Comment by nine_k

Pick used EV or industrial batteries. This must be much more efficient due to a larger cell size than in laptops.

OTOH used laptop batteries can likely be obtained for effectively zero monetary cost, while used EV or solar backup batteries still cost quite noticeable money per kWh. With laptop batteries, you pay with your time; if you for some reason have an excess supply thereof, or you just enjoy this kind of work as a pastime.