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FORGeneralMotors,agooddealofthecompany'srecoveryfromitsbrushwithbankruptcyisridingontheChevroletVolt(OpelorVauxhallAmperainEurope),itsplug-inhybridelectricvehiclelaunchedayearago.NotthatGMexpectsthesleekfour-seatertobeacashcow.Indeed,thecarcompanylosesmoneyoneveryoneitmakes.Butthe$41,000(beforetaxbreaks)ChevyVoltisa"halo"cardesignedtoshowtheworldwhatGMiscapableof,andtolurecustomersintodealers'showrooms—tomarvelatthevehicle'singenioustechnologyanditsfueleconomyof60milespergallon(3.9litres/100km)—andthentodriveoffinoneorotherofGM'sbread-and-buttermodels.

So,itisnosurprisethatGMshouldbendoverbackwardstomollifycustomersconcernedbyrecentnewsoftheVolt'slithium-ionbatterycatchingfirefollowingcrashtests.GMisofferingtoloancarstoVoltownersworriedabouttheirvehicle'ssafetywhileanofficialinvestigationisunderwayandmodificationsmadeifdeemednecessary.Thecompanyhas

Evenofferedtobuyvehiclesbackfromownerswhohavelostconfidenceinthetechnology.Therehavenotbeenmanytakers.AsofDecember5th,fewerthanthreedozenowners—outof6,400VoltssoldtodateinNorthAmerica—hadrequestedloancars.AndonlyacoupleofdozenhadaskedfortheirVoltstobeboughtback.Atasuitableprice,yourcorrespondentwouldhavewelcomedthechancetobuyoneofthosesecondhandbuy-backsforhimself,hadtheynotalreadybeensnappedupbyemployees.DanAkerson,GM'schiefexecutive,isbelievedtohaveboughtoneforhiswife.

ThetroubleallstartedinMay,whentheNationalHighwayTrafficSafetyAdministration(NHTSA)carriedoutaroutine20mph(32km/h)crashtestonaVolt—tosimulateasidewaysimpactwithatreeortelegraphpolefollowedbyarollover.Threeweeksafterthetest,thecar's16kilowatt-hourbatterypackcaughtfireinNHTSA'scarpark,destroyingthevehicleandseveralothersnearby.

Shortlythereafter,bothNHTSAandthecarmakerrepeatedtheside-impactandrollovertestonatleasttwoothercars,alltonoeffect.However,insubsequenttests—carriedoutinNovemberbyexpertsfromtheenergyanddefencedepartmentsaswellasGM—theinvestigatorsdeliberatelydamagedthebatterypacksandrupturedtheircoolantlines.Onebatterypackbehavednormally.Anotheremittedsmokeandsparkshoursafteritwasflippedonitsback.Andathirdexhibitedatemporaryincreaseintemperature,butthenburstintoflamesaweeklater.GMclaimstheinitialfireinJunewouldneverhavehappenediftheNHTSA'sengineershaddrainedtheVolt'sbatteryimmediatelyaftertheimpact.Itisoddthattheydidnot.Whencrashtestingaconventionalpetrol-poweredcar,thestandardprocedureistodrainthefueltanktopreventanychanceoffire.Itwouldseemreasonabletodotheequivalentwithanelectricvehicle.

But,then,GMdidnotadopta"depowering"protocolfortheVoltuntilaftertheJunefire.Evenwhenitdid,itfailedtosharetheprocedurewiththesafetyagencyuntilembarkingontheNovembertests.Inthewakeofthelatestfindings,GMisnowworkingwiththeSocietyofAutomotiveEngineers,NHTSAandothervehiclemanufacturers,aswellasfire-fighters,tow-truckoperatorsandsalvagecrew,toimplementanindustry-widestandardforhandlingbattery-poweredvehiclesinvolvedinaccidents.

ToyotaranintosimilartroubleswhenitsPriushybridcarwasintroducedoveradecadeago.ThoughthePrius'sbatterypackisconsiderablysmallerthantheVolt's,fire-fightersandotherfirst-respondershadtolearnhowtodisarmthevehiclefollowinganaccident—byremovingfusesfromunderthebonnetandpullingacatchbeneaththerearstorageareatoisolatethehigh-voltagesystem.Untiltheyhaddoneso,theywerewarned,theywereonnoaccounttotakeametalcuttertoanoverturnedPriustoextricatetrappedoccupants.Lurkingbeneaththefloorwasabigorangecablecarryingaheavycurrentthatwouldhavefriedanyoneslicingthroughit.

Thelithium-ioncellsusedintheVolt'sbatterypackhavemanyvirtues.Theyaremuchlighterandoperateatahighervoltagethanotherrechargeablecells—andcanthereforestoremoreenergyforagivenweight.Inaddition,theyhaveno"memoryeffect"(thetendencytoacceptlessandlesschargeeachtimetheyarerecharged)andcanalsoholdtheirchargefarlongerthan,say,thenickel-metalhydridecellsusedinthePrius.Forgoodreason,allplug-inelectricvehicles,includingtheNissanLeafandtheforthcomingFordFocusElectricplusToyota'slong-awaitedplug-inPrius,haveembracedlithium-ionchemistry.Butlithiumisahighlyreactiveelement.Ifovercharged,physicallydamagedorallowedtogettoohot,lithium-ioncellscanexperiencethermal"runaway"andevenexplode—ashashappenedonnumerousoccasionswiththelithium-ionbatteriesinlaptopcomputersandmobilephones.Also,ifallowedtodraincompletely,theycanshort-circuitandmakerechargingdangerous.Forthesereasons,alllithium-ionrechargeablebatteriescontaincircuitrythatshutsthemdownwhentheirvoltagerisesaboveorfallsbelowacertainlevel.

TohelpkeeptheVolt's435lb(197kg)batterypackattherighttemperature,GMdesignedasophisticatedthermal-managementsystem.Thisisseparatefromthemainradiatorsystem,whichcoolstherange-extendingmotor-generator(a1.4-litrepetrolengine)andfeedsthecar'sheater.Thebatterypack,mountedinaT-shapedsteeltraywithaplasticcover,runsdownthecentreofthevehicle.