As an extension to my previous performance analysis of Kentli’s PH5 Li-ion AA battery, I fully charged an unused PH5 and left it on my desk to self-discharge. Every now and then, a Texas Instruments bq27621-G1 fuel gauge is hooked up to the Li-ion battery terminals (in the case of the PH5, the recessed ring around the 1.5V terminal) and the bq27621’s default settings are used to measure the voltage and state of charge.
I started this test on June 18th, 2015 and will keep taking occasional measurements until the protection IC in the PH5 shuts down.
Since the 18th, the voltage dropped from 4.216 volts down to 4.192 volts as of July 6, 2015; the bq27621’s State of Charge reading remains at 100% for the time being. The voltage drop has been fairly linear so far, but I expect it to taper off as the battery discharges to the Li-ion cell’s “flat region”, and only after that do I expect the cell’s voltage to decline more rapidly.
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- PH5 Voltage (Jun 18 – Jul 6 2015)
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- PH5 State of Charge (Jun 18 – Jul 6 2015)
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If you had to adapt a 4x AA “moon in my room” to lithium, would you bother with a buck regulator or just wire two cells in series and call it a day?
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It would depend on the circuitry of that lamp (is it LED-based, and does it have any ICs or power converters?) and the chemistry of battery to use. Either way, I wouldn’t use raw Li-ion cells without protection circuitry. With some fiddling around with some wires, it may be possible to use LiFePO4 (lithium-iron phosphate) cells that have a 3.2 V nominal voltage and are relatively robust compared to the standard LiCoO2 or LiMnO4 chemistries.
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