Track pickups (the skates) cause drag, which uses some of your precious battery power, so reduces your running time.
If you are going to run track power AND battery (even if not at the same time), then loco's should be either one or the other. NOT BOTH. - No matter how careful you are, you or someone else, WILL get it wrong. Stuffing the current a decent battery can give into the wrong-end of a Central Station, or even an analogue controller, can be a very expensive mistake.
More than anything we are trying to protect you, and your investment, from a costly error.
PLEASE, we can tell your knowledge is limited, isolate the motor and track from your battery arrangements.