Battery all over the place

Question

I have a three month old MBP. I often browse the forums to inform myself on what's up before it happens to me. I found out about the battery update that way, and did it. I read about the possible problems that may occur, but I am not sure my problem fits - or if it even is a problem.
My battery used to last for a really long time - 4 to 5 hrs. Now it doesn't. I usually only use my MBP for surfing and occasionally downloading music or movies from itunes (haven't done that since the bat. update tho).
After experiencing shorter and shorter up time, I decided to really keep an eye on it this morning. Last night I got the message that it was running on reserve power, (and I was too) so I plugged the power cord in and shut it down for the night. This morning I unhooked the power, started it up and the time left read 5:32 for about a second or two and then read 2:32. I used it for a few minutes, walked away - the display went to sleep - I woke it up and it read 4:50. After that I let the display sleep several times and every time I woke it up I got a different reading - all over the place. 6:xx - 3:xx -4:xx and so on. I then decided to browse this discussion area and couldn't find exactly this scenario. I have been on for about 15 minutes and the time has stayed within a range of 3:05 to 2:42. I am going to let it go into full sleep and come back to finish up this post. Oh...by the way:
Battery Installed: Yes
First low level warning: No
Full Charge Capacity (mAh): 5189
Remaining Capacity (mAh): 4531
Amperage (mA): -1494
Voltage (mV): 11967
Cycle Count: 37
I'm back. Time left reads 2:22 - seems normal enough for being gone about 20 minutes. Power readings:
Battery Information:
Battery Installed: Yes
First low level warning: No
Full Charge Capacity (mAh): 5188
Remaining Capacity (mAh): 3695
Amperage (mA): -1522
Voltage (mV): 11796
Cycle Count: 37
But where has my battery life gone? I guess what it says now is a true reading, but since I have only had the computer unplugged from the power for about 1.5 hours, and it has been used for only about 25 minutes, isn't this a short period of life? It used to sit practically all day on battery power only being used once in a while. I am going on vacation in two weeks and I don't think this battery will even get me through one dvd movie on the plane - on a three month old computer! Any thoughts?
sj

Answer 1

Sarah,
The two cruical numbers for the estimate of the time remaining are the remaining capacity and amperage. If you divide capacity by amperage you will get the time estimate.
These values update relatively slowly - about every minute or so. This can lead to weird values when your computer changes its activity quickly.
Thus, right after you wake the computer up or reactivate the screen it is likely that the amperage will be low (because it doesn't use much power in sleep mode or with the screen off) and the time estimate will be high. Similarly, just after you unplug the computer the amperage estimate will be low (because it is just the amperage drawn from the battery). I have seen estimates of over 200 hours right after I unplug the AC. But as soon as it updates it all sorts itself out.
Your battery seems to be behaving normally and 5200mAh seems about right for a 15" MBP battery. They start at around 5500mAh and decline from there. Apple suggests that you should get around 300 cycles before it drops below 80% of initial capacity (4400mAh for the 15" MBP battery).
The number one hint for getting longer life out of your MacBook is to turn the brightness of the screen down. That is typically the biggest drain on the battery.