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"Bruce W.1" wrote in message ...
I found a good way to measure the capacity of a single cell. This Radio Shack multimeter: http://www.radioshack.com/product.as...%5Fid=22%2D812 It logs voltage (or current) and its software can output the log to a text file. Now all I have to to is write a little computer program to calculate the capacity. I have a similar meter that seems pretty accurate, and because the voltage is around half of one of the full-scale ranges, you don't sacrifice much because of poor resolution (as you would at, say, 2.1V). An easier way (for those of us who don't want to deal with programming access to the info) than writing a program is just to import the text file to a spreadsheet. You then have a column of voltages at uniform time intervals. If you know the discharge resistance (load resistance), then I=V/R and you can make a column of that value. The power at each interval is just V*I -- or just go to that directly as V^2/R. Then the total energy is the integral of the power over time...in watt-seconds, just the sum of the power column, if your time interval is one second. Divide by 3600 seconds/hour to get watt-hours. Sum the amps column to get amp-seconds and divide by 3600 to get amp-hours. I've done exactly this sort of thing with my RS-232-interface voltmeter. Works fine. Cheers, Tom |
Now, that looks like a handy little gadget. Thanks for bringing it to
our attention. Should do the job, all right. Roy Lewallen, W7EL Bruce W.1 wrote: Thanks Roy. I found a good way to measure the capacity of a single cell. This Radio Shack multimeter: http://www.radioshack.com/product.as...%5Fid=22%2D812 It logs voltage (or current) and its software can output the log to a text file. Now all I have to to is write a little computer program to calculate the capacity. |
Now, that looks like a handy little gadget. Thanks for bringing it to
our attention. Should do the job, all right. Roy Lewallen, W7EL Bruce W.1 wrote: Thanks Roy. I found a good way to measure the capacity of a single cell. This Radio Shack multimeter: http://www.radioshack.com/product.as...%5Fid=22%2D812 It logs voltage (or current) and its software can output the log to a text file. Now all I have to to is write a little computer program to calculate the capacity. |
On 19 Aug 2003 11:36:37 -0700, (Tom Bruhns) wrote:
"Bruce W.1" wrote in message ... I found a good way to measure the capacity of a single cell. This Radio Shack multimeter: http://www.radioshack.com/product.as...%5Fid=22%2D812 It logs voltage (or current) and its software can output the log to a text file. Now all I have to to is write a little computer program to calculate the capacity. I have a similar meter that seems pretty accurate, and because the voltage is around half of one of the full-scale ranges, you don't sacrifice much because of poor resolution (as you would at, say, 2.1V). An easier way (for those of us who don't want to deal with programming access to the info) than writing a program is just to import the text file to a spreadsheet. You then have a column of voltages at uniform time intervals. If you know the discharge resistance (load resistance), then I=V/R and you can make a column of that value. The power at each interval is just V*I -- or just go to that directly as V^2/R. Then the total energy is the integral of the power over time...in watt-seconds, just the sum of the power column, if your time interval is one second. Divide by 3600 seconds/hour to get watt-hours. Sum the amps column to get amp-seconds and divide by 3600 to get amp-hours. That is *exactly* what I suggested in another version of this thread. (Not claiming anything except that at least TWO ppl find that way easy). I've done exactly this sort of thing with my RS-232-interface voltmeter. Works fine. Cheers, Tom |
Roy Lewallen wrote:
Now, that looks like a handy little gadget. Thanks for bringing it to our attention. Should do the job, all right. Roy Lewallen, W7EL Bruce W.1 wrote: Thanks Roy. I found a good way to measure the capacity of a single cell. This Radio Shack multimeter: http://www.radioshack.com/product.as...%5Fid=22%2D812 It logs voltage (or current) and its software can output the log to a text file. Now all I have to to is write a little computer program to calculate the capacity. ================================ I wrote the program and will make it available to anyone that wants it. I runs on Windows and the .NET Framework. Works fine. The only problem I have is remembering to disconnect the battery from the resistor after it hits one volt, Doh! Some of them were pulled down to 0.4 volts (under load) but they recover to about a volt at rest. Yet one question still lingers in my mind. Is a discharge voltage to, say, 1.1 volts under load or at rest? |
Roy Lewallen wrote:
Now, that looks like a handy little gadget. Thanks for bringing it to our attention. Should do the job, all right. Roy Lewallen, W7EL Bruce W.1 wrote: Thanks Roy. I found a good way to measure the capacity of a single cell. This Radio Shack multimeter: http://www.radioshack.com/product.as...%5Fid=22%2D812 It logs voltage (or current) and its software can output the log to a text file. Now all I have to to is write a little computer program to calculate the capacity. ================================ I wrote the program and will make it available to anyone that wants it. I runs on Windows and the .NET Framework. Works fine. The only problem I have is remembering to disconnect the battery from the resistor after it hits one volt, Doh! Some of them were pulled down to 0.4 volts (under load) but they recover to about a volt at rest. Yet one question still lingers in my mind. Is a discharge voltage to, say, 1.1 volts under load or at rest? |
The end discharge voltage (generally 1.0 volt per cell for NiCd and NiMH
cells) is measured under load. After disconnecting the load, the voltage will typically rise substantially, but its value isn't of any significance. It's usually not recommended to discharge below 1.0 volt, but it doesn't seem to cause any harm, at least if it's not done really often. I think a cell is more likely to grow dendrites and short if it's left in an extreme discharged state for an extended period, so it's probably a good idea to put at least some charge back in before too awfully long if you've discharged it particularly deeply. What is harmful is reverse charging of the cell. But that happens only when you have an external source of current, like other cells in a series connected battery. Roy Lewallen, W7EL Bruce W.1 wrote: I wrote the program and will make it available to anyone that wants it. I runs on Windows and the .NET Framework. Works fine. The only problem I have is remembering to disconnect the battery from the resistor after it hits one volt, Doh! Some of them were pulled down to 0.4 volts (under load) but they recover to about a volt at rest. Yet one question still lingers in my mind. Is a discharge voltage to, say, 1.1 volts under load or at rest? |
The end discharge voltage (generally 1.0 volt per cell for NiCd and NiMH
cells) is measured under load. After disconnecting the load, the voltage will typically rise substantially, but its value isn't of any significance. It's usually not recommended to discharge below 1.0 volt, but it doesn't seem to cause any harm, at least if it's not done really often. I think a cell is more likely to grow dendrites and short if it's left in an extreme discharged state for an extended period, so it's probably a good idea to put at least some charge back in before too awfully long if you've discharged it particularly deeply. What is harmful is reverse charging of the cell. But that happens only when you have an external source of current, like other cells in a series connected battery. Roy Lewallen, W7EL Bruce W.1 wrote: I wrote the program and will make it available to anyone that wants it. I runs on Windows and the .NET Framework. Works fine. The only problem I have is remembering to disconnect the battery from the resistor after it hits one volt, Doh! Some of them were pulled down to 0.4 volts (under load) but they recover to about a volt at rest. Yet one question still lingers in my mind. Is a discharge voltage to, say, 1.1 volts under load or at rest? |
Roy Lewallen wrote:
The end discharge voltage (generally 1.0 volt per cell for NiCd and NiMH cells) is measured under load. After disconnecting the load, the voltage will typically rise substantially, but its value isn't of any significance. It's usually not recommended to discharge below 1.0 volt, but it doesn't seem to cause any harm, at least if it's not done really often. I think a cell is more likely to grow dendrites and short if it's left in an extreme discharged state for an extended period, so it's probably a good idea to put at least some charge back in before too awfully long if you've discharged it particularly deeply. What is harmful is reverse charging of the cell. But that happens only when you have an external source of current, like other cells in a series connected battery. Roy Lewallen, W7EL ======================================= Thanks for your help Roy. See you at the Dayton QRP Suite. Bruce AF8F |
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