My C14 lives on a tender (Optimate Accumate) and when plugged in the bike shows 13.2V. When I unplug the tender that drops to 12.5V. When the engine is running it shows 14.4V. The battery is a MotoBatt AGM and is now 5 years old. She hadn't been started for 5 weeks and yet she fired right up yesterday.
When the engine is running that 14.4V is running all of the electrics (computers, ignition system, lights, etc.) as well as charging the battery so it needs to be higher because of the higher load.
When she is parked and the tender plugged in, once fully charged the only drain is the batteries internal resistance, so the required voltage to maintain the battery is much lower.
Most tenders will output up to 14V when charging a nearly flat battery but that will drop to 13V once it is fully charged.
If you have one of the more modern "battery maintainers" then those are computerised and will analyse your battery before deciding what to do with it. Those can use a variety of "modes" including discharging, pulse-charging, de-sulphating, and several others. I have a cheap Chinese one of those connected to my C10 and it works just fine.
If your battery feels weak or struggles to start the bike, replace it.
I strongly recommend the MotoBatt AGM. As well as the 5 year old in the C14 I've had one in my C10 for 8 years now and it's still going strong.
You get exactly what you pay for with cheap batteries.