DC motor speed control: 12VDC 700mA fan motor powered by 7.2Ah sealed lead-acid battery
Do a Google search for "12 Volt DC PWM Speed Control".  There are many products that you can buy (eBay will sell you something for less than $10) and even more that you can build.

Is this something that you want to build yourself or are you just trying to solve this particular problem?

A 555 timer driving a N-channel MOSFET is pretty easy to build.







simulate this circuit &ndash; Schematic created using CircuitLab

You will want to use a CMOS 555 timer such as the TLC555.

Adjust the value of C1 for your desired PWM frequency.

Ratio of R1 to R3 sets the minimum &amp; maximum PWM duty cycle.  The values shown allow the PWM to be varied from about 1% to 99%.

I would set the PWM frequency to anywhere from 100 Hz to 500 Hz.  If you go above that, change D3 to a faster diode such as a UF4002 or similar.