Filter Cap for a battery pack ?

If there's RF in the vicinity, battery wiring can be an antenna; that's all the capacitor is good for. At frequencies in audio, battery wiring is much shorter than a transmission line, and those batteries are very capacitive. So, how RF-sensitive is the power rail? And, is the battery wiring paired, so it makes a transmission line, or unpaired, so it makes a loop?

There is no need to use a capacitor across the battery.

Put capacitors across the power supply on the amplifier board, especially near the output devices.

CH

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** Post one. ** Post it. **100% wrong. ** 110 % wrong.

( ship even more vomitous drivel)

..... Phil

Not so.

Learn some electronics, and tell me what the capacitance computes at, for a voltage source like a battery. I'm talking about I = C * dV/dt when you pull current from a battery, and dV/dt is close to zero (after all, batteries are sized, usually for ten hours or more of load).

It's easier to use an ESR (equivalent series resistance) meter. For example, the ESR of an Energizer E91 AA at 100KHz is about 0.15 ohms at 25C:

That's as good or better than any common filter capacitor. At RF frequencies (>1MHz) the ESR (impedance) goes up mostly due to lead inductance. Instead of absurdly short battery wires, moving the filter capacitor near the electronics works better.