Suppose you take 20 car batteries and hook 10 each in series, and hook the
two sets in parallel (one might do this to power an electric vehicle, say).
Suppose further that you accidentally take the handle of a Craftsman wrench
and inserted it in between the terminals of this mega-battery.  KABLOOEY!!!
The remaining blob of metal was cheerfully replaced at Sears, although they
had to trust our description of the original wrench.  My friend was lucky to
escape any injury!!

Be carefull,

Albrecht