I beg to differ. The self discharge rate of Eneloop NiMH LSD (low self discharge) cells is quite acceptable. I charged some Panasonic AA cells, stuffed them in my DSLR camera, and let it sit unused for about 9 months. After 9 months, they had something like 90% of full charge available. Different generations have different self discharge rates. See table at:
From almost full to dead is about a 200mv range. Expanded, it doesn't look all that flat. Linearized (with a lookup table) that's 2mv = 1% change in SoC. Not idea of cheap BMS systems, but certainly doable. Attach a good voltmeter, make sure the terminal connections aren't corroded, and you have a tolerable SoC meter. To avoid customer confusion, make the display rather grainy with 5 steps of 20% each: