Why is the Xiaomi Focus Pen not fully charged after being attached to the Xiaomi Pad 7 Pro for a long time?

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div:last-child { margin-right: 0 }To extend the lifespan of the Xiaomi Focus Pen battery and prevent overcharging, Xiaomi has implemented an anti-floating charge strategy. Once the Xiaomi Focus Pen is fully charged, the tablet will stop charging it. Charging will only resume when the Xiaomi Focus Pen battery drops to around 80% and will continue until fully charged.
Note: 
If you prefer to keep the Xiaomi Focus Pen fully charged, you can periodically remove and reattach it to the tablet, which will trigger charging until it reaches 100%.