[Elecraft] Battery Storage

[Stephen W. Kercel]

Ron:

Very informative.

Thanks,

Steve


At 10:27 AM 2/14/2005 -0800, Ron D'Eau Claire wrote:
>The old rule of thumb for an old dry cell providing 1.55 volts is based on
>the materials used, not the age or condition of the cell. A zinc-carbon cell
>produces very close to that voltage as long as the chemicals hold out.
>
>What changes with the aging of those cells is their internal resistance. It
>climbs as the chemicals are depleted.
>
>The Heathkit IM-11 tube-type VTVM specified using such a cell and provided a
>calibration marker on the scale where 1.55 volts should appear just above
>the 1.5 volt full scale range. It had an 11-megohm input resistance similar
>to 10-meghom input resistance of most modern DMM's so any "droop" was
>insignificant as along as the cell had not been allowed to deteriorate
>completely. The meter also used a 1.5 volt "flashlight battery" (zinc-carbon
>cell) for the Ohms scale, so one got the reference cell for setting the
>calibration and the Ohms bridge battery all at the same time. I used to
>check the calibration of my IM-11 whenever I changed the Ohms battery.
>Whether or not the battery had been on the shelf a month or a year made an
>insignificant difference because the meter drew only 0.00015 mA! (1.5 volts
>/ 10 megohms). So the internal resistance in any cell still functioning
>wasn't going to cause any detectable change in the output voltage at such a
>tiny current.
>
>Those batteries are still readily available. Just find the cheapest
>flashlight battery in the store and make sure it does not say anything about
>being alkaline. Many stores don't carry them because alkalines are so cheap
>themselves, but I see them around all the time marked with "Extra High
>Energy" or "Heavy Duty". Of course those terms are meaningless but they have
>to say *something* about the battery. They'd never sell if they advertised
>them as "crummy batteries that will run down quickly and may ruin your
>equipment!'
>
>The problem with those batteries is that, given enough time, they will leak
>a highly corrosive acid that will destroy whatever they are in. That's
>because one of the materials that is consumed by the cell is the case
>itself. The case is the zinc electrode. Even disconnected, there's some
>leakage current flowing through the electrolyte. You can slow it done by
>chilling it, perhaps, but sooner or later the acid will eat through the
>case. It's a paste so it moves slowly but it'll destroy everything it
>reaches.
>
>The more current being drawn, the faster the zinc case is eaten. That's why
>us OT's all knew that the moment we found a flashlight using the old
>carbon-zinc cells that had been left on for a few weeks, we had a corrosive
>mess inside the case when we opened it. The best approach was to toss 'em
>out.
>
>Ron AC7AC
>
>
>
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