[ltp] Order of system/Ultrabay battery discharge?

[Theodore Ts'o]

On Sun, Jul 13, 2003 at 09:42:42PM +0200, Juergen Stuber wrote:
> Hi,
> 
> I'm meditating whether to fork out the money for an Ultrabay battery.
> Can anybody tell me in which order the system and the Ultrabay battery
> are used?  In any case, I couldn't find a setting to change it.

In the T40p, it pulls from the Ultrabay battery first (and I think
it's the same in every single laptop I've seen).  At least for the
T40, I would *not* recommend getting the Ultrabay battery.  It's
expensive (costs as much as an extra system battery), but holds only a
fraction of the power, because of size constraints in the Ultrabay:

Ultrabay battery	2.2 Amp-hours
Slim-line battery	4.4 Amp-hours
Extended-life battery	6.6 Amp-hours

If the system drew down from the main battery *first*, then it might
be useful, since the Ultrabay battery could then serve as a reserve
pool of power, allowing you to keep the system running while you
swapped main battery.  (None of the T series laptops I've ever had
allows you to do battery hot-swap, even if the system is suspended to
memory; you have to either powerdown the machine or do a hard disk
hibernation).

My solution to this is to get an external battery, such as the
N-Charge battery from Valence Technology.  To the laptop, it looks
like it's running on AC power, so I can just briefly plug in the the
N-charge while I'm swapping main batteries.  If I need to go a long
time without power, what I will normally do is run off just an
extended-life battery that will give me 4-6 hours of run time
(depending on what I'm doing; reading e-mail takes a lot less juice
than doing a lot of kernel compiles :-).  I'll then plug in the
N-charge battery, and swap out the main battery with a new fresh
extended battery (to prevent the N-charge from using its power to
recharge the discharged main battery).  Then I'll run off the N-charge
battery, which at 10 Amp hours will give me another 6-9 hours, and
then once it is discharged, I'll run off the 2nd extended battery, for
another 4-6 hours.  This gives me anywhere from 14 to 21 hours worth
of battery, which is enough even for a flight to Sidney.  (I also have
a slim-line battery which I could use if a needed another few hours,
but I generally don't put it into the rotation.)

						- Ted