I HAVE A 12v AIR COMPRESSOR FOR UNDERWATER DIVING(SEA
BREATHE)(SEABREATHE.COM).  IT PROVIDES MORE THAN ENOUGH AIR FOR TWO
PEOPLE AND TYPICALLY BLEEDS AIR OVER AND ABOVE THE TWO USERS.  THE
BATTERY IS A GEL CELL 12V 34 AH SELED BATTERY.  IT JUST FITS INTO THE
STORAGE COMPARTMENT,SO I CANNOT USE A LARGER BATTERY.  TYPICALLY IT
WILL LAST ABOUT 1 HOUR.  THE PUMP HAS NO AVAILABLE ADJUSTMENTS, SO THE
PUMP RUNS AT A FIXED RATE. THERE IS A 6FT LONG BY 12" IN DIAMETER
BLADDER THAT FILLS WITH AIR AND THERE IS A REGULATOR THAT YOU BREATHE
THROUGH.  AS THE BATTERY LOSES POWER THE MOTOR SLOWS DOWN AND
EVENTUALLY SHUTS OFF AND THE AIR PRESSURE DROPS AND YOU CAN NO LONGER
BREATHE.  WHAT I WANT TO DO IS FIND A DEVICE WHICH I FEEL IS
AVAILABLE, THAT WILL ATTACH BETWEEN THE BATTERY CONNECTION AND THE
MOTOR CONNECTION THAT WILL ALLOW ME TO VARY (LOWER) THE VOLTAGE TO THE
MOTOR (WITHOUT WASTING THE EXCESS VOLTAGE) AND THUS THE COMPRESSOR
WILL BE ABLE TO PUMP LESS AIR (SAY FOR ONE PERSON) AND THE BATTERY
LIFE WOULD BE EXTENDED TO SAY 1 1/2 OR 2 HOURS. WOULD I HAVE TO
ASSEMBLE PARTS TO MAKE SUCH A DEVICE, OR IS ONE AVAILABLE THAT IS
ALREADY ASSEMBELED? THE PRE-ASSEMBELED UNIT WOULD NEED TO BE ABLE TO
BE ADJUSTED BETWEEN SAY 12V AND 10V. IM NOT SURE WHAT AMPERAGE. IM
GUESSING 2.5AMPS WOULD BE OK.

A Switching DC-DC converter should do it. It controls the output
voltage with short pulses. These pulses vary in duration to change the
output voltage and consequently the motor speed. The longer the
pulses, the faster the motor turns, and vice versa.
An example would be LM2650, Synchronous Step-Down DC/DC Converter by
"National Semiconductor" with input voltage between 4.5 and 18 volts
and adjustable output voltage in the range of 1.5V-16V. The output
current is limited to 3000 mA. You would have to assemble the parts
though.

See the data sheet: 
http://www.digchip.com/datasheets/parts/datasheet/321/LM2650.php

RK

P.S. if your rig last 2 hours and you have a 34 amphour battry that
would imply that the compresor pulls 15 to 20 amps there are some
varibles that are hart to compinsate for but fore a rough estamint of
how much the compressor pulls. run the compressor on a fully charged
battry till it stops. take the power rating of your batt exa 34 divide
that by how long it ran in hours so 34/1.75=19.43A
however to get the exact value you would need a curent meter and would
nead to check the pump under load at the entire voltage range exa. 8
to 14.5 volts however in most conditions the first method is suficent.
i hope this was worth the cash if you have any further questions feal
free to email me at goodfdg@hotmail.com
A PADY Advanced Open Water Diver
megreger