Sony WH-XB900N Help Manual page 15

If the headset is not used for a long time, the rechargeable battery usage hours may be reduced. However, the battery life will
improve after a few recharges.
If you store the headset for a long time, charge the battery to its full capacity once every 6 months to avoid over-discharge.
If the headset is not used for a long time, it may take longer to charge the battery.
If the headset detects a problem while charging due to the following causes, the indicator (red) flashes. In this case, charge once
again within the charging temperature range. If the problem persists, consult with your Sony dealer.
Ambient temperature exceeds the charging temperature range of 5 °C – 35 °C (41 °F - 95 °F).
There is a problem with the battery.
If the headset is not used for a long time, the indicator (red) may not light up right away when charging. Please wait a moment
until the indicator (red) lights up.
If the usage hours of the built-in rechargeable battery decreases significantly, the battery should be replaced. Consult with your
nearest Sony dealer to replace the rechargeable battery.
Avoid exposure to extreme temperature changes, direct sunlight, moisture, sand, dust, and electrical shock. Never leave the
headset in a parked vehicle.
When connecting the headset to a computer, use only the supplied USB Type-C cable, and be sure to directly connect them.
Charging will not be properly completed when the headset is connected through a USB hub.
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