Thank you for your suggestion and I have spoken to the Development Engineers and put forward your suggestion however they informed me that they did look into this however the reasons for using the 9V battery is as follows:-
The Mixmaster needs a +/- supply internally as you can appreciate voltages do fluctuate in and out of these ranges so if an external power supply was used then only the ground would be seen at 0 hence no overlay into negative voltage so this would require a lot of extra circuitry to overcome this and pull the voltage down.
Also with using an external supply, let’s say the 12 Volt Car Battery supply then again extra circuitry would need to be incorporated into the Mixmaster to filter out the noise due to ECU events seen at the battery e.g. Ignition and Injector events and Alternator Signals.
The best way to overcome this then was to use a 9V battery internally as this gave the +/- 4.5 Volts without needing the additional circuitry and filters and also keeping the cost low for the end user.
Most 9 Volt batteries do last a long time as most Multi Meters will last well over a year before they begin to show signs of needing a new battery.
I hope this helps to answer your question
Lee – Automotive Technical Specialist