Of course there were battery packs.
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The detail comes directly from Jim R, who pushed Mike into many of the shows in the wheelchair. He had the mics in his hat so he wouldn't be obstructed if the crowd was stood up which it often did. Low in a wheelchair as you describe would be blocked. He was using full size mics, very likely with right angle XLR connectors so the cable went down (his neck) not back then down.
He was meticulous about staying still and steady, but I have heard movement on his tapes many times.