It is either or. I had no problem updating the firmware with a fully charged battery. I think the idea is to make sure the camera doesn't shut down during the update which would corrupt the file system.
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EOS C100 mk2 with a Canon EF-S 18-135mm IS STM, and a Sigma EF mount 10mm-20mm EX DC HSM wide angle - Zoom H2Essential - Dell 8700 i7-4790 3.6Ghz, 24GB Ram, Win 10, Nvidia GTX 1060 6GB - DaVinci Resolve Studio 20.3 - Blackmagic Speed Editor - Presonus Faderport 1.
