Sony WH-CH520 Quick Start Manual page 19

Note
If the headset and smartphone are not connected with HFP, the remaining battery charge will not be correctly displayed.
If you connect the headset to an iPhone/iPod touch or Android smartphone with "Media audio" (A2DP) only in a multipoint
connection, the remaining battery charge will not be displayed correctly.
The remaining battery charge may not be properly displayed immediately after a software update or if the headset has not been
used for a long time. In this case, repeatedly charge and discharge the battery multiple times to properly display the remaining
battery charge.
Related Topic
About the indicator
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