Troubleshooting NiCd Battery-packs

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10.10) NiCd battery pack will not hold a charge

This applies if the pack appears to charge normally and the terminal voltage immediately after charging is at least 1.2 x n where n is the number of cells in the pack but after a couple of days, the terminal voltage has dropped drastically. For example, a 12 V pack reads only 6 V 48 hours after charging without being used. What is most likely happening is that several of the NiCd cells have high leakage current and drain themselves quite rapidly. If they are bad enough, then a substantial fraction of the charging current itself is being wasted so that even right after charging, their capacity is less than expected. However, in many cases, the pack will deliver close to rated capacity if used immediately after charging. If the pack is old and unused or abused (especially, it seems, if it is a fast recharge type of pack), this is quite possible. The cause is the growth of fine metallic whiskers called dendrites that partially shorts the cell(s). If severe enough, a dead short is created and no charge at all is possible. Sometimes this can be repaired temporarily at least by 'zapping' using a large charged capacitor to blow out the whiskers or dendrites that are causing the leakage (on a cell-by-cell basis) but my success on these types of larger or high charge rate packs such as used in laptop computers or camcorders has been less than spectacular.

10.11) What is this thing in my NiCd battery pack?

In addition to the NiCd cells, you will often find one or more small parts that are generally unrecognizable. Normally, you won't see these until you have a problem and, ignoring all warnings, open the pack. If it is a little rectangular silver box in series with one of the positive or negative terminals of the pack, it is probably a thermostat and is there to shut down the charging or discharging if the temperature of the pack rises too high. If it tests open at room temperature, it is bad. With care, you can safely substitute a low value resistor or auto tail light bulb and see if the original problem goes away or at least the behavior changes. However, if there is a dead short somewhere, that device may have sacrificed its life to protect your equipment or charger and going beyond this (like shorting it out entirely) should be done with extreme care. These may be either mechanical (bimetal strip/contacts) or solid state (Polyfuse(tm) - increases resistance overcurrent). If it looks like a small diode or resistor, it could be a temperature sensing thermistor which is used by the charger to determine that the cells are heating which in its simple minded way means the cells are being overcharged and it is should quit charging them. You can try using a resistor in place of the thermistor to see if the charger will now cooperate. Try a variety of values while monitoring the current or charge indicators. However, the problem may actually be in the charger controller and not the thermistor. The best approach is to try another pack. It could be any of a number of other possible components but they all serve a protective and/or charge related function. Of course, the part may be bad due to a fault in the charger not shutting down or not properly limiting the current as well.

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This was an interesting article. I'm going to have to re-think that "memory" problem. I'm not sure which way explains what I've experienced. That voltage depression explains some of it. Thanks.

Tektronix used to zap NiCd batteries with memory with a brief jolt of 120VAC, 60Hz on anything from 1.2 volt individual cells up to 9.6 volt packs that would leave the battery charge essentially unchanged but would burn out the dendrite whiskers in short order. The burn time was on the order of about 1/10 second. It vaporized the whiskers without warming or venting the cell. Then the battery would go through a few deep cycle discharge/charge cycles to confirm the cell memory was gone.

It may be a thermistor. Read the resistance, then charge it up. When the charge is full, it will heat up. You should then find that as it heats, the resistance will drop. The effect will be more pronounced if you wrap it to keep the heat in.

Once you get the figures, you can work out what to do with it - I used to use a resistor array & op amp to switch from fast charge to trickle.

I just clipped it to my meter and waved a cigarette lighter under it. It does act like a thermister, not a diode. I guess I'm going to have re-check my collection of zeners to separate them. Have to do this check while it/they's removed from the circuit/item, but at least I know what to look for now. I thought they were all zeners. Thanks.

The image below is a glass body thermistor, they are pretty common, and do resemble diodes, with the main difference being they will not contain a band indicating the cathode. There is sometimes a clear space in the glass, as you can see in the ones shown below, but it's not at one end of the package, and it's not on the outside of the body, as a cathode band would be.

If it's behaving like a thermistor, I would doubt this is your problem. You said the pack would charge with this out, but did you monitor the cells to see if they got warm while charging? Perhaps some cells are getting marginal, and they are heating up, and the thermistor is doing it's job? Just a thought.

Tom D

I originally had 1 battery, and 1 charger, and 1 tool. The battery pack went bad. So, I used the case to build a connector for using a lead-acid battery with the tool. Then a friend had an identical charger go bad. So, I gave him mine. Since then, he dropped the tool and broke the case for the tool. He gave me my charger back, and a couple of battery-packs (and the broken tool). One pack worked as it should, and one pack would always show fully charged (when it was dead). I replaced the cells in the bad pack, and it acted the same.

I still had the parts from my original pack. So, I put a connector on the old pack without the thermister (even tho I still had it). Then, I put it in a charger that didn't use that 3rd terminal, and both packs charged up as they should, and powered the tools. So, what I had now was, 1 good pack, 1 pack with new cells and the old thermister (always showing fully charged), and 1 pack without a case or thermister at all (not being detected). The thermister from my 1st original pack is the one I tested (but didn't use anywhere).

I now suspect that the thermister is what went bad in the pack that always showed fully charged. And, if I replace it with the one from my 1st original pack, it should work as designed. That would leave me with a 3rd pack without a case or thermister. If I knew it was the thermister from the beginning, I would have just replaced that, and not the cells. Now I have a good 3rd pack that doesn't work so good without a case or thermister.

I was just trying to figure all this out. I am making progress.

Thanks.

Lots of ordinary diodes (particularly detectors) look very similar to Zeners.

Perhaps it's in the control circuit to ascertain correct polarity ie. reverse polarity causes the diode to block control current, thus keeping the charge circuit off.

I suspect this is a basic diode, too. The voltage drop across the PN junction versus a constant current can be used as a very accurate temperature sensor.

The "diode" might be a thermal fuse. These look like a diode, but it is a one time operation over temperature safety.

https://en.wikipedia.org/wiki/Thermal_cutoff

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Simple LED “Battery-Low” warning indicator.

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