[Tinyos-help] Battery voltage curves - Estimating battery lifetime from Voltage

[Steve McKown]

On Sunday 29 April 2007 10:45, Mante Bos wrote:
> I can read the voltage from my batteries and I can transform this
> output of the ADC to the appropriate voltage of my platform (micaz).
>
> Now I would like to now of someone had some curves of the battery
> voltages of nodes while their are running typical sensorapplications.
> I know from the manual that my motes (micaz) have an operating voltage
> range of [2.7 - 3.6] V. From curves like this[1] I can estimate the
> remaining power of the battery.
>
>    Now I would like to have status of my battery, indicating of it has
> a lot of power, has medium lifetime or runs out of power. Does anyone
> has experience with trying this?
> Since the curves drops faster as it runs out of power, I suggest to
> keeping a history over time to decide in what stage it is. I guess
> that it is also important for my decision to know if my radio/sensors
> are on, because I would have a slightly greater voltage output. Has
> anyone tried this or please mail me back if you have some good
> suggestions.
>
> I would like to give my motes different behaviours depending on the
> remaining energy of the mote, so it would be interesting to have more
> than a warning that my mote runs out of energy.

Assuming you aren't periodically recharging your battery, you can use four 
variables to give you a very rough idea of battery condition: chemistry, 
temperature, current draw, and battery voltage.  If you can consider the 
first two fixed and always sample voltage at a known current draw (perhaps 
cpu running but all peripherals off/sleep), then you can create a low battery 
indicator by only reading the voltage and comparing it to a constant, with 
hysteresis.

When people begin wanting to look at the battery condition closer, very often 
the cost to do so exceeds the benefit.  Ultimately, we came to the 
realization that the best time to save power is when there is lots of it, 
since you get a multiplying effect.  Consider that 10% less energy draw over 
the operational range of a battery is a better deal, energy-wise, than saving 
30%, but only during the last quarter of the battery life.  Because battery 
life is so important in our application, we chose to give up some 
functionality to sip power all the time, looking at battery capacity only to 
generate a "service me" indication.  The end result was simpler and more 
efficient hardware and software.  Of course, this line of reasoning may not 
apply for your application...

All the best,
Steve