I had to buy some speed controllers and other EV parts for my yard machine, so I took a chance and got what was supposed to be a 36 Volt system battery SOC gauge.
kelly controller SOC gauge

I took out a variable voltage power supply, and checked the led display's SOC against a Trojan battery company equivalent voltage chart, and found that it was not even close. I could not use it as it was, and the thing was glued together with no way to get inside.I took my band saw and cut the outer tube so the back could be removed. The circuit looks like a series of resistors that make up a voltage divider and some op amp comparators to turn on the leds when the voltage rises over the voltage divider taps. This type of comparator can be quite accurate, but it depends on voltage references and precision resistors,which this unit does not use. What was most interesting was that there is a potentiometer on the circuit that can adjust the divider current to make the whole scale move up or down. I set the power supply to the 100% SOC voltage according to the trojan chart, and then adjusted the pot so the 100% led just came on. I then adjusted the power supply down until each led went out, and noted where the 50% SOC was so I knew when to get to a charger. While this setting may be a bit temperature sensitive, it at least made the gauge usable.
To gain access to the pot, without cutting open the whole case, drill a hole in the indicated place on the back cover and you should be able to adjust the gauge without cutting it open. Of course one could calculate the precision resistor values required for accurate SOC throughout the range, and replace the 10% resistors that they used, but relative SOC is all I need so simply getting the 100% and 50% points will be sufficient accuracy for me.