Teh only time a large bank wouldn't be good is if you made a habit of fully discharging them and only partially recharged them. The A4 does take some time to fully charge 2 batterys. When I find myself with dead batterys I like to use shore power to recharge. It is a good idea to watch your ammeter (charge meter) to bedcome accustomed to how it acts. If you have a full charge it will only slightly deflect to the charge side. Notice that when you start it swings to the discharge side obviously. What is important is making sure that the system is charging the batterys. As the batterys "fill up" the needle will creep towards the center. If the needle stays well deflected you have battery problems. A common mistake is to merely parallel two batterys together, this is bad news, seperate with a switch or use an isolator (as a car part 25 dollars, as a boat part double that) You can also use a switch to select the battery to use, and also have an isolator to always charge both. I like to charge seperately so I can see what is going on.