The market for micra-electronic equipment isgrowing at a seemingly

ever-increasing rate.After the era of discrete

semiconductors, it isnow integrated circuitsthat make possible the

praduction of smalland smaller portable

electranic equipment. Itis estimated that in

Britain every family hasat least one portable

radio and one in everythree people has a mo-

bile telephone. Manycars have remote carlocks, not to speak of

the remote contralunits for the video

recorder, the audio sys-tem, and so on. All

these units need bat-teries, which also haveto become smaller andsmaller (have a look in-side your quartz crystalwatch: and that batteryis not small by current

standards).

By our Editorial Staff

64

le-battefY

It may come as a surprise to somereaders that most portable electronicequipment is powered by dry batter-ies: the world market for these is gi-gantic. In this overview, however, wewill look at rechargeable batteries only.Although these corne in many shapesand sizes, there are relatively fewbasic technologies on which they arebased. During and after the SecondWorld War, the deddy of them all, thelead-acid battery, was followed inquick succession by a host of differentsystems based on different materials:silver, nickel, zine, and lithium. Thesemetals are used in combination withvarious other materials: nickel-iron:ruckel-zinc: silver-hydrogen; silveroxide-zine; zinc-air; cadmium-air: alu-minium-air: lithium-iodine (almostuniversally used in cardiac paceruak-ers); lithium-molybdenum; lithium-iron-aluminium: and so on. This arti-ele will look at a few of these.

LEAD-ACID BATTERIESThe lead-acid battery is still the mostwidely used secondary battery in the

world, mainly because of its applica-tion in the automotive field, althoughit has a growing number of other ap-plications. Its advantages are low cost,high voltage per cell and good capac-ity life. Its disadvantages are itsweight and poor low-temperaturecharacteristics.

An open-type lead-acid batteryconsists of a number of lead platessuspended in sulphuric acid con-tained in a taugh, plastic, spill-praofcase. For many years now, sealed lead-acid batteries have also been available.A1though these are not produced inthe same quantities as the non-sealedtype, their uses are increasing.

NICKEL-CADMIUMBATTERIESNickel-cadmium (NiCd) batteries havea nickel anode and a cadmium cath-ode irnmersed in potassium hydrox-ide (KOH), normally contained in atough. sealed plastic container (onlylow-cepacfty types - over 30 Ah, theyare normally of the open type wh ichrequires periodic topping up to re-

Elektor Electronics 9/96

place electrolyte).Nickel-cadmium batteries have a

lang life (typically 1000 hours at the110 rate), and good low-temperaturecharacteristics. Their cost is, however,relatively high.

Sealed NiCd batteries are normallyrated at the C, eil' or xCI! rate. Theseare, respectively, the discharge (ur-reut in A, numerically equal to therated battery capacity in Ah, whichdischarges the battery in ] hour or I'l

hours. The x indicates a dischargecurrent of x times the rated battery ca-pacity in M.

When a NiCd battery gets warm,its capacity, compared with othertypes of battery, drops. This is the rea-son that a NiCd battery is normallycharged at a low rate. Howevet; manyappliances. such as mobile tele-phones, using NiCd batteries, (ome asstandard with a fast charger,

Another problern with NiCd bat-teries is their lack of charge retention,which is why it is recommended thatthese batteries are always fuUy dis-charged before they are recharged.

The most serious aspect of NiCdbattertes is, however; an environmen-tal one. When such a battery is at theend of its life and discarded, the riskof the dangerous cadruium in it end-ing up at a rubbish tip is great. This isthe reason that environmentalists(which should be all of us) advocatetheir total discontinuance.

NICKEL-METAL-HYDRIDE BATTERIESNickel-metal-hydride (NiMH) batter-ies are a development of the NiCdbattery, and have been wideJy used inspace batteries for a number of ycars.Their structure is similar to that ofNiCd batteries, but the cadmiumcathode has been replaced by a metalalloy. During normal operation of asealed NiMH battery, hydrogenatoms ccllect at the cathode and pro-duce metal hydride. The hydrogen isconsumed again du ring discharge,leaving the metal unaltered.

NiMH batteries da not suffer fromthe lack of retention so typical ofNiCd batteries.Also, they have ahigher energy-to-volume rationthan NiCd bat-teries.

In contrast tothe decreasingcapacity of NiCdbatteries with rising temperature, thecapacity of Ni1v1H batteries increaseswith rising temperature.

A drawback of NiMH batteries istheir relatively high self-discharge.which starts levelling out only at tem-peratures below -10 "C.

Figure 2. Nicke/-cad-mium batter/es mayrep/ace primary car-bon-zinc and a/kalinemanganese in mostportable equipment.

Elektor Electronics 9/96

Figure 1. Recharge-ab/e a/kaline man-ganese batteries havebeen availab/e tromUnion Carbide(Eveready) and Bat-tery Techn%gies /nc.have been available5ince the /ate 19805.For some reason, theyhave not (yet) caughton Europe.

LITHIUM-IONBATTERIESThere are many types of lithium bat-tery; some of the more recent onesare lithium-molybdenum (liMo) andlithium-ion (Li-ion) batteries. In viewof their very small selt-discharge andlarge capacity per unit volume, Li-ionbatteries appear to have a bright fu-ture. Lithium-based secondary bat-tertes, although developed in theearly 19705, have been entering main-

stream electronicdesigns only sincethe late 19805, par-ticularly in con-sumer; portableequipment andnon-volatile mern-ory backup appli-cations, wheresmall size, lang life and low cost areprime requirements.

The practicality of lithium is

65

greatly enhanced by the progress ineMDS technology. As low-power CMDS

teehnology advanees further, gaininga larger share of the integrated cireuitmarket, lithium batteries will nodoubt prove of great value.

The anode of an Li-ion batteryconststs of a lithiu m alloy, and theeathode of carbon 01' a metal alloy.These are inunersed in an organic liq-uid eontaining a saline solution.Lithium ions move between the elec-trodes when the battery is beingcharged er discharged.

Since the marerials used in lithiumbatteries have a smaller mass per unitenergy produced than is the case, forinstance. in a NiCd battery, lithiumbatteries are much lighter.

The capacity-to-volume ratio ofLi-ion batteries is about twiee that ofNi.MH batteries.

Li-ion batteries have a life of morethan 1200 cycles.

RECHARGEABLEALKALINE MANGANESEBATTERIESAJkaline mangenese batteries wereuntil recently typical of non-recharge-able batteries. However, in the late1980s reehargeable (seeondary) ver-sions of this type of battery have be-come available in the USA (UnionCarbide; Battery Technologies) -somehow, they have not (yet) eaughton in Europe.

These batteries use a specific elec-trochemical process, are maintenance-free and hermetieally sealed. Theywere designed as an alternative toNiCd batteries for electronie applica-tions where low initial cast and lowoperaring costs are imperative. The al-kaline mangenese secondary batteryeannot be recharged as many times asthe NiCd battery, but its initial cost isvery small compared to that of anequivalent NiCd battery.

The energy per cell decreases witheach charge/discharge eyele, al-though the open-circu+t volta ge re-mains substantially the same. Manu-facturers' data suggest that alkalinemanganese secondary batteries can berecyded 50-750 times,

The degree of loss per cyde ispartly determined by the moment atwhich recharging Is commenced. Theearlier in the dtscharging process thisis, the larger the residual capadry.Also important in this respect is a nottoo heavy load on the battery: dis-charge eurrents between 100 mA and150 mA are recommended.

Secondary alkaline manganesebatteries are about twice as expensiveas primary alkalme manganese bat-teries. However; in view of the factthat during their lifespan they deliver20-50 times as much energy, the in-vestment is worth while.

66

Secondary alkaline manganesebatteries can be recharged only byspecific charging equipment.

twice as high as that of currentrechargeabJe batteries, Unfortunately,they are limited to about 50charge/discharge cycles owing toproblems with separator materialsand the platinuni on the anode. How-ever; these will, no doubt. be over-corne within the very near future.

The lithium-polymer battery alsohas an energy density weil in excessof that of for instance, an NiMH bat-tery. Here also, designers are facedwith problems, prima.rily the lownumber of charge/discharge cyeles.These appear to be caused by degra-dation of the aetive eomponents inthe cell. probably owing to the reac-tion between lithium and the elec-trolyte. producing ethylene, propy-

THE FUTUREThe evolution of new battery systemsis inexorable. As far as can be seen atpresent, two systems are likely todomina te in the next 5-10 years: zinc-alr secondary batteries and lithtum-polymer secondary batteries.

The zinc-air primary battery hasbeen in use for some time in the form01 bu tton types for the supply 01, forinstance, hearing aids.

Early models of the zinc-air sec-ondary battery have shown an en-ergy density 01 some 170 W h kg--1 atthe Cs rate of discharge. This is about

Table 1 PropertJes of some impotten: types of rechargeable battery, all R6 stzeFor companson 's sake, the table mcludes data of a pnmary alkalme manganesebattery, R6 IS the lEG reference, in whlch the letter R indicetes that the batteryis cylmdnca/; the number 6 siqnities e perticuter size

Type of baHery primary secondary lead NiCd NiMH Li-lot:alkafine alkafine acid accu

Gapacity 2.5Ah 1 Ah - 0.7 Ah 1 Ah 0.7 Ah

Energy density 3.5 Wh 1.5 Wh - 0.8 Wh 1.2 Wh 2.6 Wh

E.M.F. 1.5 V 1.5 V 2.0 V 1.2 V 1.2 V 3.6 V

No of cycles 1 50 500 1000 1000 1000

Self-discharget 1% 0,4% 6% 15% 25% 0.1%

Peak current /imited /imited good good /imited moderate

* per cell t per month

Table 2 Cnsrecteristice of chargmg currents that may be used for chargmg sec-ondary battenes: they all have advantages and drawbacks

Method Remarks

lead-acid batteries

'1Limiting of cutrent only

~and voltage measuring errorowing to transfer Iresistance

Rectified sine wave inexpensive 'VmrnI

I

Pulsating currentobviates measuringerrors

I

obviate production ,- .---.---nof gasDischarge pulses I

improves retentioncapacity

improves retention ,Pulsating peak

capacity

current reducescrystallization I

Elektor Electronics 9/96

lene and lithium carbonate.

CHARGINGSecondary batteries that are dis-charged, or nearly so, must berecharged. A number of ehergingmethods have been developed in thepast few decades. Many types of in-tegrated circuit Ior eherging apphca-tions are available from a number ofmanufacturers.

There appears to be a particularcharging method for each type of sec-ondary battery. NiCd batteries areknown for the large peak currentsthey can provide, so that, conversely,large eherging currents may be usedas well. If a NiCd battery is chargedat 4C it is fully charged in about 15minutes (this is called ultra-rapidcharging; rapid eherging. that is witha charging period ofabout orte hour, Is farmore cammon nawa-days ~ see earlier com-ment on mobile tele-phones).

Figure 3. Batteries areproduced /n a largevarlety 01shapes andslzes. Shown are /wounlts In modern ver-sions 01wh/ch NIMHbatteries are used: anotebook computerand a portable radiotelephone.

CHARGERSThe fu nction of acharger is easily de-scribed: it must pumpcurrent into the batteryuntil this is fullycharged. A good-quality charger de-tccts the moment the battery is fullycharged. The requisite electronics maydo this by monitoring the voltage atthe battery terminals, the rising bat-tery tcmperature. or the productionof gas in the battery.

Lead-acid batteries are charged byapplying to them a fixed voltage(higher Ulan the battery's e.m.f.). Cur-rent Iimiting ensures that the charg-ing current does not exceed a gtvenvalue. When the battery is fu\lycharged, the current automaticallydrops to a lower level.

In many ca ses, the charging CUf-

rent need not be regulated. nor evensmoothed. For instance. the outputvoltage of a rectifier may be appliedvia a resistor directly to the terminalsof a NiCd battery.

Another method is charging witha pulsed constant current. During thetime between two pulses. the voltageac ross the battery is measured as acheck on the stete of charge.

In another method, the battery isdischarged for a short period after ithas been charged. This is done to re-duce the production of gas in the bat-tery. An additional benefit is that itimproves the charge retention inNiCd batteries.

Batteries mayaiso be charged withvery high current pulses. In thismethod, the average level of the cur-rent depends on the pulse-pauseratio. This method also irnproves the

charge retention ofNiCd batteries. More-over; it reduces crys-tallization, which

causes the battery capacity to dropslightly with every charge/dischargecyele. The characteristics in Figure 4show the amount of crystallization asa functicn of the number ofcharge/discharge cyeles. They alsoshow that small charging currents(below OJC) darnage the battery.lt isfar better to use a high chargtng cur-rent with a smalJ pulse-pause ration.

196(018)

Elektor Electronics 9/96

1111'11'111'11

111'1"1'~-l 1

References:

Ballery Rejerence Book, 2nd Edition, byT.R. Crompton(Butterworth-Heinellla/1111995)

'Bnttcrics', Elektor Flecironics, December1994.

Figure 4. These char-acteristics show thatcrystallization is tn-creased by smallcharging currents.The larger that cur-rent, the smaller thedegree of crystalliza-tlon. The degree 01crystallization is givenon the y-axls and thenumber 01charge/dis-charge cycles on thex-axis.

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67