Battery Life Estimation
Battery life in wireless applications
People asking us to develop new wireless sensor and wireless networking products but who have very over-optimistic ideas about how small the button cell battery can be regularly contact us. I’m hoping this brief introduction to battery selection will alleviate some of the confusion.
Many electronics engineers use simple calculations to estimate the life-span of a battery based on the average current consumption and the nominal Amp-Hour (Ah) capacity of the battery. For example if they assume an average current consumption budget of 10uA, and they plan to use a 55mAh version of the popular LiMnO2 type of battery, the battery life is approximated by dividing the capacity by the average current consumption. This gives a very respectable, but probably illusory, 5,500 hours and all from a tiny coin cell measuring just 16mm in diameter and 1.6mm in thickness.
Unfortunately, the picture is unlikely to be so rosy. Battery life is heavily dependent on other factors, which include, among others,
Its internal resistance,
The minimum voltage at which the application circuit will work
Discontinuities in the current consumption profile.
Most batteries will experience an increase in their internal resistance as they age. For a LiMnO2 coin cell this can range from a few ohms when it’s new to 30Ω or more as it approaches the end of its life. This might not seem much but wireless circuits typically draw current as short pulses. While the average current might be small, current pulses of 30mA are common. At this level the battery voltage will drop by 0.9V for the duration of the pulse and this might be below the reset voltage for the circuit.
While many circuits will operate down to 2V, and below, the internal resistance of the battery will absorb much of the headroom provided by the battery. So with a battery voltage down to 2.8V a 30mA pulse will, for a 30Ω internal resistance, cause the voltage drop to 1.9V and the wireless device will reset.
The graph below illustrates the interaction of battery age, internal resistance and application circuit voltage.
This graph does not represent any particular battery but does illustrate the point. From it can be seen the voltage profile for a continuous current drain and for an average current drain of 10uA but in the form of 30mA pulses at a duty cycle of 3000:1. Not only is the battery life significantly shorter for the pulsed current profile, the minimum operating voltage, described in our example, is reached in less than half the time to that predicted using the simple capacity/average current calculation.
The change in internal resistance is very dependent on operating conditions so manufacturers are reluctant to provide hard and fast specifications for this. Instead they provide typical discharge curves for a number of scenarios. When choosing a battery use the profile closest to your requirements to estimate the battery’s life-span using the principles described above. Also, wherever possible go up a battery size to give the resulting design the best possible chance of achieving its battery life-span goal.