The battery indicator on my D500X/D1000X detector shows that the battery is ok, but the detector shuts down in a little while.

The battery indicator shows the battery voltage, not the remaining battery life time. Especially if an external battery with a higher nominal voltage is used, the battery voltage may be high enough to make all segments of the battery indicator turn on even if the battery is far from fully charged. Checking the condition of external batteries with a battery tester will provide a better indication of the battery status.

The current D500X firmware version shows the actual voltage in Volt, rather than a bar indicator. This gives a better measure of the battery condition.