Canon WFT-R10 Advanced User's Manual page 173

Battery Charging Order
You can select which battery to charge first, in consideration of the different purposes of
batteries on the left and right of the transmitter.
On the camera, press the 〈
Select [Camera first] or [WFT first].
Caution
Any over-discharged batteries are charged first, regardless of the setting.
〉 button and select [
173
: Battery charging].