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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
