Invisible <voi### [at] devnull> wrote:
> >   I = U/R has no "maximum".
>
> As an abstract equation, no.
>
> In a real system, if you decrease R far enough, eventually U starts to
> decrease too, in each a way that I remains fixed. Thus I has a "maximum
> value".
>
> Put simply, there is a limit to how fast the system can replace the
> charge you're neutralising. It's no different than the fact that a water
> pump has a maximum flow rate, no matter how slight the resistence of the
> water pipes becomes.

Your explanation is abstract as well. ;o)
The reason for the drop in voltage is a series resistance inside the supply.
This series resistance is not necessarily constant. It might increase with
current drawn.
There's no such thing as an ideal voltage source (read unlimited current supply)
in reality.
If you short a battery for exampe it will source a maximum current. Measure that
and measure the voltage without drawing current and you get the source
impedance. If you connect an load with this resistance to the battery, you'll
get half the voltage out of it.

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