I recently had to replace a battery due to the fact a cell or two was bad.
The symptom was with only a small amount of ground running of electrics I didn't have enough power to turn the engine over. Interestingly, I could tell if I was going to have a problem on my walk around. The lights would come on (strobes, beacon, nav, landing) but when I gave the stall warner a flick - no noise! So at least on my plane it fails at a higher voltage than almost anything else in the plane. But as others have said, there are lots of other warnings of stall than just the electrickey one - so potential lack of stall warning wasn't the issue that got me to replace the battery post haste!
My Bonanza has a 15.5 amp-hr 24 volt battery (70 amp hour seems like a decent size car battery). If FF has similar and threw the belt right as he started his take off roll, with a full suite of avionics and the draw from the gear retraction motor, he could well have had only a little time left. However, 10 does sound like the battery has a weak cell or possibly the belt was dropped at engine start.