[ltp] Re: Slightly off-topic: different wattage

[Theodore Tso]

On Mon, Sep 21, 2009 at 08:41:15AM -0400, James Knott wrote:
> A higher capacity adapter will not cause faster charging, as the charge
> rate is determined by computer & battery.  A power adapter supplies only
> the power that's needed, so you shouldn't see any difference in the
> power consumed by the computer with either charger.  On the other hand
> an inadequate charger may not be able to always deliver the power
> required, which can lead to problems.

I've observed charging happening faster with a higher capacity power
adapater.  I suspect what happens is that with the Lenovo 65W adapter,
as you try to draw more power than what is rating, the voltage drops,
and the battery charging circuits compensate accordingly by dropping
the charge rate.

If you compare the charge rate of the 65W Lenovo adapter, which is
what was shipped with my X61s and T400 laptops, and the charge rate
with the 90W adapter, it's quite noticeable.  However, the same can
*not* be said of the 65W iGo power adapter.  If you try to use the 65W
iGo adapter, and the battery is less than, oh, 60% or so, the laptop
will try to draw more than 65W, and the iGo power adapter will
shutdown if it is being run off the AC Mains.  (It works OK if the
same iGo power adapter is hooked up to an airplane 12V power, which
uses a different set of power circuits for 12V DC as compared to
120V-240V AC.)

So these two observations, combined, leads me to believe that if I
were to hook up an ammeter in-line with the DC output of a 90W Lenovo
adapter (which would involve splicing a cable, or at least cutting
into the outher sheath and separating out the wires to use a clamp
meter), I would find that when the battery has a low charge, and the
laptop is powered on, we would find that the total power being pulled
from the adapter is greater 65W, which is why the iGo adapter shuts
down, and why the charge rate is slower on with a Lenovo 65W power
brick.

Given that Lenovo does ship the 65W power brick with the X61s and T400
laptops, I have to assume they designed it so that it will work
correctly; just not recharge the battery as quickly when the laptop is
also powered on.

							- Ted