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the MK doesn't have a batter so confused with your question? it has a passthrough ports so that if you want to CHARGE the ipad you can do it and leave open your ipad port for something else you may need. charging through the passthrough port is not as fast... it taps out around 20-21W where direct to ipad its 26W if I am correct.
 
my 2018 ipad pro "smart keyboard folio" gets its power from the ipad itself (wireless). From what I see the magic keyboard charges via USB-C.
 
Metalsiren is correct.

The MKB does not require charging as it takes power from the iPad via the smart connector.

The USB-C port on the MKB is only functional in providing power to the iPad through the MKB (pass-through) and via the smart connector. Thus leaving the USB-C port on the iPad free for ancillaries such as SD card, HDMI, multi-port hub.
 
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