Capacity
The capacity of a NiCad is given in milliAmps-per-hour, or mAh for short. This stands for: how long can a NiCad deliver a certain current, or the capacity determent for the time you can run (or fly) your vehicle/aircraft. So, with a 1700 mAh NiCad you have more time than with a 1200 mAh Nicad.

Discharge/Internal Ri
The discharge charge is the charge (in volts) that a NiCad provides during discharge with a certain current. The higher the voltage, the more RPM from the electric motor. If the voltage is higher, then the current (Amperage) true the motor is also higher, which results in a higher torque. In short, a NiCad with a higher discharge gives an electric motore more torque than speed. the discharge resistance is in principle a direct result of the internal resistanse (Ri) of the NiCad. Thus, a NiCad with a low Ri provides a higher discharge power.

Matching/Selection
Allthough we mostly speak of the 'battery' it actually consists of several cells which are connected in seies with each other. All cells in such a pack are charged and discharged with the same current. The moment a cell is 'empty' and you continue to draw current from the battery-pack, it will be negatively charged (reverse poling). The same thing will happen to cell following the previous cell of the same pack, which will discharge until the motor completely stops.. This 'reverse poling' is noticeable, because the motor will suddenly turn slower, continues for a moment and then again turns a bit faster. This reverse poling is for most NiCads disastrous, and the cell will never reach its normal capacity again. Some cells, like the Sanyo SCR cells, retain a better resistance against this type of behavior. But there is another problem: if one or more cells are reversely (read negative) poled and the battery pack is charged again, the cells who were not totally discharged at that time will immediately accept charge again, but the cells who were negatively charged have to reach ' ' first before the actual charging process can begin. As a result, some cells will reach total charge at a certain moment while the other (who already) had a lower capacity) are not quite there yet.
So, those 'almost' charged cells will be discharge even faster and more reversely charged and again charged less than the previous time. See the problem?

Correct Choice Important
To create an usable battery pack, all the cells must have the same capacity. This selection is called matching. But that's not everything; there is still another problem to considder; cells with an equal discharge capacity do NOT all have the same charge-time or so-called 'efficiency curve'. I discovered , that cells with the same discharge curve, usually show a difference in the charge duration, sometimes more than 15 minutes! So, when the cells are only selected by their capacity, then some of the cells will definately overcharge and others will not reach total charge.
Thus to get a pack to deliver maximum power and long life, it is then very important to closely observe the discharge capacity AND the charge duration.

Is Matching Always Good?
Not all so-called matched cells are selected using this criteria. Often only the internal resistance (Ri) is measured, which is pretty much useless, or in best cases, cells are matched by their discharge curve which is also not sufficient.
Sometimes I get the impression that the cells are selected only by the color of their wires!!!
My advice is to watch what you buy. It is my experience that 'cheapie' battery packs are just that. A good matched battery pack is a timely job and you will definately find this fact back in the price of such a pack. A company in the Netherlands, called PK Model Racing, limits its cells to a 3-second difference in discharge time and 15 seconds for the charge curve. (How's that for accuracy eh?) This ofcourse provides a large matrix of the cell criteria. The PK company match their cells from batches of 10,000 cells at a time.

Is Expensive Always Good?
In principle it is (for batteries), but only in regards to the provided pack capacity. Voltage and lifespan of the battery pack have nothing to do with it.
So, at time of purchase you have to ask yourself if you really need this capacity or extra quality? Not always, because if you don't have capacity problems, a 5-minute off-road contest for example using 1700 mAh packs, you can easily buy the pack of lesser quality (the cheapie). What is important is that those cells are matched adequately. If you're not (yet) into competition or trying to break a new record, then the cheaper cells will do fine. Leave the expensive 'maximum capacity' packs for the champions!

Different Brands of NiCads
The most common battery packs are currently Sanyo and they go by the following coding:

1300SC
1400SCR
1700SCR
1700SCE

Handling NiCads
NiCads who are allowed to be fast-charge should NEVER be 'slow-charged'! Slow charging these packs is not recommended for this type of cells. Not only is the capacity less after slow charge, but also the discharge curve is allot lower. The story of slow-charge is a remnant from the early NiCad days were these cells did not have the quality and endurance of today's cells. Also not known was the knowledge how these cells were to be treated and the slow-charge was merely a precaution and prevention of explosions. They also believed that the cells would all charge equally if slow charged. With 'matched' cells is this not only totally unnecessary, it is also NOT recommended.

Slow charged cells have the tendancy to become 'lazy and start to provide very low discharge voltages. Concurrently, the current to the motor is also lower which mimics a longer use of the pack but in reality the speed is gone. (and you thought you had to look for a faster motor)...NOT! The last couple of years, however, NiCads have improved drastically. They no longer hold the so-called memory effect and are manufacturerd with a higher standard compounds. They still require your attention and good maintenance practices though! Thanks to NASA and the American space program, the NiMH (Nickel Metal-Hydrate) cells have come a long way. They are lighter, are able to supply 3 times the current, and the discharge last longer. Good for R/C, but still expensive in comparison.

First, Total Discharge!
The battery pack should be discharged totally to 0 volt. This can be done to connect one or more resistors over the battery terminals. This ensures that all cells be put at an equal discharge level. (This actually replaces the old fashion way of slow-charging). The discharge cycle happens so fast that after the 'o-volt' point is reached the cell recovers rapidly to about 0.9/1 volt, so no harm done, just total discharge.
The second reason is, that, except for the newer nicads, a cell has a so-called 'memory', which means that if a cell is discharged a couple of times to only 75% of its total capacity, it remembers this and at a certain moment will not discharge any further than to this particular level. Reason is crystallisation of the anode and cathode of the cell which in turns prevents the cell to provide a full discharge. Total discharge to 0 volts will prevent this. But be careful: as I mentioned earlier, over-discharging a battery pack with one of more cells already discharged is a bad thing and can do a lot of damage, expecially for the 1700 SCE cells. It is therefore suggested to connect a 5-ohm resistor to each single cell of the battery pack so they will all be individually discharged. This will prevent 'poling' (e.i. negative charging). SCR cells are also allowed to be discharged with a single resistor of 25 to 35 ohms, although it also would be safer to use seperate resistors for each cell.
After usage of the battery pack you can store them with resistors connected for up to a couple of months.

NOTE: This method (1980's) is outdated and not practised anymore. But I left it in to show how it was done 20 years ago.

Charging
During the battery charge a chemical reaction takes place in the cell, whereby the following sequence will happen:

The voltage will crawl up to a certain value, stays there for awhile almost constant and then goes on to the end of its maximum value pretty fast (cell is full). At this point the charge will start to lower again slowly. The cell will get warmer too at the same time untill the cell is almost full. Then the tmeperature will rapidly increase.
There are tow way to check if the battery is fulle charged:

   1 - Check the current cahracteristic (so-called Delta Peak charging method).
   2 - Via a temperature check.  (in °Celsius for accuracy).

Delta Peak Charging
This is the control of voltage charge. The moment the voltage starts to drop to a certain level, the charge is interrupted. This procedure can be done manually with a digital voltmeter. If this voltmeter is connected to the pack during charge it is noticed that it stays for a long time at 8.5Volt (6 cells) and than a certain point rapidly increases. Now this is the dangerous part; the moment the voltage drops between 0.02 - 0.04 volt the charge has to be stopped immediately. At this time the battery pack is fully charge. It can be done quite easliy but make sure you are not interrupted by something because this can lead to overcherde batteries which are useless after that and even can explode, freeing dangerous gasses inside the cells. There are these days all sorts of smart chargers which continually monitor the voltage and the charging process stop (sometimes too early). This method of charging is called 'Delta Peak'.

Temperature Charging
An other way of charging is the temperature check during charge. This can also be done these days quite easily with a digital thermometer. Just place the sensor underneath the battery's shrink sleeving and off you go. After awhile the temperature will rise sharply and the charging process interrupted at a certain value. But what is exactly that value? Well, that's the beauty of the "temperature method", you determine that value yourself.
Actually, every cell needs a different 'fall-off' value. A 1700 SCE is not allowed to be overcharged a lot, a 1400/1700 SCR has to be overcharged just a little in order to provide adequate output power.

Fig. 1 - Voltages and temperature curve of a NiCad during charge.

The nominal charge value for a 1700 SCE is 35 °Celsius while the recommended value for the P-170 SCR is 40 °Celsius.
Further more, the rule is the same for all cells, that a certain level of overcharge provides a different discharge curve. Like, if a cell is overcharged another 5 °C, it gives a bit lower capacity, but a higher boltage output, or in other words the motor will run a bit faster. This way you can determine if you like the motor run slower but longer of faster but slower. All this can be done with the Delta Peak method. There is however one danger attached with temperature charging and that is forgetting to connect the temperature sensor to the battery pack. At that time there will be no measurment of temperature and the charge will not be interrupted which will result in explosion of the cells under charge! Could be pretty dangerous!

Charge Current
As I mentioned before, if you expect optimum performance from your battery pack, it is better NOT to slow charge. Optimum means a current of 3.5 amp for a 1700 SCE, 4.0 amp for a P-170 SCR, while the Sanyo SCR's optimum charge is 4.5 amp for normal usage.
The same rule applies here that with a certain current value the charge characteristics during the charging process can be changed. A higher charge current will provide a higher discharge voltage, in other words 'more power', where the above values may be increased with another 1 amp to get a final maximum 'boost'.

Constant current, soft/hard pulse charge
Most battery chargers operate with a constant current, meaning a constant current during the whole charging process. This method is especially recommended for the 1700 SCE cells, but is never wrong for any other cell also!

Fig. 2 - Charging with a constant current is always a good method.

An other way of charging is charging with a 'Pulse' where the current constantly changes between the maximum and the minimum value. This so-called pulsing happens about 60 times per second (60Hz). There are two different types of pulses:

Soft Pulse: there is a constant current with a value of approximately 2 Amp, but halve the time there is an added current of about 6 Amp. So, 60 times-per-second the current changes between 2 and 6 amps, where the average charge current is 4 Amps. This type of charging provides a higher discharge current, for example the Panasonic P-170-SCR cells.

Fig. 3 - Charging with a soft pulse provides some specific cells with a bit higher discharge current.

Hard Pulse: with this method the current changes 60 times-per-second between say, 0 and 12 Amps. If the charge current is 33% of the charge tme and the rest of it is 0, than the average amperage is again 4 Amps.
This is often used to charge the Sanyo SCR packs, which are given a maximum boost using this method. This way of charging is a bit more dangerous because I noticed that there are chargers available with a too high maximum which can be harmfull for the battery packs. So be careful with the purchase of such a charger, or a charger which has this as a feature. If you do decide to get this type of charger I strongly recommend spending a bit more money (no made-in-china crap!) and get a quality product.

Fig. 4 - Charging with a hard pulse provides the Sanyo SCR cells with a maximum 'boost'.

Current Source
A so-called 'charger' is a circuit which does nothing more than adjusting the current, with a relay to stop the charge. The energy which is pumped into the NiCad, has to be provided by some sort of current source and this is the one most important to provide high quality charge.
Most of the time this is a 12 volt car battery or the standard 110 vac house current.
During 'charge' there has to be a voltage-level difference between the NiCad and the charger in order to get the current flow going. When this voltage level is large enough, the current flow will deminish drastically and this way the quality of the charge cycle. Most of the time this goes unnoticed, because the cutoff circuitry in the charger will interrupt the charge-cycle 'anyway' and we think: "ahhh, my NiCad is full".
What really happens is: if the current flow drops, it also drops the voltage of the NiCad. A Delta-Peak charger will notice this voltage drop and stop the charging process all together. With temperature charging a similar situation occurs, because the cell is internally warmer than externally, even if the cell is not charging correctly anymore the temperature will still increase untill the cuttof point is reached. In this case this type of charger will thus just stop the charge cycle also without the cell being 'fully charged'.
So the solution is to provide a sufficient voltage level difference between the charger and the NiCad(s). A standard car battery will quickly be unable to provide more than 11.5volts after a couple of charges at the field. The voltage drop over the charger is also 1 to 1.5 volt so that the cells also don't get more than 10 to 10.5 volt. This is really not enough to sufficiently charge a 6-cell battery pack. You can see how important it is to choose a charger which provides a large enough voltage level difference for a completed charge. For a Delta-Peak charger this means an almost ripple (noise) free voltage, because the cutoff circuitry would otherwise react to this ripple and interrupt the charge. In this case, those who use the temperature method have it a lot easier since they can just use a simple transformer (12V/120 watt minimum) with a cheap ac-to-dc circuitry (4 diodes or less). If you really have to depend on charging your packs via the car battery it is advised to use a charger with a FET (Field Effect Transistor) controlled end stage because the voltage drop 'over' the charger is then less than a normal charger with transistors, and will provide the extra 1 volt or so to finish the charge cycle sufficiently.

The performance of your 'electric-model' starts with the charging of your battery pack, because: "what you don't put in it, you will not get from the electric motor!"

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