Battery　thermal　management　system　(BTMS)　is　important　for　the　battery　pack　in　electric　vehicles.　Existing　literature　focuses　on　the　structure　of　BTMS　but　not　on　the　selection　of　the　optimal　cooling　surface　of　the　battery.　To　the　authors＇　knowledge,　this　study　first　investigates　the　optimum　cooling　surface　for　prismatic　lithium　battery　based　on　anisotropic　thermal　conductivity,　dimensions,　and　metal　shell.　The　specific　heat,　thermal　conductivity,　and　heat　generation　are　measured　experimentally　to　establish　a　three-dimensional　(3D)　shell　cell　separation　numerical　model.　Results　show　that　aluminum　shell　plays　the　role　of　fin　and　enhances　the　cooling　effect.　Cooling　on　surface　B　has　better　effect　when　the　aluminum　shell　thickness　is　less　than　0.3　mm;　otherwise,　cooling　on　surface　A　is　better.　The　cooling　effect　can　be　improved　by　increasing　the　thickness　and　area　of　aluminum　shell.　Battery　temperature　rise　reduces　by　67.5%　when　the　thickness　changes　from　0　mm　to　1　mm.　A　negative　linear　correlation　exists　between　the　temperature　rise　of　the　battery　and　the　cooling　area.　Double　surface　cooling　reduces　the　temperature　rise　by　24.1%　compared　with　single　surface　cooling.　The　investigation　on　optimal　cooling　surface　can　guide　the　selection　of　cooling　form　of　BTMS.