CHARTER Low Power Design Guide
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Low Power Design Guide
4 Techniques to Maximize Battery Life A battery is often seen as a storage component that contains a certain amount of power which it is able to release. Reality, once again, is not that simple as is going to be shown in the following. The power that can be drawn from a battery is not depending on its charging only. To illustrate this, have a look at figure 4 (the data has been taken from Panasonic Nickel-Metal Hydride Batteries Handbook):  figure 4: Battery discharge lines The figure shows a battery being charged under the same circumstances but being discharged at three different rates. Have a look at equation 8 to remember the physical magnitude energy respectively work. equation 8: W = P * t = U * I * t Where W is the drawn energy over the time t, P stands for power and U for the voltage at which the battery is discharged with the current I. Notice that I is constant during discharging. The battery is considered empty, when U drops below 1 V. By applying this knowledge to the diagrams, we get three totally different values for W:
table 3:Evaluation of figure 4 What do these numbers tell us? The total power that can be drawn from a battery is highly depending on the average current. The higher I, the more power is lost (22% in the given example). Therefore average current should be reduced to a minimum. Further on, cells recover when allowed to relax between discharge. It makes an energy source more efficient, when it is discharged with current bursts instead of a steady flow. This is as far as this paper is going into the subject. The field of battery design takes complete books to cover.
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