I am having problems with my spec2 when I added the 4th cell. I am charging it with the apex ac optional charger with the stock charging jack. The problem started 3-4 runs after running with the 4th cell (extremely fast). I then started to see a decrease in speed and later on the car would act as if it had only 3cells to power it. So I was wounder if I could find out what the problem is.
particularly if you added a fourth cell to three cells that you already had run for a while.
the voltage of the 4 cells will start to drop after a few minutes so it will start to run like a 3 cell. under the same conditions the 3 cell would run even slower.
also, when you run 4 cells you are using more current so the battery's voltage will sag more quickly.
Actually if you look at the discharge charateristic of NiMH batt., you will know more why it slow down and act like a 3cell. When it is fully charged, each cell will around 1.4v (total 4cell ~5.7v) which has a higher top volt. and thus higher top speed.
After a while run each cell drops to a level ~1.1v which act like a fully charged 3cell(~4.3v).