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LettGtSConi. from page 5

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rect connection is not available across thc battery tcrminals and/or opcration is required in a moving vehicle..

‘ Setting output RMS voltagc: Output RMS voitage may be set using an aver-age reading AC rectifier mcter. When the average reading meter registers 255 volts, the output RMS value will be close to 250 volts. This occurs because thc form factor for the output wavcform is close to 1.0 while the form calibration factor for an average reading rectifier instrument is 1.11. This method is mid-way in accuracy between the true RMS meter and the light bulb comparison methods, but far morę convenient than the latter.

* The additions deseribed above en-hance the versatility of the inverter without detracting in any way from its performance.

R. Beaumont,

Pcnnant Hills, NSW.

Comment: Thanks for the information Mr Beaumont. Vm surę other builders of this project will find it of great ihterest. *

NiCad batteries

I am very concerned about the Dick Smith Electronics advertisement for the NiCad cells, which appeared on page 78 of thc June EA, and again in the company^ insert in the July issue. The text contains statements and draws conclu-sions which are at variance with well known facts about these cells. I enclose copies of technical articles on NiCads which have appeared in EA, ETI and other publications, and have highlighted relcvant portions.

It appears to me from studying thc advertiscmcnt and a conversation I had with its author that he is not well in-formed on the subject of batteries and their applications.

Firstly, I consider the text of the ad to be vcry vague (c g., figures are quoted without reference to celi typc, size or brand). However, thc generał thrust is obvious and might be summa-rised as . . . “replacc the dry batteries in your portable electronic equipment with NiCads and enjoy large savings in cost, improvements in performance and longer running time per chaige than be-fore.” There are many well known problems with this proposition, making it doubtful advice at best. Taking the four paragraphs in order the objections are:

1. Four hundred plus charges might be obtained, but only under laboratory controlled test conditions. Series dis-charge of unmatched cells and recharge beforc complete dischargc will scverely limit the life of a celi. This high cost of changcover to NiCads ($50 to $300 in-cluding charger/s) means a break-even point of about 50 to 100 cycles. In many cases this may not be reached, due to cells deteriorating. Also many users will find waiting 12 to 14 hours before re-,use very inconvcnient, necessitating pur-chasc of a second set of cells so as to have a charged set on hand. Morę cost, hcnce longer to the break-even point.

2.    The constant discharge vo!tage of a NiCad is not disputed and may be good in a few applications, BUT the normul terminal voltage of only 1.2 volts is very close (within 0.1 volt) to the end point voltage of a dry celi. Your torch or cas-sette player will perform as if it had nearly fiat dry cells in it. Some equip-ment will barely work at the rcduced voltage offered by NiCads.

3.    The intemal resistance of a NiCad is very Iow, allowing high discharge cur-rents — sometimes dangerously high if the equipment was not designed for them. Stalled motors in toys can be burnt out, flash units may overheat and fail. (Sce National flash unit operating instructions). Accidental fires are likely if shorts occur inside equipment or to cells. A “C” size NiCad can dcliver 100 amps plus if shorted.

4.    The cnergy capacity (amp hours) of a NiCad is superior only to the poor-est grade of dry cells. The figures in the Plessey brochure (quoted as a source by DSE) were obtained by comparing a

Books

Continued from page 84

because of the difficulty in obtaining it elsewhere.

The author Richard Miller is appar-ently widely recognised in the USA as an expert in the fields of artificial intel-ligence, computer-aided engineering and robotics. He has published quite a few other books on these subjccts, and has acted as consultant to over 250 compa-nies. He has also madę pioneering con-tributions in the field of CAE, including an algorithm for electro-acoustic trans-ducer design.

In this book he has produced a practi-cal manuał for use by manufacturing en-gineers and engineers, covering the use of robots in many different applications. These include metal fabrication, elec-tronics manufacturing, plastics, food Processing, health care, textiles, printing and packaging. There*s a wealth of information given, although most of it seems to consist of illustrative cxamples

NiCad with a light duty dry celi, in a heavy duty situation. Arguing from a single example is logically erroncous.

I performed a 90mA discharge test on Eveready “Red” cells (as sold in Dick Smith Stores) and found they lasted over 5 hours to 1.1 volts. The specifica-tions for Evercady “Black” and Alka-line cells indicate 10 hours or morę is to be cxpected. ANYONE considering in-vesting in NiCads would have to be using their equipment regularly and heavily, and would be most unlikely to be using light duty dry cells. The exam-ple givcn in the ad is not typical, not explained (what dry celi?) and is there-fore misleading.

There is a further misleading asser-tion in the text accompanying the “Multi-Cell Chargcr”, where it is claimed one can “properly” check a NiCad celi by measuring its voltage under load. This is nonsense. The para-graph 2 statement about constant volt-age discharge means that the State of charge CANNOT be found by measuring the voltage under load. It is likely that users of this device will believe cells to be fully charged when they are not, and/or to overcharge only partly discharged cells. Either of these errors will shorten the life of a NiCad celi.

A charger design which overcomes this difficulty appeared in Electronics

Australia in March this vear.

¥

I would hopc that some form of cor-rection or retraction appears shortly.

Phil Allison,

Sydney, NSW.    ® of existing installations and applications, rather than design details.

Fairly obviously the artwork for thc book itself has been produced using a word processor — no doubt to save time and keep it as timely as possibje. This is fair enough, but to be honest it could have done with a bit morę proof reading; there are quitc a few typo’s in places. Some of the illustrations leave a lot to be desired, too — giving every appearance of being rcproduced from rather poor photocopies. Not too im-pressivc in a book of this very signifi-cant cost.

On the whole, I suspcct that the peo-ple in manufacturing concerns for whom it’s mainly been written will find it of great interest and value, despite these limitations. But it will have limited ap-peal to others.

The review copy came from distribu-tors Prentice-Hall of Australia, w ho ad-vise that it is available from major technical bookstores. (J R )    <Z>

122 ELECTRONICS Australia. September 1987