Do you have the charger safety charge time set correctly. You want to make sure that it is not timing out before the full charge. If you charge at the 5000 7.4 rate that will take just a little longer than 1 hour to charge the battery if it is nearly discharged. If they will not charge to 8.4 volts the batteries are probably no good.

Be very careful charging these batteries. Make certain that they do not get hot or that they are not swelled. Charge outside on concrete.

8.4 volts is the voltage of a fully charged 7.4 lipo pack. Under load the pack will drop below 8 volts in a short time. But they should stay within the 7 volt range for most of the run time. Most ESCs for a 7.4 LIPO are set to shut off the motor at 6.0 volts.

Go to one of the suppliers like FMA Direct and look at a discharge curve on a 7.4 LIPO pack. They have some 4800 mah packs. You will note the initial drop at the beginning and then the discharge will be flatter, and then at the end a big drop. These curves are generated using a substantial load on the battery.

The problem with weak LIPO batteries is that they can go completely bad and cause loss of control. And they are dangerous when charging. So be careful.

What happens when you run the motor on these batteries? If you have the ESC set correctly it will shut down the batteries at the correct voltage.

Is there someone in your local area that can help you analyze your batteries using a different charger?

Be careful.

bob