3500mah?
Or buy this for €10:
dx.com/p/replacement-high-capacity-3-7v-3500mah-battery-pack-with-back-case-for-sonyericsson-xperia-arc-x12-69083
I use this battery for a year now and I can still use my phone for 2 days when charged 100%, including a bit of navigation and browsing, normal use of whatsapp and calling...