[Galaxy S II Themes and Apps] kahvitahra:
This is what original stat_sys_battery.xml looks like

<?xml version="1.0" encoding="UTF-8"?>
<level-list
xmlns:android="http://schemas.android.com/apk/res/android">
<item android:drawable="@drawable/stat_sys_battery_0" android:maxLevel="2" />
<item android:drawable="@drawable/stat_sys_battery_5" android:maxLevel="7" />
<item android:drawable="@drawable/stat_sys_battery_10" android:maxLevel="12" />
<item android:drawable="@drawable/stat_sys_battery_15" android:maxLevel="17" />
<item android:drawable="@drawable/stat_sys_battery_20" android:maxLevel="22" />
<item android:drawable="@drawable/stat_sys_battery_25" android:maxLevel="27" />
<item android:drawable="@drawable/stat_sys_battery_30" android:maxLevel="32" />
<item android:drawable="@drawable/stat_sys_battery_35" android:maxLevel="37" />
<item android:drawable="@drawable/stat_sys_battery_40" android:maxLevel="42" />
<item android:drawable="@drawable/stat_sys_battery_45" android:maxLevel="47" />
<item android:drawable="@drawable/stat_sys_battery_50" android:maxLevel="52" />
<item android:drawable="@drawable/stat_sys_battery_55" android:maxLevel="57" />
<item android:drawable="@drawable/stat_sys_battery_60" android:maxLevel="62" />
<item android:drawable="@drawable/stat_sys_battery_65" android:maxLevel="67" />
<item android:drawable="@drawable/stat_sys_battery_70" android:maxLevel="72" />
<item android:drawable="@drawable/stat_sys_battery_75" android:maxLevel="77" />
<item android:drawable="@drawable/stat_sys_battery_80" android:maxLevel="82" />
<item android:drawable="@drawable/stat_sys_battery_85" android:maxLevel="87" />
<item android:drawable="@drawable/stat_sys_battery_90" android:maxLevel="92" />
<item android:drawable="@drawable/stat_sys_battery_95" android:maxLevel="97" />
<item android:drawable="@drawable/stat_sys_battery_100" android:maxLevel="100" />
</level-list>

This is what you want it to look like.

<?xml version="1.0" encoding="UTF-8"?>
<level-list
xmlns:android="http://schemas.android.com/apk/res/android">
<item android:drawable="@drawable/stat_sys_battery_0" android:maxLevel="0" />
<item android:drawable="@drawable/stat_sys_battery_1" android:maxLevel="1" />
<item android:drawable="@drawable/stat_sys_battery_2" android:maxLevel="2" />
<item android:drawable="@drawable/stat_sys_battery_3" android:maxLevel="3" />
<item android:drawable="@drawable/stat_sys_battery_4" android:maxLevel="4" />
<item android:drawable="@drawable/stat_sys_battery_5" android:maxLevel="5" />
<item android:drawable="@drawable/stat_sys_battery_6" android:maxLevel="6" />
<item android:drawable="@drawable/stat_sys_battery_7" android:maxLevel="7" />
<item android:drawable="@drawable/stat_sys_battery_8" android:maxLevel="8" />
<item android:drawable="@drawable/stat_sys_battery_9" android:maxLevel="9" />
<item android:drawable="@drawable/stat_sys_battery_10" android:maxLevel="10" />
<item android:drawable="@drawable/stat_sys_battery_11" android:maxLevel="11" />
<item android:drawable="@drawable/stat_sys_battery_12" android:maxLevel="12" />
<item android:drawable="@drawable/stat_sys_battery_13" android:maxLevel="13" />
<item android:drawable="@drawable/stat_sys_battery_14" android:maxLevel="14" />
<item android:drawable="@drawable/stat_sys_battery_15" android:maxLevel="15" />
<item android:drawable="@drawable/stat_sys_battery_16" android:maxLevel="16" />
<item android:drawable="@drawable/stat_sys_battery_17" android:maxLevel="17" />
<item android:drawable="@drawable/stat_sys_battery_18" android:maxLevel="18" />
<item android:drawable="@drawable/stat_sys_battery_19" android:maxLevel="19" />
<item android:drawable="@drawable/stat_sys_battery_20" android:maxLevel="20" />
<item android:drawable="@drawable/stat_sys_battery_21" android:maxLevel="21" />
<item android:drawable="@drawable/stat_sys_battery_22" android:maxLevel="22" />
<item android:drawable="@drawable/stat_sys_battery_23" android:maxLevel="23" />
<item android:drawable="@drawable/stat_sys_battery_24" android:maxLevel="24" />
<item android:drawable="@drawable/stat_sys_battery_25" android:maxLevel="25" />
<item android:drawable="@drawable/stat_sys_battery_26" android:maxLevel="26" />
<item android:drawable="@drawable/stat_sys_battery_27" android:maxLevel="27" />
<item android:drawable="@drawable/stat_sys_battery_28" android:maxLevel="28" />
<item android:drawable="@drawable/stat_sys_battery_29" android:maxLevel="29" />
<item android:drawable="@drawable/stat_sys_battery_30" android:maxLevel="30" />
<item android:drawable="@drawable/stat_sys_battery_31" android:maxLevel="31" />
<item android:drawable="@drawable/stat_sys_battery_32" android:maxLevel="32" />
<item android:drawable="@drawable/stat_sys_battery_33" android:maxLevel="33" />
<item android:drawable="@drawable/stat_sys_battery_34" android:maxLevel="34" />
<item android:drawable="@drawable/stat_sys_battery_35" android:maxLevel="35" />
<item android:drawable="@drawable/stat_sys_battery_36" android:maxLevel="36" />
<item android:drawable="@drawable/stat_sys_battery_37" android:maxLevel="37" />
<item android:drawable="@drawable/stat_sys_battery_38" android:maxLevel="38" />
<item android:drawable="@drawable/stat_sys_battery_39" android:maxLevel="39" />
<item android:drawable="@drawable/stat_sys_battery_40" android:maxLevel="40" />
<item android:drawable="@drawable/stat_sys_battery_41" android:maxLevel="41" />
<item android:drawable="@drawable/stat_sys_battery_42" android:maxLevel="42" />
<item android:drawable="@drawable/stat_sys_battery_43" android:maxLevel="43" />
<item android:drawable="@drawable/stat_sys_battery_44" android:maxLevel="44" />
<item android:drawable="@drawable/stat_sys_battery_45" android:maxLevel="45" />
<item android:drawable="@drawable/stat_sys_battery_46" android:maxLevel="46" />
<item android:drawable="@drawable/stat_sys_battery_47" android:maxLevel="47" />
<item android:drawable="@drawable/stat_sys_battery_48" android:maxLevel="48" />
<item android:drawable="@drawable/stat_sys_battery_49" android:maxLevel="49" />
<item android:drawable="@drawable/stat_sys_battery_50" android:maxLevel="50" />
<item android:drawable="@drawable/stat_sys_battery_51" android:maxLevel="51" />
<item android:drawable="@drawable/stat_sys_battery_52" android:maxLevel="52" />
<item android:drawable="@drawable/stat_sys_battery_53" android:maxLevel="53" />
<item android:drawable="@drawable/stat_sys_battery_54" android:maxLevel="54" />
<item android:drawable="@drawable/stat_sys_battery_55" android:maxLevel="55" />
<item android:drawable="@drawable/stat_sys_battery_56" android:maxLevel="56" />
<item android:drawable="@drawable/stat_sys_battery_57" android:maxLevel="57" />
<item android:drawable="@drawable/stat_sys_battery_58" android:maxLevel="58" />
<item android:drawable="@drawable/stat_sys_battery_59" android:maxLevel="59" />
<item android:drawable="@drawable/stat_sys_battery_60" android:maxLevel="60" />
<item android:drawable="@drawable/stat_sys_battery_61" android:maxLevel="61" />
<item android:drawable="@drawable/stat_sys_battery_62" android:maxLevel="62" />
<item android:drawable="@drawable/stat_sys_battery_63" android:maxLevel="63" />
<item android:drawable="@drawable/stat_sys_battery_64" android:maxLevel="64" />
<item android:drawable="@drawable/stat_sys_battery_65" android:maxLevel="65" />
<item android:drawable="@drawable/stat_sys_battery_66" android:maxLevel="66" />
<item android:drawable="@drawable/stat_sys_battery_67" android:maxLevel="67" />
<item android:drawable="@drawable/stat_sys_battery_68" android:maxLevel="68" />
<item android:drawable="@drawable/stat_sys_battery_69" android:maxLevel="69" />
<item android:drawable="@drawable/stat_sys_battery_70" android:maxLevel="70" />
<item android:drawable="@drawable/stat_sys_battery_71" android:maxLevel="71" />
<item android:drawable="@drawable/stat_sys_battery_72" android:maxLevel="72" />
<item android:drawable="@drawable/stat_sys_battery_73" android:maxLevel="73" />
<item android:drawable="@drawable/stat_sys_battery_74" android:maxLevel="74" />
<item android:drawable="@drawable/stat_sys_battery_75" android:maxLevel="75" />
<item android:drawable="@drawable/stat_sys_battery_76" android:maxLevel="76" />
<item android:drawable="@drawable/stat_sys_battery_77" android:maxLevel="77" />
<item android:drawable="@drawable/stat_sys_battery_78" android:maxLevel="78" />
<item android:drawable="@drawable/stat_sys_battery_79" android:maxLevel="79" />
<item android:drawable="@drawable/stat_sys_battery_80" android:maxLevel="80" />
<item android:drawable="@drawable/stat_sys_battery_81" android:maxLevel="81" />
<item android:drawable="@drawable/stat_sys_battery_82" android:maxLevel="82" />
<item android:drawable="@drawable/stat_sys_battery_83" android:maxLevel="83" />
<item android:drawable="@drawable/stat_sys_battery_84" android:maxLevel="84" />
<item android:drawable="@drawable/stat_sys_battery_85" android:maxLevel="85" />
<item android:drawable="@drawable/stat_sys_battery_86" android:maxLevel="86" />
<item android:drawable="@drawable/stat_sys_battery_87" android:maxLevel="87" />
<item android:drawable="@drawable/stat_sys_battery_88" android:maxLevel="88" />
<item android:drawable="@drawable/stat_sys_battery_89" android:maxLevel="89" />
<item android:drawable="@drawable/stat_sys_battery_90" android:maxLevel="90" />
<item android:drawable="@drawable/stat_sys_battery_91" android:maxLevel="91" />
<item android:drawable="@drawable/stat_sys_battery_92" android:maxLevel="92" />
<item android:drawable="@drawable/stat_sys_battery_93" android:maxLevel="93" />
<item android:drawable="@drawable/stat_sys_battery_94" android:maxLevel="94" />
<item android:drawable="@drawable/stat_sys_battery_95" android:maxLevel="95" />
<item android:drawable="@drawable/stat_sys_battery_96" android:maxLevel="96" />
<item android:drawable="@drawable/stat_sys_battery_97" android:maxLevel="97" />
<item android:drawable="@drawable/stat_sys_battery_98" android:maxLevel="98" />
<item android:drawable="@drawable/stat_sys_battery_99" android:maxLevel="99" />
<item android:drawable="@drawable/stat_sys_battery_100" android:maxLevel="100" />
</level-list>

Notepad++ is good for editing.

edit. REMEMBER that you need to

decompile

framework-res.apk to make that .xml file to look like that. Like Nitroz said use apkmanager or apktool for that

edit2. you need to have framework-res decompiled when you put those battery images inside that drawable-hdpi folder. That way when you compile in puplic.xml will be generated ids for those.

This is basic stuff, google is also full of tutorials for this :)