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I have some old NiCd cells, and I have a bench power supply.

From what I read about NiCd batteries, I understood that a psu alone can’t charge them fully because it can’t measure the delta V.

I saw some circuits for that, but I couldn’t understand how do they work. Can you explain?

I also have old fast 1 hour NiCd charger with voltages 7.2-14.4. (Does it mean I can connect some cell combinations in between these ranges? Like 7 cells for 8.4V).

Another question: after I fully charged a battery of 6 cells, their voltage was 8.5V. I wanted to know how good they are, so I connected them to a resistor and monitored the current and voltage, and started a stopwatch to measure the time. The resistor draws about 1A, and I thought if the battery will supply 1A for an hour, it has 1Ah capacity. But the current is slowly dropping and I don’t know how to calculate that way, and I don’t know at what voltage to stop discharging. I still monitoring the volt/amps of the battery by the time I’m writing this question.

The battery supplied 1A for 25 minutes, and then the current slowly fell down, and now, the voltage is 6.13 and the current is steady at 0.77A for a while.

toolic
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noname delete
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  • If I remember well, "charge voltage" can be at max 1.45 V per cell, and then charging should be switched off. https://www.batterystuff.com/kb/articles/battery-articles/what-the-heck-is-a-nicd-battery.html#:~:text=Charging%20NiCd%20batteries&text=The%20charge%20rate%20for%20a,2%20(50%25%20capacity). – Antonio51 Aug 25 '22 at 10:50
  • Current through a resistor will fall as battery voltage drops. R = V/I so 6.13/0.77 =~ 8 Ohms. For 1A V = 8V so V per bat = 8/6 = 1.25V. This is about what you'd expect. You need a constant current load if you want a steady current with time. An eg LM317 with R = 1.25/I and reference grounded will give you 1A. Here for 1 A R = 1.25 Ohm. Dissipation in LM317 will be high - a series input resistor will help. – Russell McMahon Aug 25 '22 at 13:54
  • See circuits [here](https://www.google.com/search?q=LM317+CONSTANT+CURRENT+LOAD&rlz=1C1CHBF_enNZ834NZ839&sxsrf=ALiCzsZMNVOCxd8vU5TbaP9rRnVY6Lz2fg:1661435583585&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjJjJvukeL5AhVTwzgGHdVWC5MQ_AUoAXoECAEQAw&biw=2194&bih=1153&dpr=1.75) – Russell McMahon Aug 25 '22 at 13:54
  • @Antonio51 Charging only has to be switched off if it's being done at a high current. You can charge NiCd (and NiMH) batteries all the way up to 100% and keep charging them, and autocatalysis will prevent them from overcharging. As long as the charge rate is low enough that autocatalysis can keep up, the cells will just get a little warm. – forest Aug 26 '22 at 00:14

3 Answers3

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You can't fast charge NiCds with a bench power supply, as you can't detect the deltaV. However, you can time charge them with a suitable current limit.

If you discharge through a resistor, the discharge current will vary as the terminal voltage varies. If you want to compute Ah, then read the current every minute or so. You can manually integrate the charge, or put all the readings into a spreadsheet, and have it integrate them and draw a graph. Once in a spreadsheet, you can graph voltage against charge rather than time if you wish.

A NiCd cell will have little useful charge in it below 1 V, so it's sensible to stop discharging at this level.

An individual NiCd cell can be taken down to 0 V without damage, but it will be damaged if reversed. If you are discharging a multi-cell battery, it's possible that one weaker cell could be reverse charged by several stronger cells, even if the total battery voltage is still above 1 V per cell. If you are discharging a NiCd battery aggressively, especially an unknown one, check the individual cell voltages, as well as the total battery voltage.

winny
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Neil_UK
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NiCd cells were around long before fancy computer-controlled chargers were invented. And unlike LiIon cells, they aren't inclined to catch fire.

Traditionally, NiCd cells were charged at 0.1C for around 12 to 14 hours. C is the amp-hour, or milliamp-hour, rating of the cell, with the "hour" removed. For example if you have a 1200mAh cell, then C is 1200mA. So 0.1C would then be 120mA.

So set the current limiter on your bench supply to limit at 0.1C, and set the voltage at about 1.4V per cell. At that current, NiCd cells can tolerate being trickle charged for many hours, without worrying about when to stop charging.

If that's too slow, bump up the current a bit, but then don't overcharge the cells. So 0.2C for 6 hours would do.

If you're really in a hurry, and if you know the cells are flat, charge at 1C for no more than 1 hour, then switch to trickle charge. Overcharging at that speed can damage the cells.

Simon B
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Does it mean I can connect some cell combinations in between these ranges?

Yes. As long as they fit properly in the charger.

Another question: after I fully charged a battery of 6 cells, their voltage was 8.5V. I wanted to know how good they are

Under discharge, NiCd discharge voltage is 1.2V. It varies little. Once there is a noticeable drop (to 1V), then it is discharged. Under charge, the charging voltage should be about 1.4 V. Fast charge >0.5C is more efficient energy wise than slow charge. They can be trickle charge at 0.1C continuously.

Fast charge must be terminated to prevent overheating and thus damage. While charging the batteries remain cool. The charge completion can be measured by a small decrease in voltage (5mV per cell) following a small plateau. After this point heating starts to occur.

RussellH
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