My reason to post it on EcoModder first was the vast experience of people there with EV builds and conversions.
Limiting the current to safe levels has always been the object, and just to be sure I will start with a big resistor in the circuit. A current limiting step-down DC converter would be great, but even if such a thing exists it will probably be too expensive to meet its ROI point ever.
I will monitor pack temperature (I have a thermometer for it ready and waiting)
My choice for LiFePO4 is no flight of fancy.
I like their relative safety; they may overheat when severely maltreated, but won't catch fire and burn.
They can be recharged over 2000 times without losing much of their capacity, even on near full cycles.
Unlike other lithium battery chemistries LiFePO4 cells keep the voltage quite steady throughout almost their entire range. For example:
This 12S pack start out at 39 Volt right from the start and keeps the voltage above 38 Volt until it has lost 80% of its charge.
The Insight IMA battery voltage hovers around 107 Volt, 1/3 of that is 35.33 Volt. This pack reaches that at about 95% of its capacity spent.
I would not want to discharge it deeper than that so it seems spot on.
I do not expect high currents to and from the parallel pack, at least not for long. The OEM pack buffers those.
Once the OEM packs voltage has been raised the current flowing from the booster pack will dwindle to less than 1C. When the Insight uses EV mode the voltage will drop, but not by as much as now because of the higher SOC and being fed from the parallel pack.
And if I can recharge the parallel pack before it is fully discharged, the OEM battery will still be in higher SOC and will not draw much current from it even from the start.
So I see no reason to use anything else than LiFePO4.
Using another NiMH hybrid pack is out of the question. That would be too heavy to be portable and takes up too much space.
Portability is a must, I will not charge the pack while it is in the car and at times I do need the space the pack would take.
The OEM NiMH pack is capable of handling the big discharge on acceleration (even better so when constantly trickle fed from its booster pack).
It does not need more NiMH.
Things to do on short notice:
Search out the right cabling, batteries, frame, docking connector (portable, right?), BMS or cell monitoring stuff, etc.
While I'm at it and to test the waters with one supplier of batteries I think about replacing my 12V battery with a 20 Ah 4S LiFePO4 block like this
Any thoughts? I mean, "What could possibly go wrong"?