[RedDevil]

Hi Planetaire, thanks!
Your awesome conversion is one of the main reasons I dare try this project.
Knowing it will come nowhere near yours in effectiveness and sophistication...

I was thinking about using this contactor: EV-Power | DC Power Contactor 100A, Coil 12V.

I may use a couple of them, to set up one circuit with a resistor to reduce the current when the voltage between both packs differs too much or the temp is low.
I could use one contactor to short the resistor when all is well, then the other to cut off all when the LiFePO4 pack hot or low on voltage.

Another point is, should I allow recharging the parallel battery on braking?
When the Insight recharges the main pack voltage can peak above 120 Volt.
When it has been pulled to an artificially high SOC by the parallel pack, it may not have enough capacity left to store the regenerated power.
Bleeding it off to the parallel pack may be a necessity; and that should be able to handle it, as this can only occur after it spent some of its energy already
(unless I'd put it in fully charged at the top of a mountain - but I would never do that).

Nevertheless, there again I'd need to take the max current the pack can take into account, maybe switch to the resistor circuit on high current either way?

I still need to find out what cables, collision breakers and current sensors to use. Safety is a must and a professional look to the project a bonus.
I don't want my dealership to refuse working on my car.

Tips on where to buy my batteries etc. are welcome!
I'd prefer EU based due to tax, transport and customs restrictions btw.