Things you need:
- Tool for decompiling/compiling work (I will not cover this, there are many guides for this)
- Some common sense
Go in res/drawable
Open stat_sys_battery.xml and replace all with this
Now open stat_sys_battery_charge.xml and replace all with this
Now put your all battery png images in res/drawable-hdpi
Images should be named as- "stat_sys_battery_0", "stat_sys_battery_1"..........."stat_sys_battery_9 9", "stat_sys_battery_100"
For charging images- "stat_sys_battery_charge_anim0", "stat_sys_battery_charge_anim1"................... ."stat_sys_battery_charge_anim99, "stat_sys_battery_charge_anim100"
You can get some battery images from here. Extract package to get battery images, dont flash them.
You can find more in forum.
At last recompile SystemUI.apk