Well I did a
practical experiment earlier:
I have a car battery, it's on the car and it works fine.
I also have a few AA NiCads (or are they NiMH?)
The AA cells don't read anythingV at all when I measure them, in fact they don't take a charge, no matter how long you have them in the charger for. On ohms each AA cell battery measures about an ohm from + to - (I wondered what the internal resistance was, now I know
)
So each AA cell, one at a time, I connected flashed directly accross the car battery, for about 1/2 a second each. As I put each one down, they were quite hot.
When I measured each one, they each (3 of them) read near 1.2 V. A few minutes later, I took them out the freezer where I put them to cool down, and I put them in the charger, with another partly flat AA cell I found.
I now have 4 working AA cells
So when you connect something lower voltage to a higher voltage..., it can: blow it up, or do nothing, or it could even mend it!
The clever (or dangerous) bit is knowing (or not knowing) which it is going to be.