Diagnosis
The lithium-ion battery is degrading, which is accelerated by consistently draining it to zero or leaving it on the charger 24/7.
How to Fix It
Recalibrate the battery meter. First, fully charge the handle until the light on the charger base turns off. Then, use the brush until it completely dies and won't turn on. Finally, charge it again, uninterrupted, back to 100%. This can often restore accurate battery level reporting.
If That Didn't Work
The battery itself is likely worn out. For the 9700, this is not a user-replaceable part. If under warranty, contact Philips. If not, the fix is a new handle.
Heads Up
For longest battery life, store the brush off the charger in a dry place. recommends charging it only when the battery indicator shows one bar.