Fubarino Contest: Battery Capacity Tester

fubarino-contest-battery-capacity-tester

Here’s a project that we sadly let slip through the cracks a couple of years ago. Luckily [Brian] dusted it off and added an Easter Egg to the firmware in order to include it in the Fubarino Contest. The device is a rechargeable battery capacity tester. It discharges NiMH or NiCad batteries through a load resistor at about 1 Watt. [Brian] includes a discussion in his write-up about the hardware’s inability to work with 14500 Li-Ion cells. He includes enough info for you to figure out how to make changes to the circuit if you want to enable this option.

There is a MOSFET for switching each of the three battery positions. The ATmega168 takes readings from the cells once per second. It displays status information on a Nokia 5510 cellphone screen. This is where he chose to inject the Hackaday URL. When a cell’s discharge is complete, the image above scrolls onto the screen and remains there for a short time. See for yourself after the break.

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Battery capacity tester lifts the veil on manufacturer capacity claims

battery_cap_tester

[Nick] was tasked with building a battery capacity tester by one of his teachers in order to test some aftermarket batteries that were purchased for their Vex robotics lab. The batteries were cheaper than the official version, but boasted more than twice the capacity. Fairly skeptical of the rating, he got to work designing his circuit.

He originally planned on discharging the battery through a resistor and measuring the voltage with a PIC microcontroller. After prototyping the circuit, he found that the PIC did not have enough storage space for the data he was collecting, and that there were issues with fluctuating current as the voltage decreased.

Undeterred, he built a new tester using a Teensy microcontroller and a different discharging circuit using a LM317T. This second version not only included an LCD screen to track the discharging process in real-time, but it also dumps all of the data and calculations to a spreadsheet on the computer connected to the Teensy.

The capacity tester works pretty well, according to [Nick]. He says that most batteries overestimate their capacity, and that his meter is getting readings within an acceptable variance when testing known good batteries. What about those knock-off batteries from China?  He discovered that they can hold about half the charge that they claim – it’s a good thing he decided to test them out!

While he provides the software he used for the tester, there are no schematics to be found. Check out some of the other battery capacity testers we have featured in the past for tips on building one yourself.

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