the first step was to connect the new battery in parallel with the existing one. Now the total consumption is allocated to both of them and a small improvement in performance is already visible. Looks like this factory controller can deliver a little more than a single factory battery can.

As it is now very important that the batteries are on the same voltage and equal charge, the second step was to bring the power supply to the parking space so that I could charge the batteries without taking them out to apartment.

The third step will be to replace the controller ... probably kls7218s

p.s. Today I have tested range ... 34km with 32% of battery. I was pushing to maximum without any calculation to save battery for longer range.