Battery life sudden deterioration.

Hi,
I have a dell 1520. When I bought it I got 3 hours 30 mins use out of a 6 cell battery.
After 9 months that fell to 1 hour 45mins. Seemed to happen all of a sudden. The battery deteriorated gradually (was down to 2 hrs 50 after 6 months), but between about 7 and 8 months lost almost an hour.

I decided to buy a new 9 cell battery expecting about 4 hours use but all I get out of this is 3 hours (less than I used to get out of a smaller battery).

Not sure whats changed. I recently took vista off and installed XP (no effect) - still 3 hours under light use. Passmark battery monitor quotes battery wear at 6% (not much).

I have done bios upgrade and I'm wondering if this latest upgrade to A09 is to blame. The high pitched core 2 whine I once had is now noticable by its absence. Could this have had a very bad effect on battery life disabling C4 CPU state somehow? Is there anyway to go back to A08 to find out?
I'll put up with whine most of the time and use RMclock when I want it to be silent.

 

You should take a look at your system. How many programs are running or using resources? How many processes are running in the background? What power settings are you using? Is your brightness higher than it needs to be? Are you using wireless, bluetooth, or leaving a disc in the drive that you don't need?

The Bios upgrade will run the fan more often, but that did not impact my battery life in a way that was noticeable.

 

Given its a clean install of XP, very little processes. I checked before I installed anything.
Looking at reviews my 3 hours for a 9 cell is normal. I can only think that before when I was getting that from a 6 cell the battery monitor was woefully inaccurate or I got a particularly good battery which is no longer at full health. My 9 cell did come via ebay so may not be as fresh as it should be either.
Current life = 3 hours with 9 cell, 2 with 6