off topic question! mA/h rating and expected lief span of coin cell battery

[1231]

hi folks

well, thanks for all the help here i got no labview. im just wondering how i can calculate an approximate lifecycle for coin cell batterys on the sensor motes i got here. if i know the average current it draws, say 30mA. and the input voltage (say 3volts), how can i approximate the life span of the device with 3.3v coin cells, i think 220mAh rating.

at the minute all my testing was done when i was sending data blocks so i was always using current ,unless i was in idle mode. so my lifespan was just a couple of hours. i want to know how long the device would last under realistic working conditions, like sending packets of data when required, etc.

sorry that  this is way off topic!

eric

[johnsold]

Eric,

The simple answer is that it is complex. The ampere-hour rating of a battery is typically based on a fixed discharge, either constant current or constant resistance. The most commmon rating is calculated from the constant current which discharges the battery to a specified voltage over 20 hours. The complexity comes from the fact that batteries do not provide the same amount of charge (current*time product) for different current levels.

Looking at your case: 220 mAh capacity with load of 30 mA and variable duty cycle. If the capacity is based on a 20 hour discharge (coin cells may be rated differently because their typical use is long term slow discharge), the standard discharge rate would be 220/20 = 11 mA. If the capacity were linear with discharge current, the life with your load would be 220/30 = 7.3 hours. As you observed the actual life was shorter. This may have been due to less capacity at higher currents, internal resistance, or your load may require a higher voltage than that at which the capacity rating was specified.

If your duty cycle was reduced to 10% and idle current was negligible, the life span would nominally be 220 mAh / (30 mA * 10%) = 73.3 hours. Depending on the characteristics of the battery and how long the load was on each time, the lifetime might be longer or shorter than that.

Lynn

[1231]

thats very useful info, these ni people really know the bizz.....!

regards

Eric

[johnsold]

Eric,

The NI people are the ones with blue names and status bars. The rest of us are users who are willing to help.

Lynn

[Conseils]

Most of the people contributing to the site are not NI staff...
it may be true of Johnsold ... give stars.... give lots of them....

Anyway the following information may well be of further interest... here
http://www.duracell.com/oem/primary/silver/silver_oxide_prod.asp