Is this the charger ?
http://www.wheelspinmodels.co.uk/i/62617/
max 3amps charging, still less than a hour charging (always really topping the battery up).
I don't think 7.5volts is a discharged battery, someone may correct me.
You should be able to monitor the amount of charge that is put back into the battery. Remember your esc should have a 'lipo cutoff' I would have thought nearer to 6.5volts.
Perhaps you can find a local club, all will be pleased to help out and give hands on advice, perhaps something rubbing and over working the esc, thermalling issue as Jim has suggested.