Android - How to show the battery percentage remaining in numbers?

if you use cyanogenmod, there's an option to show numeric battery percentage, and numeric signal strength in dbm on the status bar

You can try Battery Notif free from Android Market.

Bardcode Scanner compatible store link

Battery indicator is a free app that does this: https://www.appbrain.com/app/com.darshancomputing.BatteryIndicator

Unfortunately, it doesn't replace the stock icon, it just adds another that displays the numbers. I'm not aware of any way to replace or change the stock icon, even with root access.