In a link, I posted above, data sheet for MAX11068. I didn't look myself earlier, 82 pages...., yet now I get curious and open it. It took a moment to find out on page 10 all analog front end in full details, plus a lot of useful information.
Instead of electromechanical relay - MOSFET multiplexer. The same company produce a chips, high voltage mux, MAX378/379.
It's not 150V, but could be other producers/manufacturers. Or just one ! electromechanical relay to switch a half battery.
It never hurt to study/research first what is already exist on a market, instead of inventing a bike. 