I have two 9 volt alkaline rechargeable batteries. The actual voltage is something less than 9 volts. Alkaline batteries are approximately 1.25 volts per cell and alkaline batteries are approximately 1.5 volts per cell. Most chargers for rechargeable batteries are constant current chargers. Properly designed they will charge a variety of cell voltages.
Several years ago when the alkaline rechargeable batteries came out they were rechargeable only in special recharger’s. The recharger’s were intermittent chargers. They would shoot a little voltage into the battery and then stop and then do it again and again. They would read the voltage and temperature of the battery as they charged. It used to be a popular science to charge them but now with the advent of all the special rechargeable batteries that really do work most people have ignored recharging alkaline batteries. I still have a charger that has an alkaline setting.