Sorry but your not doing your batteries any favours by running them till they are so flat they can only move the servo.
Most nihm batteries should not be discharged to less than 0.9v per cell. If you are continuously doing this your are buggering them up.
You should just run the normal 5 mins race plus maybe 1 min before and after then let them cool and put them in a balancing board to equally discharge them all to the same voltage 0.9v then recharge them. The way you are doing it now means that not all the batteries in the pack are at the same voltage so the charger will false peak meaning just one battery may be fully charged and the rest way under charged, hence your low run times.
Next time you flatten your pack measure the capacity of each cell with a volt meter and you will see the difference in all the cells. Buy yourself a decent discharge board, should be plenty around pretty cheap now most people have gone lipo. Also get a better charger, pm me if you want a decent nihm charger.
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