[Al Skierkiewicz]

Very carefully, using a small flat blade screwdriver, you can gently pry the contacts out until they firmly contact the battery posts. You might also want to clean them with a little denatured alcohol and a q-tip.
What is taking place is the small current required to keep the processor happy is not enough to overcome the high resistance of the bent contacts. So although everything appears to be fine, the processor thinks it has a dead battery. The higher current for the motors, reduces the voltage drop across the bad contact(s).
If you can remove the battery case, you will get a better look at the contacts and where to bend them and on occassion I have found a cracked solder joint. Loose contacts are a sure sign of a dropped RCX.