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Power supplies 259NiMH cells are available in a wide range of standard sizes, including button cells formemory back-up, and are also frequently specified in multi-cell packs for commonapplications such as mobile phones, camcorders and so on.Lithium-ionThe Lithium-ion cell has considerable advantages over the types described above.Principally, it has a much higher gravimetric energy density (available energy for a givenweight) – see Figure 7.26, which compares approximate figures for the three types,drawn from various manufacturers’ specifications. But also, its cell voltage is aboutthree times that of nickel batteries, 3.6–3.7V versus 1.2V. Its discharge profile withtime is reasonably flat with an endpoint of 3V (Figure 7.25), and it does not suffer fromthe NiCad memory effect.4.5Li-ion discharge curves43.5Volts32C CC/52.50.1 Hours 1 10Figure 7.25 Li-ion discharge characteristics201612Wh8Gravimetric energy densityLi ionNiCadLi-ion: 0.127Wh/gNiMHNiMH: 0.063Wh/g40NiCad: 0.037Wh/g0 50 100 150 200Weight gFigure 7.26 Comparison of energydensity versus weight(approximate values)
260 The Circuit Designer’s CompanionThese advantages come at a price, and Li-ion batteries are more expensive than theothers. Also, they are much more susceptible to abuse in charging and discharging. Thebattery should be protected from over-charge, over-discharge and over-current at alltimes and this means that the best way to use it is as a battery pack, purpose designedfor a given application, with charging and protection circuits built in to the pack. Thisprevents the user from replacing or accidentally degrading individual cells, and givesthe designer greater control over the expected performance of the battery. Since thehigh cost of a Li-ion battery pack makes it more suited to high value applications suchas laptops and mobile phones, the extra cost of the integrated control circuitry ismarginal and acceptable.7.5.4 ChargingThe recharging procedures are quite different for the various types of secondary cell,and you will drastically shorten their lifetime if you follow the wrong one. The greatestdanger is of over-charging. Briefly, NiCad and NiMH require constant currentcharging, while lead acid needs constant voltage.Lead-acidThe recommended method for these types is to provide a current-limited constantvoltage (Figure 7.27). The initial charging current is limited to a set fraction of the Cvalue, generally between 0.1 and 0.25. The constant voltage is set to 2.25−2.5V per cell,depending on whether the intention is to trickle charge or recover from a cyclicdischarge. The higher voltage for cyclic recharging must not be left appliedcontinuously since it will over-cook the battery. The actual voltage is mildlytemperature dependent and should be compensated by −4mV/˚C when operating withextreme variations. This charging characteristic can easily be provided by a currentlimitedvoltage regulator IC.Charged voltageCharging currentFigure 7.27 Current limited constant-voltage chargingtimeAn elegant modification to this circuit is to arrange for the output voltage to dropfrom the cyclic charge level to the trickle charge level when the charging current hasdecreased sufficiently, typically to 0.05C. This two-step charging gives the advantagesof rapid recovery from deep discharge along with the benefit of trickle charging withoutthreat to the battery’s life. It does not work too well when the load circuit is connected,though.The cheap-and-cheerful charging method is to charge the battery from a full-waveor half-wave rectified ac supply through a series resistance. This is known as “taper”charging because the applied voltage rises towards the constant voltage level as thecharging current tapers away. Since it is cheap it is very popular for automotive use, butmanufacturers do not recommend it because of the risk of over-charging, and the
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The Circuit Designer’s Companion
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NewnesAn imprint of ElsevierLinacre
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vi Contents2.2 Design rules 452.2.1
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viii ContentsChapter 5Analogue inte
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x Contents7.5.3 Secondary cells 256
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xii Contents
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2 The Circuit Designer’s Companio
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10 The Circuit Designer’s Compani
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Grounding and wiring 25Cable type R
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Grounding and wiring 27Shielding an
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Grounding and wiring 29successive h
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Grounding and wiring 31(a) Two audi
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Grounding and wiring 331. Side-by-s
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Grounding and wiring 35ABV pZ out =
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Grounding and wiring 37times will d
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Grounding and wiring 39of Z o . Thu
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Printed circuits 41material. The co
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Printed circuits 43board cost you s
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Printed circuits 45Board ABoard ABo
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Printed circuits 47which they can w
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Table 3.3 Survey of capacitor types
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100 The Circuit Designer’s Compan
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334 Indexde-rating 310dielectric ab
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336 IndexFlash ADC 209Foldback curr
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338 IndexMultilayer PCB constructio
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340 Indexlimiting element voltage 7
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