The answer is yes. Basically the timer outputs a servo position signal and the position is in essence a ratio demand of the supplying battery voltage at the ESC. Without changing also the motor the voltage output will need to be reduced from the new battery to the old battery voltage value, ie turn down the timer output. Think of it like this, the timer puts out a signal that goes from 0-100% of the Vin or for a 14.8V 4s battery 75% would be 11.1V and for a 18.5V 5s battery 75% would be 13.9V. So, without changing the motor to maintain the same speed the timer output has to be reset to 11.1V/18.5V = 59.5 %, "turned down to 59.5%". In other words the pot on the FM0c would have to be turned down from roughly 3/4 to just under 5/8 turn thereabouts.

It did not on mine.  I flew a 5 cell 2200 back to back with a 4 cell 2800 at a contest and did not notice any difference.  What I did learn was to bring more batteries to a contest.

Ken

I just read Mark's post so I need to add that my ESC autodetects the cell count and makes that adjustment.

I mostly run the elcheapo ESC's and the FM0c. I find them both to be very adequate for most installations although I do run a Jetti Pro and Fiorroti in my PA models. My new timer project is being bench tested using the ZTC Beatless ESC. Provided the response of the timer and elcheapo work together, that is my future direction. Flight testing is going to begin in the near future.

Tim is right again. I switched to a 5 cell battery and ran it briefly. The rpm stayed at the programmed in value. I'm happy. Learning has taken place.