BATTERIES

Nickel-Cadmium (NiCad) is a very well-known rechargeable battery technology, used in electronic equipment.  It is also known by its famous memory effect, which makes this kind of battery lose its charge quicker when it is old than when brand new. This article will explain more in depth how NiCad batteries work what is the memory effect how it happens and how to prevent it.

NiCad batteries are made of two chemical elements, Nickel, Hydroxide, and Cadmium. A third element used as electrolyte, a solution of Potassium Hydroxide (KOH). Cadmium is the big problem. It is the element behind the memory effect, it is a heavy metal and very toxic.

Newer rechargeable battery technologies do not contain Cadmium anymore (e.g., Nickel-Metal Hydride [NiMH], Lithium-Ion [Li-ion] and Lithium-Ion Polymer [Li-Pol]).

Charging method batt2

A NiCad battery requires a charger slightly different then the regular charger. A charge termination method is needed if a fast charger is used. Most often battery packs have a thermal cut-off inside that feeds back to the charger telling it when to stop the charging. This is done once the battery has heated up and or a voltage peaking sensing circuit is triggered. When charging at room temperature under normal charge conditions the cell voltage will increase from an initial 1.2 V to a point of about 1.45 volts. This rate of rise increases quickly as the cell begins to reach full charge. The end point voltage reduces slightly with increasing temperature.

Overcharging

Sealed NiCad cells have a pressure vessel that is supposed to contain a portion of oxygen and hydrogen gases until they can recombine back in to water. Usually this occurs during a quick charge and discharge and when an overcharge conditions exist. When the pressure passes the limit of the safety valve, water in the form of a gas is lost. Being the unit is designed to have an exact amount of electrolyte this loss will quickly affect the capacity of the cell. This will also affect its ability to receive and deliver current. To be able to detect these conditions of overcharge demands you need great sophistication from the charging circuit. A cheap charger will damage even the best quality cells over time.

What is memory effect?

Memory effect is when your battery thinks that it is fully charged but it isn’t. Let’s say that is 60% charged but it thinks that it is 100% charged. When installed on its charger it will stop recharging, because it is thinking that it is already full. When you start using the battery, it will last shorter, since it is only 60% charged – this is why you assume that older NiCad batteries last less than new ones. This is the case, but there are ways of preventing the memory effect to happen.

This happens because of the formation of Cadmium crystals in the battery. These crystals are difficult to dissolve and these are responsible for the memory effect. The trick to avoid the memory effect is to avoid the formation of these crystals in the battery.

To avoid this only recharge the battery when it is discharged and not when it is partially discharged. Also high temperatures help the crystals to be formed.

This brings another problem to the table: NiCad batteries cannot be fully discharged or they will be damaged. Fully discharged means your voltage is below 1 V per cell (NiCad batteries are usually formed by grouping multiple 1.2 V cells.

The trick that is recommended by many people to fix the memory effect is by fully discharging NiCad batteries by shorting them or any other sort of quick discharge. This does more damage than good to the batteries, even though several people claim that they can recover NiCad batteries with memory effect by doing this. This kind of trick won’t dissolve the Cadmium crystals, which are responsible for the memory effect problem. The right way to discharge NiCad batteries and prevent memory effect is to discharge them by using them normally until your equipment complains that the batteries are low. The problem with this is we are flying remote control planes. If we wait we lose them.

Another thing some people claim is to recover NiCad batteries by zapping them, performing a high-current quick charge on them.

Monitoring the current charge status is very hard, because NiCad batteries don’t present a linear discharge ramp. The voltage found on a NiCad cell stays at 1.2 V until the battery is discharged. So even if the battery has only 25% of its charge, it will keep providing 1.2 V on its output.

To explain this, non-rechargeable 1.5 V have a linear discharge ramp, so when it has 50% of its charge, it will provide only 0.75 V on its output. So you can see why it is easier to monitor the current charge status of regular batteries. Just measure it with a voltmeter.

When a NiCad is partially charged, we can’t tell if it is really partially charged or fully charged, because on both scenarios the battery will provide 1.2 V on its output.

NiCad batteries are discharged whenever they present a 1.0 V voltage on its output. The problem, like we said before, is that if you keep using the battery below this point, you will damage it.

The correct way to recharge a NiCad battery is to fully charge it, use it and wait for it to achieve its 1 V level per cell, and only then recharge it. This is also known as a full charge cycle. NiCad batteries can only be recharged about1000 times. After that you will start to see battery problems.

Like I said before it becomes more of a problem with RC plane flight. If you don’t have the money to purchase a battery charge that can bring your batteries to the 1.0V, I will show you how to create a discharger and tell when your batteries are ready to be charged.

Dead Batteries

Some people said to recover a dead NiCad battery; this is a battery showing 0 V on a voltmeter and doesn’t revive by hooking them to a battery charger. They try doing a high current quick charge, a process known as zapping, and then put them back in to the charger for a regular charge.

This will work if the battery has an internal short circuit caused by a small dendrite, which is a crystal that develops with a typical multi-branching tree-like form connecting the two battery poles internally. What the zapping does is to burn this dendrite, like if it were a fuse, solving the short-circuit problem.

The problem with that is it can come back, not only other dendrites can be formed but also the material that was vaporized is now inside the battery, this can act as a resistor, making the battery hold a lesser charge than when it was good.

If you zap the battery and the dendrite was not the problem then an internal short circuit can damage the battery even more. As we said, if you let your NiCad battery completely discharged, it can be damaged.

Cycling

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Cycling a battery means charging it to capacity and then letting it discharge almost completely. This needs to be done in moderation. Excessive cycling will damage NiCad batteries and should be avoided. We only cycle batteries to measure the capacity and changes. By always applying the same load, we can find any difference in the discharge time which indicates a change in the battery. A pack with a sudden reduced discharge time could run out unexpectedly during a flight. To avoid this you should only cycle when the battery is new, at the beginning of a flying season, every six or so weeks during the flying season, and when you think there may be a problem. Recording and comparing the discharge times will give us advanced notice of a battery failure. There are many ways to cycle your battery. I will give you several and tell you what to look for.

Cycling needs to be done with an expanded scale voltmeter and a couple of flashlight bulbs. Radio Shack sells bulbs in many voltage and milliamp combinations.

Pick a six volt bulb for testing the receiver pack and another bulb rated at twelve or fourteen volts for the transmitter battery. Solder a charge connector and jacks for the probes to each one. This way you can tell how things are moving at a glance. We need to measure the time a bulb takes to discharge a pack until the average cell voltage reaches l.0 volts, 4.0 volts for the receiver pack or 8.0 volts for the transmitter. Let’s call this the “discharge time”. This is a simple measure of battery capacity. The time a battery can keep the bulb lit without going below l.0 volts per cell slowly decreases with age. Let say the light bulb discharge time on a new pack was five hours. When that time diminishes 20%, about one hour with the bulb we chose, I’d replace the pack. You can go below this but 80% is a safe number. A few bucks for a new battery is cheaper than a new plane. Another thing to look for is sudden drops in discharge times. This can cause more concern than the gradual drop just described above. When this happens, cycle the batteries two more times to be certain the change wasn’t causes by not having the battery at full capacity. A sudden drop of 6 percent indicates six percent of the plate area stopped working. I would watch this pack very carefully, or better yet replace it.

Using the same method above you can discharge your batteries using the light bulb until the voltage reaches 1 volt per cell. At this point you can now do a full recharge and not worry about the memory effect.

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