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Batteries in a Portable World: A Handbook on Rechargeable Batteries for Non-Engineers, Second Edition
Batteries in a Portable World: A Handbook on Rechargeable Batteries for Non-Engineers, Second Edition, by Isidor Buchmann (Cadex Electronics Inc), starting at $7.20



Home » Power
The Truth About NiCd Batteries
Author: Gabriel Torres
Type: Articles Last Updated: March 6, 2006
Page: 2 of 4
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.

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