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I am making a small binary counter circuit which will toggle states of an RGB LED as a small design project to practice both design and PCB layout. My design will incorporate two CR2032 batteries in series to provide power to the circuit.

I have calculated that at times (when all three LEDs are on) the current will be at ~20mA.

I have been reading about putting a capacitor in parallel with the batteries very close to them in the circuit to help with some current pulses in the circuit. It seems that there is some controversy about doing this. I was wondering if anyone can advise if this would be a good idea and if so how to select a proper capacitance value.

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If your objective is to provide a supply for fast transient loads the capacitors should be as close as possible to the loads. That's why pretty much every digital device has power supply bypass capacitors right at the power pins.

The link you have there was looking at the leakage through the capacitors, it doesn't quite matter where there are. Leakage through the capacitors is a parasitic load regardless. Those leakage currents are tiny compared to the LEDs you're driving. If your goal is to extend battery life when the circuit is sitting, that problem can be solved with a switch.

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  • \$\begingroup\$ Is there ever a reason to put a cap in parallel with a battery close to the cell? \$\endgroup\$
    – Alex
    May 3, 2017 at 4:00
  • \$\begingroup\$ I haven't ever had a reason to. Putting capacitors close to loads provides a fast reacting power supply that can supply quick surges faster than the battery will. If you had a battery that came as a complete unit which had circuity inside that was known to be sensitive to HF noise then it might be necessary as a band-aid. But generally, no. \$\endgroup\$
    – alphasierra
    May 3, 2017 at 4:15
  • \$\begingroup\$ I have seen people claim that a very large cap right at the battery extends battery life by smoothing out any loading transients on the cell. But they were talking about the number of cycles for rechargeable cells rather than life of a coin cell and I never saw any real data to convince me, just some logical sounding arguments as to why this would help in some situations. \$\endgroup\$
    – Andrew
    May 3, 2017 at 8:13
  • \$\begingroup\$ If suppressing transients is the objective the capacitors should be as close as possible to the loads drawing those transients. I've never heard of anyone putting a large cap right next to a battery for that purpose before. \$\endgroup\$
    – alphasierra
    May 3, 2017 at 18:35

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