[RedDevil]

I am considering to turn my Insight into a mild PHEV by adding some 1 kWh op LiFe(Y)PO4 (Lithium Iron Phosphate or LFP) batteries and stuff. Charge that at home, gently (all but) deplete on the move, to aid acceleration and to allow for P&(e)G-ing on the highway. Every time I can put 1 kWh into my commute it should save me at least half a liter of fuel, so it has the potential to save me over € 200 each year just on commutes.

The idea is to feed the IMA system with 500-1000 W continuously once power is engaged, maybe making it voltage-dependent to feed more into it when the system needs it and maybe even charge back a bit under braking.

Maybe could add some thin foil solar cells on the roof later, if the setup works fine.

The batteries have to be LiFePO4 or LiFeYPO4 type as those seem ideal for this kind of use. But I haven't looked into how to feed the IMA yet. Any tips?

Maybe I can compensate for some of the extra weight by exchanging the under hood lead acid battery for a smaller LiFePO4 battery.
Then it shoud not have a BMS system as that interferes with the high current draw while starting, but I wonder if I can safely do so. If the battery gets charged at over 14.4 V (3.6V per cell), then I guess not.
If it (the lead acid battery) gets charged at less than 12.8 v then the LFP wouldn't get charged at all.

Of course I could use a small BMSed LPF battery and a string of supercaps to circumvent these issues. But I just wanted to keep it simple.

So... what would be my chances to make this successful?
Should I create a low-amp high-voltage pack slightly over the IMA's operating voltage and just bleed it in?
Or should I use a DC-DC converter, controller and stuff?
Or is this all just a bad idea?