rapid power loss power dives from 100% to 3% in seconds

the power reading reduces from 100% to " critical battery power" in about 10 seconds after take off any advice . tried two new batteries same result. Charger charges back up to green within 15 mins. switch off copter and reboot shows the power at a level you would expect (80-90%) than does the same again when you you press take off. any advice ANYONE cheers