My first Arduino project was to build a battery capacity tester. I’ve got a box of rechargeable AA batteries, and it seams they’ve been less and less effective. Since most applications require 4 batteries, invariably one problem battery makes the rest of them look bad.
The Atmel ATMega328 microcontroller has 6 analog inputs with 10-bit A-to-D converters and a external AREF that allows you to define what voltage 0x3FF represents. In other words, it’ll give you ~1.4mV precision measuring 0-1.5V when given a 1.5V analog reference. Plenty accurate for a battery capacity measurement.
The principle is fairly simple. Apply a known load to a battery, record the voltage periodically while the battery discharges, stop recording at some point, and integrate to arrive at the area under the curve in order to derive the amp-hours delivered from the battery.
Enough theory, let’s see how it works. The UI starts with a helpful message:
While the battery discharges, the UI prints the voltage, real-time capacity measured thus far, and the duration that the measurement has been taking place.
Schematics, excluding the LCD display..
The Atmel ATMega328 microcontroller has 6 analog inputs with 10-bit A-to-D converters and a external AREF that allows you to define what voltage 0x3FF represents. In other words, it’ll give you ~1.4mV precision measuring 0-1.5V when given a 1.5V analog reference. Plenty accurate for a battery capacity measurement.
The principle is fairly simple. Apply a known load to a battery, record the voltage periodically while the battery discharges, stop recording at some point, and integrate to arrive at the area under the curve in order to derive the amp-hours delivered from the battery.
Enough theory, let’s see how it works. The UI starts with a helpful message:
Insert Battery
While the battery discharges, the UI prints the voltage, real-time capacity measured thus far, and the duration that the measurement has been taking place.
Battery In
When the cut-off voltage of 0.9V has been reached, the “usable” capacity is saved at the top. The real-time data continues so the capacity below 0.9V is measured. For the NiMH batteries I’ve tested, the capacity below 0.9V is minimal (~100mAH).
After the cut-off is reached, an LED starts blinking to attract attention that the test is effectively over.
Done
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