A voltmeter will tell you the condition of your charging system and how well the bike is handling accessories. At 3000rpm and above, the corrected output of a charging system with no added electrical load should be between 13.5V and 14.8 volts.
No matter what the load or rpm, 12.8V and above is charging the battery but more than 14.8V is overcharging and possibly damaging components. The higher over 14.8V, the more likely damage is being done and the more likely the damage will be bad, like a fried ECU. 12.6-12.8V is breaking even.
Below 12.6V is discharging and an indication of a charging system problem or an accessory overload if at 3000rpm or higher. The farther below 12.6V, the sooner you'll have a dead battery.
I said corrected output earlier because a mounted voltmeter will often show a small difference between its reading and that of a hand held meter at the battery terminals, usually a bit lower. After mounting a voltmeter, check for any difference and apply the difference when checking the meter for the most accurate reading. A real problem will show a large difference from the desired range so, if you decide not to bother with figuring or doing a correction factor when checking your output, it's not a big deal. Some people are happy will the colored lights indicators to check the charging output and that's probably fine. I just like to know where in the range my bike is running rather than being happy with a green light.
