I was curious what the reviews were like for those new long lasting
ultra-thin batteries that sit under your laptop.
 
Here is one link.  
http://www.pwrplant.com/notebook.html 
 
My questions are: 
1 – Where do I buy one of these things?  Is there more then one
manufacturer?
2 – Do they work?  What do the reviews say?  How much longer then a
regular battery do they last?

3 – Are they compatible with the Sony PCG series laptops (optional)?

The leader in this product category is Electrovaya, formerly known as
Electrofuel.  The reviews on these products are favorable, other than
the price and obvious consideration of carrying the extra weight of
the battery on your trips.  The highest capacity model offers 12-16
hours of usage.  You can order directly from the Electrovaya web site.

Below I will list the statistics for the different manufacturers and
models.  I personally would tend to choose Electrovaya, though the
Valence product is also interesting (since it allows you
simultaneously recharge a second device like a cell phone or PDA). 
The others I have included for the sake of completeness, though their
web sites inspire less confidence and I would tend to avoid them
(notably the Bix product is the older NIMH battery technology).

Electrovaya lists these Sony models as compatible: 
Sony Vaio PCG-F590, PCG-R505GL
From what I have read, most laptops should be compatible.  There is a
note that the Sony laptops are not compatible with the PowerPad 80
Model 3, however, it appears you can specify a model when you order.


               PRICE   WEIGHT   WATT HRS  VOLT/AMP HR  DIMENSIONS
Power Plant     $ 99   3+  lbs     60     12V  5.0Ah     
10"x7.25"x0.86"

Electrovaya   
 PowerPad  80   $279   1.4 lbs     80     15V  5.5Ah   
6.63"x8.75"x0.375"
 PowerPad 120   $399   1.9 lbs    120     15V    8Ah  
11.75"x8.75"x0.375"
 PowerPad 160   $499   2.4 lbs    160     15V   11Ah  
11.75"x8.75"x0.375"

Valence
 N-Charge  65   $199   1.8 lbs     65      *     5Ah  
11.81"x9.06"x0.512"
 N-Charge 130   $299   2.9 lbs    130      *    10Ah  
11.81"x9.06"x0.512"
 * The Valence allows you to charge two devices simultaneously.
   One high voltage at 16-24V, one low voltage at 5-12V.

Bix PowerBase   $179   3.5 lbs     90     12V  7.5Ah   11.8" x8.9"
x0.82"

 
Electrovaya (Electrofuel) Reviews:
TechTV Fresh Gear: "Electrofuel PowerPad" by Hahn Choi, September 18,
2001
http://www.techtv.com/freshgear/products/story/0,23008,3347162,00.html

Eye On Technology:  "Electrofuel PowerPad 160"
http://www.apmsafe.com/Laptop%20Magazine%20Review_files/eye.htm

ZDNet:  "Electrofuel Serves up Longer Battery Life" by Bill Machrone,
March 13, 2000
http://www.zdnet.com/anchordesk/story/story_4557.html

Valence Review:
Mercury News:  "Rechargeable batteries are not keeping pace with tech
travel needs [N-Charge is a Solution]" by Jon Fortt, June 12, 2002
http://www.siliconvalley.com/mld/siliconvalley/3457030.htm

Other Articles:
PC Magazine:  "A Long and Happy Life for Notebooks" by Bill Howard,
October 15, 2002  (Article about battery life briefly mentions
Electrovaya and Valence)
http://www.pcmag.com/print_article/0,3048,a=31232,00.asp



Manufacturer Links:
Electrovaya
http://www.electrovaya.com/

Valence N-Charge
http://www.valence.com/ncharge.asp

Power Plant
http://www.pwrplant.com/notebook.html

MobyPower External Li-Ion battery brick
http://www.mobypower.com/menu02_04_01.html

Bix PowerBase
http://www.bixnet.com/unpowbat.html


Search Strategy:  Search for Electrovaya and its competitors

Search Terms:
Electrovaya laptop
://www.google.com/search?q=electrovaya+laptop

Electrofuel laptop
://www.google.com/search?q=electrofuel+laptop

Electrovaya Valence laptop
://www.google.com/search?q=electrovaya+valence+laptop

I hope that you have found this information useful and easy to
understand.  If you have any questions about this information, please
do not hesitate to post a clarification request prior to rating the
answer.

thanks for the excellent review

I see that the dimensions listed above have decided to wrap in an
unfortunate way.  Here is the same table again, without the
dimensions:
 
               PRICE   WEIGHT   WATT HRS  VOLT/AMP HR  
Power Plant     $ 99   3+  lbs     60     12V  5.0Ah     
 
Electrovaya    
 PowerPad  80   $279   1.4 lbs     80     15V  5.5Ah   
 PowerPad 120   $399   1.9 lbs    120     15V    8Ah  
 PowerPad 160   $499   2.4 lbs    160     15V   11Ah  
 
Valence 
 N-Charge  65   $199   1.8 lbs     65      *     5Ah  
 N-Charge 130   $299   2.9 lbs    130      *    10Ah  
 * The Valence allows you to charge two devices simultaneously. 
   One high voltage at 16-24V, one low voltage at 5-12V. 
 
Bix PowerBase   $179   3.5 lbs     90     12V  7.5Ah