Laptop external power from batteries (DC/DC)

michael miller k4ghp at comcast.net
Mon Aug 18 20:55:49 EDT 2008


I went through this same process a couple of years ago.  Initially it
made no sense that a computer that uses components running on 12, 5, 3.3
and maybe 2.5vdc couldn't run off of a 12 vdc car battery.  Ultimately
it turned out that it was the vendors choice of laptop battery that was
the problem.  Unlike lead acid (2v), alkaline(1.5v), NiCad(1.2v) and
NiMH(1.2v) batteries, the basic lithium cell is nominally 3.6v.  Dell
(and probably some others) use a stack of 4 of these in series giving a
nominal voltage of 14.4v, somewhat higher when fully charged.  Li
batteries should never be discharged below 3v/cell, so not below 12vdc
for Dell laptop batteries.  Car alternators are limited to 14.4v which
is the most that lead acid batteries can handle on a regular basis.
Most prefer to float around 13.8v.  When you shut the engine off but
leave a load on the battery the voltage drops down to 12 to 12.8.  Not
nearly enough to charge a 4S lithium battery pack. To get the ~20v
needed to operate the 4S lithium charger you can either use an expensive
DC to DC converter or an ac inverter and charger.  I was going to try to
modify a dead laptop battery to see if I could keep the laptop happy
with 13.8v, but got some feedback that there is internal hardware that
checks supply voltage and impedance and shuts down the laptop if it
doesn't look like the factory battery and charger.  Maybe sometime I'll
pick it up again.

Mike Miller 
On Mon, 2008-08-18 at 19:07 -0400, Bill McGonigle wrote:
> On Aug 18, 2008, at 18:24, Ben Scott wrote:
> 
> >   Someone brought up the idea of powering a laptop from DC sources,
> > rather than the traditional AC power bricks.  This is strictly about
> > external power, not the laptop's built-in batteries.
> 
> I'll admit to it and just mention here that it came up in the context  
> of doing linux demos in the wild, so it's not especially off-topic.
> 
> I've since found this guy:
> 
>    http://store.mp3car.com/Carnetix_CNX_P1900_DC_DC_Regulator_p/ 
> pwr-010.htm
> 
> which looks promising (my laptop is also 19VDC).  I'd need to  
> repurpose a power cord.
> 
> I've tried doing audio recording with my laptop running off of the  
> car with an inverter and the electrical noise ruined the effort.  It  
> would be nice if this worked.
> 
> But there are actually folks on this list who understand these  
> things, so I'll stop and listen now.
> 
> -Bill
> 
> -----
> Bill McGonigle, Owner           Work: 603.448.4440
> BFC Computing, LLC              Home: 603.448.1668
> bill at bfccomputing.com		Cell: 603.252.2606
> http://www.bfccomputing.com/    Page: 603.442.1833
> Blog: http://blog.bfccomputing.com/
> VCard: http://bfccomputing.com/vcard/bill.vcf
> 
> 
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-- 
michael miller <k4ghp at comcast.net>



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