Phonak Naida P30 PR drops hearing volume after loosing 50 -60% of charge

Hi,

I have a Phonak Naida P30 PR (Its a rechargable battery using USB). Hearing volume drops after 50-60% of charge is lost on the device. I have to manually increase the volume again to hear better.

Is this a common issue on all hearing aids or just my device is faulty ?