Flutter background timer service

I am trying to make a real time timer that charges an amount per minute. The timer has to count in the backround (when the app is in the background). Any suggestions on how to do it? My timer function:

void _startTimer(int timePassed) {
if (timePassed == 0 || timePassed == null) {
  charge.value = 1.20;
  timer = Timer.periodic(const Duration(seconds: 1), (timer) {
    seconds.value++;
    if (seconds.value == 60) {
      seconds.value = 0;
      minutes.value++;
      charge.value += 0.20;
      if (minutes.value == 60) {
        minutes.value = 0;
        hours.value++;
      }
    }
  });
} else {
  seconds.value = timePassed;
  charge.value = (((timePassed / 60).ceil()) * 0.2) + 1.0;
  if (seconds.value >= 60) {
    minutes.value = seconds.value ~/ 60;
    seconds.value = 0;
    if (minutes >= 60) {
      hours.value = minutes.value ~/ 60;
      minutes.value = 0;
    }
  }
  timer = Timer.periodic(const Duration(seconds: 1), (timer) {
    seconds.value++;
    if (seconds.value == 60) {
      seconds.value = 0;
      minutes.value++;
      charge.value += 0.20;
      if (minutes.value == 60) {
        minutes.value = 0;
        hours.value++;
      }
    }
  });
}

}