Battery life for my Thinkpad T520

[raczkaat]

I have a ++27 9 cell battery and an external ++55 9 cell battery connected, too. How long should my battery be? I don't know if I should have let it die or whatever that myth is. I charged them to 100% while using it and let it go down to 5% but it wouldn't die. It would just hibernate I think. So now it jumps from 6 to 10 hours, even though it lasted 15 the first day. I don't know much about computers, I'd appreciate an estimate or what I could do differently. I have had it for 4 days.