Last changed: 23 July 2000. (Latest changes marked like this.)
This discussion represents my thoughts on practical, and meaningful, test measurements for NiCd packs. The information here is correct to the best of my knowledge, but I accept no responsibility for errors and omissions. Above all, remember that NiCd batteries are potentially dangerous. A short circuit can cause burns or start a fire, and over-charged NiCds can explode!
Uebrigens, ich kann auch deutsch.
One of the key parameters, and one which can be tested relatively easily, is the capacity in mAhr. Basically, all you need to do is to discharge the battery pack at constant current, and time how long it takes for the battery voltage to drop to 1 Volt per cell. The capacity is the current in mA multiplied by the time in minutes, divided by 60.
This can be automated quite easily. My own design was published as a circuit idea in Electronics World, October 1997, and I won both a PicoScope A/D converter and a PicoScope ADC200-50 two channel 50Mz oscilloscope for my efforts.
The unusual feature of this design is that it is powered entirely from the NiCd pack under test, for packs of 4 or more cells. I have to admit that I have not actually built this circuit, other than bread-boarding it to confirm that it does work. However, as it is based on a circuit that I have been using for several years I am confident that it is sound, even if a few details may need changing. (I have also sketched out a circuit design that will test down to a single cell, but that requires a separate power supply.)
In fact, you can simplify this considerably by removing the restriction of constant current discharge. A car headlamp bulb (12V 40W) makes an excellent discharge load for Sub-C cells, and a stop lamp (12V 21W) for smaller cells, in both cases for packs up to 9 or perhaps 10 cells. The drawback is that you have to monitor the current throughout the discharge, and calculate the capacity as the area under the current/time curve. (Technically, the time integral of current.)
Since we need to measure the voltage to determine when the battery is flat, you may think that this means you need two meters, one for voltage and the other for current. Not so! In principle, the voltage applied to the lamp determines the current, so the trick is to 'calibrate' the lamp. Admittedly, this does require two meters, but with luck you can borrow one for a short time to do this. You apply varying voltages to the lamp, perhaps from a number of packs with different cell counts and different states of charge, and draw a graph of current against voltage. I found that over a moderate voltage range of say a discharged 7 cell pack to a fully charged 8 cell pack, the graph is almost straight. You then fit an equation of the form I = aV + b (a,b constant) to the straight line. From now on, if you measure the voltage throughout the discharge, you can readily calculate the current that goes with it. For the headlamp I use, I found I = 0.215V + 1.27 . (If you are really keen you can fit a quadratic or even higher order polynomial that works well over a very wide range of voltages. eg I = 1.211 + 0.2476 * V + -0.00324 * V * V .)
In practice, measuring the voltage every minute for the first five minutes, every five minutes during the flat part of the discharge curve, and every minute once more during the rapid voltage drop to exhaustion, is generally good enough. If, like me, you possess a PicoScope or similar, you can automate this - I use it to log the voltage every 12 seconds. The program I use is available on my Free Programs page.
Logging the discharge voltage has another important benefit - it lets us detect that a cell is failing, in that it becomes flat before the others. In this case, we will see a premature fall-off in voltage before the main fall-off. Detecting this is important, because a faulty cell can prevent a peak-detection charger from cutting off at the appropriate point. The weaker cell becomes over-charged, and if you insist on really flattening a pack at the end of a flight (a bad idea!), the weak cell is reverse charged by its stronger companions. The result of both over charge and over discharge is to rapidly destroy the weak cell. Provided the problem is detected early, a few cycles of slow charge can generally partially recover the pack and delay the day that it must be retired.
In fact, the mAhr capacity of a pack is not the whole story, because it doesn't take into account the voltage that the pack is delivering. Clearly, a pack with an average voltage during discharge of 1.25V is better than one with an average of 1.15V, even though they may have the same capacity. One way of quantifying this is to calculate the total energy (in Joules) available from the pack. For each measured point during the discharge, we multiply the current by the voltage, to obtain the power in Watts. Then we plot this against time (seconds), and find the area under the graph. This is the total energy, in J, or for more convenient numbers, divide by 1000 for kJ (kilo Joules). A Sanyo 600AA has a capacity of about 2.2kJ, for example. If you feel more comfortable with voltage, divide the energy capacity in kJ by the mAhr capacity, multiply by 1000 and divide by 3.6 to obtain the average voltage. Incidentally, simply taking the time average of the voltage discharge profile is not a valid thing to do unless the pack is discharged at constant current.
This may seem like hard work, and a lot of arithmetic, but it lends itself very well to a spreadsheet. Making the voltage measurements is more work by far than extracting the maximum amount of information from the voltage profile.
When discharging a pack via a headlamp, it is very useful to have an automatic switch to turn the lamp off when the pack gets down to one volt per cell. This need not be complex, requiring little more than an 8211 comparator and a power FET. I have been using an own design for several years, and have recently constructed an improved circuit, for handling packs of between 4 and 10 cells.
One point to be aware of when using lamps as loads is that they have very high starting currents. I have measured pulses of 20A for a headlamp, and 15A for a stop lamp, taking 100 mSec or more before settling to the steady state current. This can drop the pack voltage to such an extent that the low voltage cutoff operates immediately.
For measuring the capacity of single cells I offer this circuit. I built it mainly to check domestic NiCd cells - of the thirty cells I had in use around the house, only ten were worth keeping! The rest had much reduced capacity or lost in excess of 25% of their charge in a week. (I intend to add a description and circuit board layout for this circuit - e-mail me to remind me if you need it quickly!)
I have found an excellent page describing NiCd batteries, their properties and problems, by some people who sound as though they know what they are talking about, in an article entitled 'NiCd Battery FAQ'. (Link revised 23/1/1999)
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