Creation of multiple power cells to be controlled by Arduino
OK. Looks much better now. And I see one problem. It seems that the normal way to do the speed control is with a pot. A variable voltage at the input depending on the rotation. The Arduino low end designs have an analog out capability but it really is not analog. It is a pulse width variable signal that produces an on percentage between 0 volts and the CPU positive supply voltage. The boards do not have a real digital to analog converter. (maybe some higher end boards but not boards based on the Uno CPU. 

It may still work but you need to integrate the pulse output and produce a real analog output voltage. A resistor on the output connected to a capacitor to ground will do the integration and filter out the pulses. 



See http://www.ontrak.net/pwm.htm for more details. 



The problem you might encounter is to much loading on the capacitor by the pot input circuit. Without more details about the module I couldn't say. The signal can be buffered with an amplifier but that adds to the complexity. Still would be doable though.

Or you can add a real digital to analog converter to the Arduino board. That would be my preference rather than having to add a buffer amplifier if a simple RC circuit wasn't good enough.

Do you have a link for that board?

From the picture of the linked speed module. It uses a LM324 opamp to generate the PWM signal. I did a quick look and found a typical circuit. 



http://www.pcsilencioso.com/cpemma/graphics/erg_pwm_sch.gif

Typical but not the exact one. It is much as I expected. And if you did the RC only filtering then it may just work. The opamp input would not be too much loading on the capacitor. The concern that I have is that the 5 volt maximum from the Arduino may not be high enough to turn the motor on at its maximum value.