Battery Troubles

[Twista89]

Ok well I bought me a second battery off ebay a couple of days ago to help with this battery life but I have a question...I was boasting with my phone today lol and was using the navigation feature to show the street level googe maps and i remember the battery was down to 15% at one point and the next like 3-4 minutes it completely cut off. Now I have rooted the phone and put bugless beast on and OCed to 800mhz, and I have heard that once the voltage goes below a certain point the phone will cut off. But being at 15% would that cause the voltage to be to low even though its still got a little bit of life left? and also Ive been charging the phone for the past 20 minutes and I cant get past the Boot Loader screen where it keeps saying Battery Low Cannot program...Is everything alright???