iBook battery life

[Tim Seufert]

On Thursday, April 3, 2003, at 10:03  PM, Greg Hamilton wrote:

> I have an iBook 500. I'm currently running OSX 10.2.4. I'm 
> experiencing severely reduced battery life. A lot of people have been 
> complaining of the same problem lately. eg. Have a look at this forum 
> on Apple's own web site : 
> http://discussions.info.apple.com/WebX?14@22.cZL1aAD3j0Z.0@.3bbf6b95
>
> There's some suggestion that the problem is not just hardware. OSX may 
> be over charging the battery or reporting incorrect battery charge.

In most PowerBooks the battery is not directly controlled by the OS.  
The PMU (Power Management Unit) chip is what does the real work.  The 
PMU is an 8- or 16-bit microcontroller responsible for all sorts of 
things related to power use.  Since the PMU's program code is stored in 
ROM (or, in recent Macs, in flash) its behavior is mostly independent 
of the OS you run on the Powerbook's main CPU.  ("Mostly" because that 
OS can send commands to the PMU which alter its behavior, but in 
general the OS does not or cannot intervene in fundamental PMU 
decisions such as when to turn charge current to the battery on or off.)

I should also note that the batteries themselves play a role in the 
charge/discharge process.  Lithium Ion batteries for notebook computers 
always have a minimum amount of onboard intelligence for safety 
reasons.  LiIon is a fairly nasty battery chemistry and is easy to 
explode if not carefully controlled (especially during charging), so 
they like to put a temperature sensor and controller inside the battery 
which allows it to disconnect itself from the outside world if high 
cell temperatures are detected.  In many cases the internal controller 
is even smarter than that, to the extent of having storage for 
historical information on the battery and even control over charging 
current level.  I don't know exactly where on the scale from mere 
thermal protection to everything-but-the-kitchen-sink Apple's batteries 
fall, though I'm fairly certain they do more than just monitor 
temperature.

IMO, OS X is not overcharging the battery.  If it could do that (which 
I don't think it can), your battery would probably catch fire or 
explode when overcharged.  (Seriously.)  As for reporting incorrect 
charge, it is quite possible that OS X might interpret data from the 
PMU incorrectly.  But with no less than three "intelligent" agents 
interacting (the battery, the PMU, and the OS), it is somewhat futile 
to guess where the blame should go.

These days, Linux is likely to copy any power control logic in OS X, 
since Linux/PowerMac kernel code is now frequently written using Darwin 
source code as a reference for how the hardware works.  (When the 
Darwin driver for a given bit of Mac hardware is unencumbered by IP 
licensing issues, it's much easier for Ben H. & company to write 
drivers than in the pre MacOS X days.)