Sunday, February 26, 2012

Accurately Measuring Battery Capacity

In this post we will look at solution for measuring and characterizing the capacity of rechargeable batteries for devices such smart phones, portable game systems, and wireless sensors. The following video made by a colleague of mine explains the challenges of measuring battery capacity and an ideal solution made of three products: N6781A SMU unit for battery drain analysis, N6705B DC power analyzer mainframe, and the 14585A analysis software. I have blogged about the N6781A in the past. One feature that I want to point out that the video doesn't cover is its seamless current measurement ranging. This is a patented Agilent only technology that allows the N6781A to seamlessly move through its measurement ranges without any discontinuity or glitches in its output. This means you get 18 bits of measurement resolution no matter if you are measuring microamps or amps.

The video demonstrated measuring the battery's capacity by using the N6781A as a load, meaning it simulated the device that the battery would be powering. Since the N6781A is 2-quadrants and can regulate its output at 0 V it can also act as a zero ohm measurement shunt to measure and characterize the battery's capacity while powering the device it will be used in. To use the N6781A as a zero ohm shunt you simply connect it in series with the battery and the device it is powering so the current flows through the N6781A. The below figure shows the zero ohm setup.

Notice in the above figure that the N6781A has a built-in separate DVM for measuring the battery's voltage even though it is not directly connected to the battery. 

In this post we have been focusing on characterizing a battery's capacity, of course the Above zero ohm shunt setup could be used to characterized the device's power needs too. And the N6781A can also be used as a source to simulate the battery to characterize the device's power needs. If you have any questions regarding this post feel free to email me and if you have any comments or personal insights to share please use the "Comments" section below.


  1. Obviously, we would not want to spend many hours manually taking voltage readings every 10 seconds, therefore a power analyzer is a very useful tool. I use the Power Analyzer PRO as it does everything automatically. The fully charged battery is connected to one side, and the power resistor is connected to the other. Every 0.3 seconds it measures the voltage and current, displays instantaneous power, and calculates and displays the total Watt-hours of energy dissipated since the discharge began.
    Sincerely, @Kingkinu from Ac Dc Adapter for mobile phone

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