Keyestudio KS4028 Manual page 301

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BAT, the output port of power, is interfaced with the lithium battery
holder(rechargeable batteries) and saves the electric energy into batteries.
This is the switch. Slid to ON end, then the external lithium battery will be
connected, supplying to the expansion board; on the contrary, slide to OFF,
then the current of lithium battery will be disconnected.
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