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I purchased a battery 4 weeks ago and have had trouble starting the car. I think it is the starter, however, I have had the battery on the charger for 3+ days and it is reading 13.1 volts open circuit. I seem to remember a new battery should be at 13.7V and that by the time it is at 12.0 it is time for a new one.
When connected to the car it is reading 12.8 volts.
Should a new battery be well above 13.1 volts (i.e. 13.7)?
Your new battery voltage sounds ok if its been on the charger for that long. Depending on the battery 12.6 - 12.8v is around the right area for an open circuit battery, yours will be higher as its just off the charger by the sounds of it.
Once connected to the vehicle and charging the battery voltage should increase to between about 13.8 and 14.7v as the alternator starts to charge it.
If you're engine is spinning over energetically when you attempt to start it, that's a good thing and you may have another starting issue.. If however the engine spins over slowly or sounds laboured when you're attempting to start it you may be right about the starter motor.
