i am trying to read the voltage of a lipo accumulator attached to a Teensy Feather Adapter on a Teensy 4.1.

Reading works, but i have no clue how to calculate the voltage from the values read.

What i do:

setup() {

uint32_t last = 0;

void loop() {  
  float vbat = analogRead(A7);
  if (millis() > last + 30000) {
    last = millis();
Raw values are 686.00...

According the the Schematic there is a voltage divider with 2x 100k resistors attached to Teensy Analog 7. This seems to be correct as different accus display different values and over time (on the lower valued one)
the value rises.

But how is that correct value calculated?