The notification will only happen when the Miles Tracker spots that the voltage on your car’s battery has dropped below 12 volts for a while. It doesn’t necessarily mean your battery health is poor, because even a healthy battery can drop below 12 volts (if, for example, the lights were left on).
If your car starts, it may be just a specific event that caused the notification. It could also be because you have only made short journeys recently (which means the battery doesn’t get fully charged up by the car).
If you’re not sure, it’s always worth getting your battery looked at or replaced if it’s older than three years.