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?
Quick Rules of Thumb
- 10mA is your maximum current draw.
- 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:
- Surpassing 10mA per cell greatly reduced battery life.
- 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.
- 1mA was as high as you could go without significantly degrading rated capacity(get nearly the published capacity).
- Staying below the .1 mA suggested line would normally meet the rating of the capacity.
- 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.
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.
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.
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.