Laptop external power from batteries (DC/DC)

Ben Scott dragonhawk at gmail.com
Thu Aug 21 09:48:21 EDT 2008


On Wed, Aug 20, 2008 at 8:03 PM, Bill McGonigle <bill at bfccomputing.com> wrote:
> I'm curious if the voltage drops as the battery discharges or remains fairly
> level.

  I know that voltage does drop as a battery discharges, and that the
pattern of the voltage drop depends on the type of battery.  I've
think I've read (could be wrong) that Li-ion drops on an S curve,
while most other types have a more steady slope.

  However, that's academic.  The batteries in every laptop I've
checked don't run at the same voltage as the line cord.  Presumably,
the power electronics in the laptop are designed to compensate for
battery voltage varying.  I would not expect the same for the line-in.

> ... many laptop vendors offer a 15.5v adapter to be used on airlines ...

  I believe it is the airplane power sockets which provide 15 VDC.

  With our Dell's at work, the travel adapters use the same input
power cord for auto (cig) as for airline.  The cig plug pulls off the
end of the cord to reveal an airline plug.

  I've assumed that what happens is the DC/DC converter in the travel
adapter can operate at 12 VDC or 15 VDC input, and in both cases
output the voltage expected by the laptop.

  So I don't think simply wiring a 12 VDC source into a laptop's line
connector would work, for our Dell's, at least.  Maybe other brands
use more sophisticated power electronics in the laptop.  Given market
price pressure, though, I would expect not.

> These power the laptops but not the laptop's chargers.  So there are
> probably extra pins or smart power control board to factor in.

  I know the Dell line cords have three conductors.  Power supply,
control signal, and common.  The control signal lets the laptop talk
to some intelligence in the adapter.  I've seen this demonstrated a
few different ways:

When attempting to use a 65-watt travel adapter instead of the 90-watt
AC-only adapter:
* With a high-power-draw laptop, the laptop will pause during POST,
warn that the battery will not charge or charge slowly, and ask for a
keystroke to continue.
* With a docking station, the laptop will stop during POST, state the
adapter is under-powered, and refuse to boot.

  I've also had a laptop where the on-board electronics which talk to
the adapter apparently failed.  The laptop would pause during POST,
warn that it could not talk to the adapter, would not charge the
battery, and things might not work at all, and ask for a keystroke to
continue.  (The motherboard was replaced to fix the problem.)

-- Ben


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