Definition
Theratioofthepoweroutputbythebatterytoitsweight.Alsoknownas"powerdensity".Note:Thespecificpowerisusuallyexpressedinwattsperkilogram(W/kg).
Introduction
Becausethepowergivenbythebatteryisnotdeterminedbyitself,itismoredeterminedbytheload,sothetotalamountofpowervariesgreatly.Althoughthebatteryhasthemaximumpower,itisunwisetomakeitworkinanyworkingconditionnearthemaximumpowerformorethanafewsecondsunderanycircumstances,becausethemaximumpowerwillnotlastforalongtimeandtheworkefficiencyisrelativelylow.
Somebatterieshavehigherspecificenergybutlowerspecificpower,whichmeansthattheycanstorealotofenergy,butreleaseenergyataslowerrate.Inelectricvehicles,theycandrivethevehicleforalongdistance,butthespeedwillbeveryslow.Foranyparticulartypeofbattery,ahigherspecificpowerusuallyresultsinalowerspecificenergy.
Fordifferenttypesofbatteries,thisdifferenceinspecificpowerandspecificenergyisveryimportant,andyoucanusethisdifferencetocomparethem.Asetofgraphsisusuallyusedtocomparespecificpowerandspecificenergy,andthisgraphiscalledtheRagonecurve.Sincethepowerofthebatteryvariesgreatlyindifferentapplications,theRagonecurveusesalogarithmicformtoexpresstherelationshipbetweenspecificenergyandspecificpower.
Whenthespecificpoweris1~100W/kg,thechangerateofspecificenergyofnickel-cadmiumbatteriesisslightlysmallerthanthatoflead-acidbatteries.However,whenthespecificpowerisabove100W/kg,thespecificenergyofnickel-cadmiumbatteriesdropsfarfasterthanlead-acidbatteries.
Ignoreotherfactorssuchasthecostofuse,iftherequiredspecificpowerislessthan100W/kg,theperformanceofthenickel-cadmiumbatterywillbebetter.However,whenthebatteryneedstoreachaspecificpowerof250W/kgorhigher,lead-acidbatteriesaremoreattractivetousers.ThepointemphasizedbytheRagonecurveisthatitisimpossibletogiveasimpleanswertothequestionof"whatisthespecificpowerofacertainbattery".