If it read 4.8 volts in the plane it was time to charge it. This is actually a fairly low voltage. With ANY load it will get sucked down to the cut-off point. The battery that comes with the system is fairly marginal IMO and a slightly larger one is a good idea as I said before. A 6v is even better but you would need a new charger for it. PLEASE have someone experienced check your installation too. Pushrods that bind greatly increase current demands and therefore reduce battery life.

Also, when you checked your battery did you do it with a loaded meter or a plain one? Only readings under load are valid.