actual battery health algorithm as of ios 13.3

yeet.c

//requires: com.apple.private.iokit.batterydata entitlement

extern "C"
CFArrayRef IOPSCopyPowerSourcesByType(int type);

int healthPercent;

NSArray *sources = (__bridge NSArray *)IOPSCopyPowerSourcesByType(1);
NSDictionary *batteryDict = sources[0];
if (sources && sources.count && batteryDict[@"Maximum Capacity Percent"]) {
    healthPercent = [batteryDict[@"Maximum Capacity Percent"] intValue];
} else {
    healthPercent = -1;
}

double constraintedHealthPercent = fmax(fmin(healthPercent / 100.0, 1.0), 0.0) * 100;

int finalPercent = (int)constraintedHealthPercent;
NSLog(@"correctedHealthPercent: %i", finalPercent);

returns '0 %' for me, iOS 13.7