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Battery Discharge Test -- "Rest & Rejuvenation"?

After reading the "Battery Saver Kit" instructions, I wondered if the "Rest and Rejuvenation" approach really added to the battery life. That is, if the battery is allowed to rest between uses, will the total "on time" of the radio be increased? And will longer rests be even better? The battery under consideration is the Eveready 412, 22.5 volt battery. It is the correct physical size for the TR-1 and matches the original specifications for the radio.

Previous Research
I decided to look into this on two fronts. One was to see if there was any existing research literature to support this. I came across a key paper that addresses this issue:

E.J Podlaha and H.Y. Cheh, "Modeling of Cylindrical Alkaline Cells, Part VI. Variable Discharge Conditions",
Journal of the Electrochemical Society, vol. 141, no. 1, January, 1994, pp.28-35.

While this addresses alkaline cells, which are a different chemistry than that in the Eveready 412, it was believed there may be some connection. The authors do determine that the rest and rejuvenation approach has merit, and that it will extend the useful life of the battery. Portions of the paper are quite technical, employing partial differential equations, but other parts are very readable.

Battery Discharge Experiments
Next, I decided an experiment was in order. The Regency Battery Saver Kit implies an experiment of running the radio 2-3 hours per day, using the same battery only every 6th day, or so. With their estimated battery life under these conditions of around 40 hours, this experiment could take 20 sessions of 6 days each, or 4 months. While an automated setup could be constructed to perform the tests, the likelihood that a summer lightning storm could cause a power outage was significant, and this would render the experiment useless, requiring it to be restarted.

Instead, two experiments were proposed -- one allowing the battery to discharge without turning off the radio. This is a harsh operating condition and not likely in practice, but would yield a boundary condition on the discharge capabilities of the battery. A fresh Eveready 412 was purchased (from Batteries Plus) and connected to the radio. The battery terminal voltage was read every minute and recorded automatically. The result shown here indicates the battery is able to power the radio for about 19 hours, until the voltage drops below an arbitrary level of 12 volts. The radio still plays at under 12 volts, but this level yielded a quiet sound from the speaker, and was chosen as the minimum operating condition for these tests.

Next, another fresh battery was used in a setup that turned the radio on for just 4 hours per day, allowing it to rest for the remaining 20. The radio's volume control was set the same as in the first test, and power was applied using this control apparatus. The power supply on the left provides power for the digital voltmeter, which is interfaced to the laptop computer's serial port. Custom software collects the data from the DVM. The power supply on the right is turned on and off by a standard household light timer. The supply then turns a relay on and off, making the connection between the battery and radio. To accomplish this, a piece of paper was inserted between the battery's "+" terminal and the radio's battery clip. Then a piece of aluminum foil was inserted between the battery and the paper, providing a contact to the battery. The result of this experiment shows that the battery regains some of its voltage during each resting period, but loses much of that quickly during the next discharge period.

A main question, though, is whether the radio is actually powered longer than the 19 hours indicated above, before it falls below 12 volts. To investigate this, only the "on time" of the last experiment is plotted along with the result from the first experiment. This comparison shows that the rest and rejuvenation approach yields about 27 hours of "on time" before dropping below 12 volts -- an increase of 8 hours, or 42% -- certainly significant.

The remaining question is whether a longer rest period would improve the results even more. The fact that even after 20 hours of rest the battery is still gaining voltage suggests this possibility. I'll leave this experiment to someone else.

If you have any suggestions (corrections?) or just comments,
please send them along to me, Steve Reyer.
I'd like to hear from you!