The battery fitted to the NC30 and NC35 as standard are what's known as SLA (sealed lead-acid) and are designed to be maintenance-free in use. An SLA battery has a self-discharge characteristic of 1 percent per 24 hours (Note - the self-discharge in a Gel MF battery is 0.1 percent per day). In other words, the battery will go from a fully charged to discharged state in 80 days. You should never discharge a battery past 80 percent (60 percent for Gel MF) hence the 80 days, and not 100 days as your simple arithmetic would suggest.
If you're likely to have the bike laid up for long periods, over the winter for instance, its best to charge the battery intermittently. I'd recommend every 60 days maximum put the battery on a suitably designed motorcycle charger (Optimate or similar) and let it run through its cycle.
To check the general condition of the battery, make sure its fully charged then disconnect it from the charger and any possible load and let it rest for 30-45 minutes. Measure the voltage at the terminals using a suitable DVM. If you get less than 12.7 volts DC, the battery is not as healthy as it could be and it is time to consider replacing it. If you get less than 12.3 volts DC, it's definitely time to replace it. Also, if the battery is more than three years old, likewise replace it.
Incidently, you can do a rough and ready check of your regulator/rectifier in a similar manner. With the battery properly connected on the bike, start the engine, increase rpm to 5,000 and check the DC voltage at the terminals - you should get approximately 15.5 volts DC. Any more and you may have a regulator problem. Similarly, you can check for AC voltage at the terminals. If you get any AC voltage at all your rectifier is beginning to fail (and killing your battery while its failing - double jeopardy!). The rectifiers used are known as full-wave rectifiers, and any AC reading at all is a bad sign.
