Battery tester for the Dutch Cyclists' Union

My name is Atte Bakker and I am a volunteer with the Dutch Cyclists' Union. Every autumn these volunteers repair bicycle lights of people who can't do it themselves. I have developed a measuring box with which you can quickly and effectively test a circuit. In it are two Li-ion batteries that must be charged correctly. To that end, I found a battery tester that fits exactly what is needed: a DIY kit with a KA2284 (please see attachment). In it is a variable resistor that I want to replace with a fixed resistor. Because otherwise I have to tune each unit separately, which can cause large differences. I have already done tests with a combination of fixed resistors of 8479 Ohm, but they gave a bad result: the LEDS either all lit up or not at all. Would you be able to modify the block diagram in such a way that the unit works as it does with the variable resistor? Thank you in advance for your indispensable response!

Best regards on behalf of the Dutch Cyclists' Union,

Atte Bakker