOC, and so could have similar challenges. In general the Leaf's BMS / controller seems quite a bit smarter though, as it seems like it has the ability to adapt to changing capacity over time to account for battery degradation. It would be really interesting to see if this capability extends to revising the capacity of the battery upwards
For example, as I understand it the BMS already manages 2 cells in parallel at every leg of the battery stack. What would happen if you simply added a third (or third and fourth) identical cell in parallel (being careful to match them first of course)?
If the Leaf uses simple coulomb counting and a fixed capacity (or is unable to revise capacity upward) on the next charge it would simply put in the right amount of Ah to fill the original pack, and leave the new expanded pack less than full. But if it uses some sort of adaptive technique (maybe particularly on a 100% charge?) that looks for some other markers of charge completion, its possible it might just be smart enough to revise the capacity of the pack up to account for the new cells
Maybe it would take multiple charge/discharge cycles to get it all, but that would be really cool!