The PSP battery use

In the short few months I owned a PSP, I know now that after about two hours of playing on the unit, I check the battery settings and it indicates that around 40% of the battery power was used.
     In fact, I had to charge my battery recently, two days ago.  When I played on my PSP today, the charge level went from 100% to 62% for the 1 hour and 50 minutes or so that I used it.  This means that one hour equals about 20% of the PSP's battery power--that is if you are using a UMD (that's pretty much the only way I'm using my PSP now.)
I'm hoping I won't have to replace the battery pack too soon, though;  if it gets too warm where I live, the charge level decreases on its own.  It's how from September 29th to October 9th the charge level of my battery went from 33% to...well, it didn't turn on, which means it was 0%.

So how is it for everyone else?  Do you keep track of how much battery power is used when you use your PSP?  It may depend on the frequency of use and the length of time, wouldn't you agree?