In the 1st place, the original battery space may not be able to fit in a big 60Ah battery unless with some layout modification. Not knowing your car age, the low initial charging of 13.2V is on the low side and maybe due to wear and tear condition of alternator and original thin alternator wiring and thus, take a while to resume normal charging from 13.5V to 14.5V. Or when you initially drive off your car with loads like aircon, heavy sound system, headlights and etc will cause the voltage to drop as well.
Your car alternator may not able to charge big 60Ah battery sufficiently especially if you don't drive long enough to charge.... whenever you start your engine, you are draining 20% to 30% of your battery so you need to charge it up by driving at least 30 minutes or more to charge to 90% or more power to the battery. Nissan Latio/Sylphy is using at most 60Ah alternator charging so to charge up 60Ah, your alternator have to work very very hard. Sometimes, it may not able to charge fully to your almost 50% bigger Ah battery (from 45Ah to 60Ah). Do remember that the main task of alternator is not just to charge up your battery but also power up the car loads like aircon, lightings, ECU, sensors, spark plugs and many others....
Which is why my product, UTR Power Charger Evo Series, comes with alternator positive cable connection to battery will help the charging of alternator.
More information of UTR Power Charger Evo Series:
http://www.box.net/shared/oos93jl7ei
