static float estimate_battery_level(uint16_t voltage) {
    int p[] = { 100, 60, 10, 0 };
    int v[] = { 4200, 3870, 3690, 3270 };
    int size = 4;
    
    float percent = 0;
    for(int i = 0; i < size; ++i) {
        // if >= then 100%
        if(voltage >= v[i]) {
            percent = p[i];
            break;
        } 
        
        // if not last
        if(i < size-1 && voltage >= v[i+1]) {
            percent = p[i+1] + (voltage-v[i+1])/(v[i]-v[i+1])*(p[i]-p[i+1]);
            break;
        }
       return percent;
}