X4 Battery drain very fast at standby

less than a year ago (Aug 12th) i've got from you as a replacement set for the X4, due to defect X3.

The problem started last week after the X4 is fully charged (by PC USB connector), the LED is green and the announcement says the "battery is fully charged".

About 3 hours without any use (they sit on the table standby- power off) the battery is almost at 50%.

needless to say that I've tried resetting them, changing the charger and nothing helped.