First things first, how old is your battery? Also I would actually run the car and turn on accessories, and with the car at a fast idle, read what the VDC is across the battery terminals. Just to make sure it's just not the gauge that is reading improperly. You should be anywhere from 13.5-14.2VDC. This means your alternator is doing it's job in simple terms. An alternator output test would be difficult unless you have a VAT-40 machine laying around. To be honest, I would do a load test on the battery. They sell many devices to check this, or simply bring your battery to your local auto-parts store. They will generally test your battery free of charge.