The Truth About NiCd Batteries
By Gabriel Torres on March 6, 2006


Introduction

Nickel-Cadmium (NiCd) is a very well known rechargeable battery technology, used by several electronic equipments, such as laptop computers, cell phones, cordless phones, old motherboards, etc. It is also very well known by its (in)famous “memory effect”, which makes this kind of battery to lose its charge faster when it is old than when it was brand new. In this article we will be explaining more in depth how NiCd batteries work, what and why the “memory effect” happens and how to prevent it.

As the name implies, NiCd batteries are made of two chemical elements, Nickel, under the form of Nickelic Hydroxide, and Cadmium. A third element used as electrolyte, usually a solution of Potassium Hydroxide (KOH). The Cadmium is the big villain. First, it is the element behind the “memory effect”, and second it is a heavy metal and thus very toxic.

That’s why newer rechargeable battery technologies do not use Cadmium anymore (e.g., Nickel-Metal Hydride [NiMH], Lithium-Ion [Li-ion] and Lithium-Ion Polymer [Li-Pol]). Laptop computers, cell phones, cordless phones and motherboards found today on the market don’t use NiCd batteries anymore and you won’t have any kind of problem or “memory effect” if your electronic gadget uses a different battery technology from NiCd. Just to make sure, if you take a look at your battery you will find a sticker showing which battery technology it uses. If it isn’t NiCd, you won’t have the “memory effect” problem.

What is this “memory effect” anyway?

“Memory effect” is when your battery “thinks” that it is fully charged but it isn’t. So let’s say that is 70% charged but it “thinks” that it is 100% charged. Under this condition, when installed on its charger it will stop recharging, because it is thinking that it is already full. When you start using your gadget, it will last shorter, since it is only 70% charged – and thus the assumption that older NiCd batteries last less than brand new ones. Which is true, but that are ways of preventing the “memory effect” to occur.

Why Does Memory Effect Occur and How To Prevent It?

Technically speaking, it occurs by the formation of Cadmium crystals inside the battery. These crystals are hard to dissolve and the ones responsible for the “memory effect”. So the trick to avoid the “memory effect” is to avoid the formation of those crystals inside the battery.

This is typically accomplished by recharging the battery only when it is discharged and not when it is partially discharged. Also, high temperatures help the crystals to be formed.

However this brings another problem: NiCd cannot be fully discharged or they will be damaged. Fully discharged usually means having a voltage below 1 V per cell (NiCd batteries are usually formed by grouping several 1.2 V cells; typical NiCd batteries are 3.6 V packs using three 1.2 V cells).

So the “trick” that is recommended by many people to solve “memory effect” by fully discharging NiCd batteries by shorting them (or any other sort of “quick discharge”) does in fact more damage than good to the batteries, even though several people claim that they can recover NiCd batteries with “memory effect” by doing this. The bottom line is: this kind of trick won’t dissolve the Cadmium crystals, which are responsible for the “memory effect” problem. The correct way to discharge NiCd batteries and prevent “memory effect” is to discharge them by using them normally on your gadget until your gadget complains that the batteries are low.

Another thing some people claim is to recover NiCd batteries by “zapping” them, i.e., performing a high-current quick charge on them. We will talk about this in the next page.

Monitoring current NiCd charge status is very hard, because NiCd batteries don’t present a linear discharge ramp. The voltage found on a NiCd cell stays at 1.2 V until the battery is “discharged”. So even if the battery has only 30% of its charge, it will keep providing 1.2 V on its output, for example.

Let’s explain this better. Regular non-rechargeable 1.5 V present 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 easily monitor the current charge status of a regular battery, it is just a matter of metering it with a voltmeter.

So when a NiCd is partially charged, when 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.

NiCd batteries are “discharged” whenever they present a 1 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. That’s exactly when your cordless phone starts beeping telling you that its battery is discharged, for example. Time to recharge it right away.

So the correct way to recharge a NiCd 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 recharge cycle”. NiCd batteries can only go through 500 full recharge cycles. After that the battery starts presenting problems.

Like we mentioned, usually electronic gadgets will let you know when the battery achieve this state: is the exact point when you gadget starts complaining that its battery is low.

Some old laptop computers using NiCd battery packs used a trick to let the user know how much battery was remaining. The manufacturer knew how much time the computer would last with the battery fully charged. So on the minute you disconnected tyour laptop from the wall, it started measuring the time it was running on batteries and made a calculation on how much battery was left. So it didn’t really measure the battery status (because, like we said, it is almost impossible to know NiCd charge status) but instead showed a “guess”.

Another big problem with NiCd batteries is that they lose charge when not is use, at a 1% per day rate. This means that a NiCd battery that is not is use will lose its charge at a 30% per month rate. In three and a half months it is totally gone, what can permanently damage it – like we said, NiCd batteries cannot be fully discharged.

Dead Batteries

Some people claim to recover dead NiCd batteries – i.e., batteries showing 0 V on a voltmeter and that doesn’t revive by hooking them to a charger – by performing a high-current quick charge, process known as “zapping”, and them putting them back to the charger for a regular charge.

In fact, this will work if the battery has an internal short circuit caused by a small dendrite, which is a small piece of material 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.

But the problem can come back, because not only other dendrites can be formed but also the material that was vaporized is now inside the battery, which can act as a resistor, making the battery to hold less charge than when it was good.

However, zapping the battery if the problem isn’t an internal short circuit can damage the battery even more. As we mentioned, if you let your NiCd battery completely discharged, it can be damaged – i.e., completely discharged forever – and the problem here won’t be any dendrite inside creating an internal short-circuit.

Keep in mind that this technique has nothing to do with “memory effect”. Some people can claim that they solved the “memory effect” of a battery by doing this but in fact the battery problem was another one (internal short circuit).

Summary

Originally at http://www.hardwaresecrets.com/article/The-Truth-About-NiCd-Batteries/292


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