The greatest idea ever, or the worst idea ever? ("Free" BMS)
 

NiMH batteries are known to not need balancing even in series packs. Liion and LTO chemistries are known to need balancing.

Here's the idea: Use NiMH batteries in parallel with other batteries to keep them balanced!

When building a Liion pack, use three NiMH cells for each Lithium cell, or two for each LTO cell.

"But NiMH aren't free!"

No, but you get to use their capacity too! The BMS:ing they provide is free!

Stupid crazy - or crazy stupid?

Above my pay grade. Cell voltages the same?

I assume a BMS is designed and implemented based on the battery chemistry. Mixing battery chemistry sounds like a daunting task given the particularity and optimization for each battery technology.

What about energy density, discharge rate and operating temperature ? They all differ based on the chemistry. You would potentially be faced with a big compromise and possibly operate at reduced overall capacity.

Erm. NiMH batteries absolutely do need balancing. Top balancing is the easiest way.

NiMH is about 1.1-1.4v so 3x for Liion is 3.3-4.2v and 2x for LTO is 2.2-2.8v.

It needs to happen, but it does so automagically when the pack is kept full, no external cirquit needed.

Would be cheaper to buy a BMS. And/or one of the new(ish) active balancers. They're both pretty cheap...you couldn't buy components to makeshift your own for what you can buy one ready-to-go for.