Addendum :

While replying in this thread I belatedly realized that in the original post I forgot to credit one very important contributer : CrossBow.

While I did not reproduce exactly his own hack(s), his detailed explanations were decisive and convinced me of the dangers of working "blind" (ie without appropriate instruments) and placing too much faith in the specs of batteries, rechargeables et al.

I followed his good advice and bought a basic multimeter, and have been having so much fun with it since that I now wonder how I've survived so long without it in my toolbox :-)
I certainly couldn't have done all those hacks on the same Saturday without it and without melting something or other... so thanks CrossBow !


PS: a case in point to underscore the sense in his cautious approach of these things, which sounds like paranoia but isn't...
While learning to use my new tool I tested everything within reach that looked like it might carry an electron... like verifying that 4 new AA batteries deliver well over 6V, and 4 freshly charged AA cells well over 5V, etc.
Also, I came across an older made-in-china (are there any others ?) wall wart, the sort with a slider to select voltage between 3 and 9V, another to select polarity, and almost every power plug form factor out there. It was in my son's room, where I used it to power his various toys and gadgets when their batteries ran out. Thus it's had a rough life over the years, but although it had a battered look, it seemed to be still working (in that it powered things).
Looking to test its accuracy at the various settings, I was shocked to find that no matter what the buttons were supposed to select, it now always unflinching delivered between 18 and 20V...
Now I know why those fixed-track racing cars were hard to control in the bends :-)