[Tamir]

my 2016 cbr250r that gets infrequent use.
I have a battery charger to prevent the battery from dying and I check the voltage frequently.

Hi. If you are still with the original battery,
then it is likely has reached the end of its life.
Start with the simple things.
Battery testing It's simple and done with a load, just as jkv357 explained:

...check it before trying to start, where you should see over 12.7V(12.3V is fine too), and also read the voltage as you hit and hold the starter button. You should still see over 10V and closer to 11 during cranking if it's in good condition.

After a minute from the start-up attempt the voltage should recovered and return above 12V close to the initial value.

Battery is the most common problem.

Bad Fuel after a year is less common (And I have never used a stabilizer):
1. The tank was full (less possibility of oxidation).
2. The CBR stored in a shady and dry place.
3. Start once a month to prevent fuel from drying in the narrow passages.