So why the powering off with apparently good batteries? Best I can determine from another source is that with Alkaline batteries you have a linear discharge rate and their charge shows more accurately on the VIO display. However, when using NiCads, they will retain a somewhat lower voltage, for a longer period, but then voltage will drop suddenly as the charge begins to weaken. This might not show on the charge display, but could cause sudden shut offs and thus the Invalid clips.
That's exactly the problem; in fact, I remember reading somewhere that you cannot accurately use battery meters/gauges on NiCad rechargeables precisely because of the nonlinear discharge.
I got burned on my first day out this season, because my NiCads took a dive in capacity over the summer (I didn't use them and I guess they discharged too low, then couldn't take a full recharge later). Didn't bother me too much, as I was only trying to dial in my new camera mount that day, but I did have to go buy a new set of NiCads. Not sure what I will do to maintain the new set over the summer, but at $12 for four, I suppose I could afford a new set each season.