Laptop external power from batteries (DC/DC)

[Bill McGonigle]

On Aug 20, 2008, at 14:58, mike ledoux wrote:

> A good (and sadly, expensive) DC-DC voltage
> converter is only about 85% efficient, so to get the 19.5Vdc @ 4.62A
> you need, you will draw ~8.63A @ 12Vdc.  ~5 hours becomes ~4.5.

Yeah, that does sound a bit wasteful if one is starting from scratch.

OK, so how about hooking up a 1.5V a 6V and a 12V deep cycle battery  
in series?  Assuming one could find cells of comparable AH ratings.

Or, if one were more serious... IIRC the last time I looked inside a  
battery it was made of several plates lined up together in a row,  
sitting in an acid bath.  Actually, alternating repetitions of  
different kinds of plates, which I assume were cells of some voltage  
in series.  So, if one bought two marine batteries, a bigger box, and  
was familiar with proper acid handling techniques, ought there be an  
electrical reason that 'just' making a 19.5v battery with the  
required number of cells would be insufficient to power a laptop  
(finding a charger might would be a separate challenge)?  I'm curious  
if the voltage drops as the battery discharges or remains fairly  
level.  Come to think of it, laptop battery voltages aren't 100%  
level, so I wonder how precise that 19.5V really needs to be (12V and  
6V are easily sourced, for instance).

Also I've noticed that many laptop vendors offer a 15.5v adapter to  
be used on airlines, which have some sort of under-seat connectors.   
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 think the new answer to "what can I do with my old obsolete  
laptop?" is "give it to somebody playing with batteries". :)

-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