I read somewhere that in the very earliest experimental versions of Plug-In Prius Toyota used two 'ordinary' HiMH Hybrid batteries, one on top of the other (half filling the boot).  It was said to be an early attempt to develop the changes to the rest of the Hybrid system to manage the different charging and discharging requirements, and the changes necessary to allow higher cruising speeds and better acceleration in EV mode.  I guess they had other teams working separately on developing the LIO
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