JLCPCB Prototype for $2(Any Color): https://jlcpcb.com

Very clever active cell balancer for lithium-ion battery packs, which can transfer energy from any cell to its immediate neighbour using tiny switched-mode energy transfer modules. The transfer modules are bi-directional, so energy can be transferred either up the series string, or down.

Active Balancer BMS 3S 4S 6S 7S 13S 16S Equalizer Battery Energy Transfer Board https://ebay.us/G4Jpu7

2S ~3S 5S 10S ~17S Li-ion Lifepo4 LTO Battery Active Equalizer BMS 1.2A Balancer https://ebay.us/gxPguo