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I am working on a small wireless sensor platform and trying to figure out power supply options. My power budget is approximately: .2mA constant, ~15mA for .1s every 2-3s, and 60mA for 10s very intermittently (once or twice per hour). This works out to about 50mAh for the lifetimes I'm looking for, and ideally it would be rechargeable.

I've found some button cells by Panasonic that fit the bill (VL2330 - PDF), but can't find much information on the Lithium Vanadium Pentoxide chemistry. The data sheet suggests a "continuous standard load" of .1mA, which presumably is just the rate required for the nominal capacity. Beyond that there's no more information as to maximum pulse currents, derating, or otherwise. A 1C pulse would be nothing for a regular Lithium chemistry, but I haven't used these before.

Does anyone have experience with these, or know of another direction to look for this kind of application?

Thanks, John

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3 Answers 3

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Quick Rules of Thumb

  1. 10mA is your maximum current draw.
  2. Buy fresh batteries. Always verify this by running lifetime tests on your batteries when you buy from a new supplier, or a new batch.

Our General Conclusions

When testing with coin cell batteries at my last job we found a number of things:

  1. Surpassing 10mA per cell greatly reduced battery life.
  2. 20-30mA was normally around your "maximum" current you could draw, but this is not dependable, the 10mA line being the highest current we could pull for deterministic function.
  3. 1mA was as high as you could go without significantly degrading rated capacity(get nearly the published capacity).
  4. Staying below the .1 mA suggested line would normally meet the rating of the capacity.
  5. Someone selling you old batteries can do more to harm your tests than almost anything else. The batteries purchased directly from china gave the best results, the nice company in new york had battery cells that performed worse that batteries we had on site in storage for 4 years.

Capacity Rating

These results all seemed relatively consistent across different rated capacities, giving the same results with 50mAh batteries and 400mAh batteries. We chose to use 200mAh batteries on devices that needed to pull 20mA, they would significantly outperform 800mAh coin cells(the big ones you can buy) as passing 10mA hurt the battery life significantly.

When we passed the 10mA line the lifetime of the battery had a very large variance, we attributed this to manufacturing differences, but the same batch could have vastly large differences.

How I know this.

We did testing on batches of 20-50 coin cells to choose our rating and package. After this point we ran tests on more than 500 batteries under different conditions to verify results and predict lifetime. We ran tests where the current was pulsed, tests in different temperatures, and tests were we let devices run for months to try to test what our expectation would be. I am sorry I do not have references, as this testing was done at a company and was not published in any form, I have nothing I can reference for this.

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  • \$\begingroup\$ Thanks for your experience. Out of curiosity, were those standard CR2450-type cells (basic consumer lithium), or a rechargeable variant? \$\endgroup\$
    – John Laxson
    Mar 3, 2011 at 6:40
  • \$\begingroup\$ @JohnLaxson, ours did not need to be recharged. They were the standard consumer type. During our testing we testing other variants and sizes, our final choice was a CR2032. We were better off with multiple of these than a higher capacity option with only one! We only needed to pull 20mA also, but two batteries just beat everything else out of the water to stay below the 10mA threshold per battery. \$\endgroup\$
    – Kortuk
    Mar 3, 2011 at 6:42
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Button cells perform horribly on large currents. The 15 mA will be a problem, the 60 mA won't work. Consider a supercap; they are great in this application.

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    \$\begingroup\$ @brianCarlton, I have to disagree on the supercap note. Supercaps have very high self discharge rates, this means that if you need a long term battery source without recharge you cannot use a supercap. If you need only a day or so, you will still have to significantly over-spec your supercap, the cost will normally be a deterrent for this to be a feasible solution. \$\endgroup\$
    – Kortuk
    Feb 26, 2011 at 6:57
  • \$\begingroup\$ @BrianCarlton, If you mean use a supercap as short term power for the power bursts and a battery to give a trickle charge, this is a great solution, but the complexity of circuitry and cost can be a deterrent in low complexity/low cost systems. \$\endgroup\$
    – Kortuk
    Feb 26, 2011 at 6:58
  • \$\begingroup\$ Assuming capacitor across battery: To limit the battery drain to 15mA, limit the voltage droop to 150 mV. (assuming 10 ohms internal resistance.) So we need enough capacitance to drop 150 mV in 10 seconds of 60 mA. That's 10mV/second. I=C * dv/dt, so C = 60 mA / 10 mV, or 6 farads. Moral: Put a resistor in series with the battery, so that heavy currents are drawn from the capacitor. \$\endgroup\$
    – markrages
    Feb 27, 2011 at 5:24
  • \$\begingroup\$ @Kortuk, what complexity of circuitry? Wouldn't a 100 ohm resistor fromt the coin cell to the supercap be sufficient? (and power the circuit from the supercap.) \$\endgroup\$
    – markrages
    Feb 27, 2011 at 5:29
  • \$\begingroup\$ @markrages, have you seen the size/cost of a 6 farad supercap? I think if a coin cell battery is on the order of acceptable cost a 6Farad cap is adding quite a bit. I am not saying one solution is correct for all cases, I just find that when a coin cell battery is on the order of acceptable cost for your power an extra 6Farad supercap is probably not an option. 6F superap I will say is 2 dollars, looking up they are more, but rounding down for very very large order. 6 coin cells for about 20 cents. We got them for 2 cents a piece(only buying a couple thousand), but you can add a little. \$\endgroup\$
    – Kortuk
    Feb 27, 2011 at 7:23
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Lithium cells are reported to have a maximum current that can be drawn from the cell before damaging it. TI did some tests and were not able to confirm this.

TI document (pdf)

Discussion on PIClist

I think you may have problems drawing 60mA out of that 50 mAh battery. While the datasheet doesn't give the resistance, based on my experience with other coin cells, I will guess about 50 ohms. 60 mA * 50 ohms is 3V. In other words, at 60 mA current draw the output voltage is zero.

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