What is the maximum pulse current from a watch battery?

Question

I know that a comprehensive answer probably spans 1-2 orders of magnitude. Reasons include different battery sizes, chemistry, etc.

I can't access a lab at the moment or I would just measure it.

I want to draw 10 mA from a watch battery, but only for a few seconds. The duty cycle is < 10%. The series resistances I've been able to find seem to be < 8 Ohms, so the voltage drop due to internal impedance should be < 0.08 V typically, which is fine.

As far as continuous current goes, the numbers I've been able to find range from 0.01 mA to 0.1 mA. I don't think that 10X that for pulsed is asking a lot, but 100X might be a stretch which is why I'm asking the question.

I know not to actually short the battery for more than a fraction of a second, but if you do short it, is 10 mA a current that most watch batteries can meet or exceed? Duty cycle will be < 10% for about 10 seconds. After that, device shuts off and condition of battery is irrelevant. Is this feasible?

My 2nd question concerns power. It would be nice to draw 12 mW during that short time battery is sourcing power. I realize that's a lot but, is it doable?

I assume larger batteries have a better chance of hitting these numbers than smaller batteries, but if some battery chemistry makes this especially easy that might be more important than mass. Chemistry is also a weak point of mine so pretend you're talking to a 6 year old if you get into chemistry.

Answer 1

Requirements:

3V Primary button cell: 10 mA Duty cycle < 10% for about 10s only.

The question rephrased could be what size button or coin cell will guarantee this at room temp?

From my research in electrolytic capacitors and batteries, I have come to realize power density vs ESR has a fairly constant product within a family of slightly different sizes.

Based on Panasonic Specs for a new battery

CR2032 0.19 mAh, Isc=190 mA, ESR 15.8 Ohms , ESR * mAh= 3.0
CR2025 0.163 mAh, Isc=140 mA, ESR 21.4 Ohms, ESR * mAh= 3.5
CR-AGZ 2700 mAh, Isc=3000mA, ESR = 1 mOhm, ESR * mAh= 2.7

I use ESR*mAh as a constant with some tolerance.

For maximum power transfer Load if R = ESR, for a 10 mA load, 2*ESR=3V/10mA, ESR=150 Ohms @ Vbat = 1.5V for MPT

Thus pulse power is \$Pd=V^2/(2*ESR)\$= 7.5 mW (?)

• Your 10 mA load possibly assumed Vbat=3V which at max. power transfer is only 50% or 1.5V may not fit your assumption.

Conclusion

From this, you can decide what size you need by computing Max Power Point (MPP) or Vmin vs Rload to determine ESR max from mAh using ESR*mAh=3 and compute what is possible. Then test to verify margin with samples and compute confidence level or success from Std Dev and design thresholds.

other info

For LED's I know you can get about the same brightness of RED LED and White LED from a CR2032 5mm right across the cell. The voltage decays in a day approx. with gradual declining current limited by the rising ESR of the cell and slight reduction in Voc.

For low ESR e-caps in the < 100 uF range ESR*C = 1 to 20 us for a good product. For Std. e-caps it is ESR = 100 to 250 us. Since RC=T , size and voltage will impact results.