davesainsbury.com

Nicad Madness

Introduction

I am feeling much better now. Windows 95 is running like a dream. For example, I select ‘Send’ from the ‘File’ menu when I finish typing a letter. Win95 transfers the document to the Mail Exchange program and asks me to choose the recipient from my address book. If the recipient has a FAX number, Win95 starts the modem, rings up and sends the document as a FAX. If the recipient has an EMAIL address and I am at home, Win95 starts the Modem, rings the university, connects to my mail server , “posts” the document and hangs up. If I am at work ,Win 95 ‘sees’ the network connection in the docking station, connects directly to my mail server and again the document is “posted” without any help from me. It doesn’t get any better than this! To add icing to the cake Netscape has just brought out a Web Browser containing a WYSIWYG web page editor, newsreader and mail server. Oh heart be still within my breast!

I am still under doctor’s orders not to write about installing Win95. The article for the last news letter was so disturbed the Editorial committee considered it was unsuitable for the general readership. Thrill seekers can find that article at “http://www.ozemail.com.au/~dsainsbu/asa/asa8.html”.

NiCd (nicad) batteries are now endemic; laptop computers, cordless drills, video cameras, marital aids, cellular phones, portable patient monitors… There are lot of myths and very few facts concerning the care and feeding of NiCd’s. The truth emerges from a basic understanding of their anatomy and physiology.

Anatomy and Physiology of a NiCd Battery

A NiCd battery is a collection of NiCd cells. A NiCd cell consists of a positive nickel hydroxide terminal and a negative cadmium terminal in an electrolyte solution containing hydroxyl (OH-) ions.
During discharge, cadmium is oxidised by the loss of electrons (anode…oxidation) These electrons pass through the external circuit doing work. The nickelic hydroxide is reduced to nickelous hydroxide by the acceptance of electrons (cathode…reduction). The electrolyte allows the movement of charge within the cell.

In the early phase of charging the discharge reactions are simply reversed. The current that will completely discharge a battery in one hour is known as C. Theoretically you could recharge the battery in one hour using a current of C. In reality you need 1.2 to 1.4 C. Most overnight chargers run at C/10. As a result they will take 12-14 hours to recharge a flat battery. What happens when the nickel is all converted back to the nickelic state (Ni+++) and the cadmium to metal (Cd)? The current starts to split water into hydrogen and oxygen. The pressure inside the battery goes up. The gases are released through a vent to stop the cell exploding. The cell is damaged anyway by because the loss of gas and water dries out the electrolyte.


The cell is made with an excess of cadmium hydroxide to prevent hydrogen gas from forming. Oxygen is formed at the positive terminal as expected but cadmium metal is formed at the negative terminal. The oxygen diffuses through the electrolyte to combine with this metallic cadmium and regenerate the cadmium hydroxide. Energy is not stored in this process, it is released as heat. You can tell if your battery has reached this state because it gets warm.

This protection mechanism has its limits however. The maximum current it can absorb is about C/10. Now you can see why this is chosen as the overnight charging current. You can accidentally leave the battery in the charger for the next day. The worst that can happen is that it will get a bit warm.

If you leave the battery in the overnight charger for days at a time the slow charging and increased temperature will enlarge the crystals of cadmium at the negative plate. You can also do this by repeatedly overnight charging a battery that is only partially discharged. The cadmium crystals cause a ‘dip’ to form in the operating voltage of the cell. There are also changes in the crystalline structure of the positive plate that will produce a dip in the operating voltage. The longer the overcharge the closer the dip gets to the beginning of the discharge cycle.

Curve A represents the voltage during the discharge cycle of a new 12 volt battery (10 X 1.2V cells) Curve B represents a battery that has been consistently overcharged. If electronic equipment is sensitive to voltage levels it will interpret this as a failing battery and switch itself off at point C even though the battery still contains a full charge. It will appear to have developed a ‘memory’.

Some people claim this memory is a consequence of repeatedly discharging the NiCd to a certain level and then recharging it. They claim the NiCd ‘remembers’ that level. Now you know the truth!

How to fix NiCd ‘Memory’

Don’t get into trouble in the first place. Run your battery until it is fully discharged. (You will need a back up battery.) Trickle charge overnight and then remove the battery from the charger. If your battery is warm it is fully charged. Take it out of your computer if it is not needed because your computer may be continuing to charge it. If the battery starts to run out sooner than expected take it through 3 complete discharge-recharge cycles. Here is catch 22. If your computer is shutting down at 20 % discharge how do you fully discharge the battery? Do not take it down to zero volts as you risk a nasty phenomenon called ‘cell reversal’. The cell that runs out first is fried by the remaining cells. You need to take the battery down to about 1 volt per cell. (A nice little NiCd discharger kit is available from Dick Smith.) There is no advantage in deep cycling NiCd’s routinely. The batteries have a limited number of charge/discharge cycles and you will be wearing them out for no good reason. (Battery and recharger manufacturers may not agree !)

References

There is a huge folklore behind NiCd’s. You will find more information in these locations http://www.bath.ac.uk/~bspahh/bikelights/lights.html ,http://www.cc.columbia.edu/~fuat/cuarc/NiCd.html . Special thanks to John Russell (Adelaide) for a truckload of information.

Next month Hot Java!

Dave Sainsbury

EMAIL: david.sainsbury”AT”adelaide.edu.au  Last Update:02/05/2005