something to starve voltages for fuzzes (with psu)

[Peter Snowberg]

If it were the impedance raising as the battery dies, I would think every circuit built with a big cap across the supply would be essentially immune that effect thanks to the impedance of the cap. Is this the case?

I always thought it was the voltage and instantaneous current availability over time.

[R.G.]

To a first order, yes, it is the internal impedance and decoupling does help with the problem of high supply impedance.

To a second order, the voltage and impedance changes speed up as the battery gets closer to exhausted. I believe that is what you're calling "instantaneous current capability over time".

The "instantaneous current capability" is, of course, just the effective series resistance. The "over time" part is (I think) the change in impedance and voltage.

At least I'd model it that way.