laptop keeps turning off

I have a 3 year old HP dv5118 and now on checking the battery life its says it needs to be replaced and to ckeck if under warranty unfortunaltely its not and so was planning to buy a new battery. Is 6 cell better or 12 cell better?
The main problem is that for the last 2 months laptop keeps shutting off when its doing something like writing a cd/dvd etc. It only works for basic things like websurfing etc.
If I try to convert a movie into another format is just switches off. I never had this problem before. I tried to remove the battery and work with main supply but still same problem happend
Please give me some valuable information for the same.
Thankyou