You are right in that on short journeys the amount of charge via a split relay will be minimal. The split relay will help, You will still need to charge the battery regularly though.
The amp per hour charge rate is dependant on a number of factors for example the length of time the engine/alternator are running and how quickly they run.
Most alternators are designed to trickle charge a battery so the longer and faster they run the higher the charge rate. (Up to the maximum capacity of the alternator and battery) There is also a restriction based on the maximum capacity of the alternator and battery in any system plus any drop off through the wiring due to resistance.
The ampage per hour rate will drop off as the battery nears full charge, the batteries capacity to take amps drops as the voltage capacity nears its maximum. The only way of physically pushing higher current into the battery would be to increase the Voltage capacity and increase the maximum generating capacity of the Alternator.
Ohms Law
http://en.wikipedia.org/wiki/Ohm%27s_law