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Weight ratio power



Synonympowerdensitygenerallyreferstotheratioofpowertoweight.

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".

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