First thing I'd check is the charge voltages, very easy to do with a multimeter.
Connect the multimeter (set to volts) direct to the battery terminals, making sure the main connections are are clean and tight. The voltage should be around 12v. Start the engine and the volts at idle may be around 11.5v, start reving the engine, and by 3500 rpm the voltage should settle at about 13.8v or slightly above regardless of the headlight on or off. If it refuses to raise higher than that, even with reving higher, there may be a problem with the charging system.
A daily 20km run each way should be enough to keep the battery charged, as long as there are no extended periods of stop start during the run. It may be better to invest in a small float charger that can simply plug into the accessory socket on the side of the bike, no need to disconnect the battery. It's what I use to keep the battery at its peak especially during winter.
I wouldn't try to fit any sort of what is euphemistically called 'high o/p regulator', as they can cause more damage as they simply boost the charge voltage and can over charge the battery on any longer runs.
The alternator doesn't actually 'grow tired', but can be a collection of cheap fixes, such as connections, that can have everything running fine. Try the above check first and get back to us with the results.
I hope I spoke s l o w l y enough for a Kiwi to understand!!
Bill............................;-)