The first thing to check is to make sure the battery will hold a charge. Use a battery charger to charge it and see if it will keep a charge. If that is not the problem then check your connection. You may have a loose ground keep a circuit from being made.
Well your mercury might nnot be charging necause your battery is bad. Just because you alternator is good dosent mean that your battery will hold a charge form the alternator.
You may have a bad battery. After some time the battery will not accept a charge any longer and no matter how hard your alternator tries to charge it. The battery is not going to get charged because it is worn out and may need to be replaced. A new battery may fix it.
