[Elecraft] Battery Storage

[Ron D'Eau Claire]

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