I'm riding 2014 Dyna Street Bob. Everything is stock condition, nothing much added. My genuine Harley Davidson AGM battery is just over two years old.
I used a voltmeter to test the battery:
It is 12.5v when the bike is off
It is 12.2v when the light is on
It does not able to crank start

I used the CTEK MXS5.0 to charge it over night
I removed the charger and wait for one hour to test it again:
It is 13v when the bike is off
It is 12.6v when the light is on
It is 14.2v around 2000rpm

But it does not hold charge for ~12hrs and the cycle repeat again. I tried the "Recond" function from the CTEK MXS5.0 but it still don't hold charge more than ~12hrs

During these two years, there were two to three times I forgot to turn off the emergency light for about 20minutes and the battery not able to crank start. And there were two to three times I didn't ride the bike for two weeks, it didn't put on charger and it can't crank start. The past few months, the battery holding charge from around one week to three days to two days and this week around 12hrs. Is there anything else that I can test before replace the battery. It is a pity that the battery is just over two years. The bike had the alarm and ABS install from Harley Davidson when it is bought.

Can I say that the two to three times I forgot to turn off the emergency light for about 20minutes and the battery not able to crank start and the two to three times I didn't ride the bike for two weeks, it didn't put on charger and it can't crank start, is it the issue cause the battery to degraded and not able to use more than two years. And is it that it is better to charge one every few days if not riding for weeks because it is the alarm that draw the battery.

 

From my experience the new AGM Batteries are made with cheaper materials to compete in price range. This causes the battery to last less. Just over two years was not bad.

Keep in mind that AGM batteries loose their peak charge capabilities once they get discharged past a certain amount. It is probable that happend to your battery.

 

Its done...get a new one. If it wont hold a charge for a day your out of luck. Same thing happened to me recently. At 2yrs my battery was okay but still wasnt like new, at 3 years...i cant go 24hrs without charging or at least riding. Battery tender most of the time but i left it sitting for 6weeks once. I think that was the straw that broked the camels back.

if u want...check rhe voltage during when u start it. See how far it dips down during that second.

 

The data you gave makes sense.
14.2 V at 2000 rpm is a perfect charging system.
13 V when on is a good resting voltage
12.6 with light on means death,,,,,

The battery took a charge but it was a surface charge. That means the battery is sulfated and will not hold a full charge, more like a false appearance of a charge. The stock HD batteries are garbage. 2 years on the average. Find an Interstate Battery dealer and get the GYZ20HL. It has more ***** than the HD or HD aftermarket battery and made more durable. I have had the GYZ32HL in my FLH for 5 years and it tests great with a load tester. My son has one in his 2016 Road Glide Ultra and it spins that 103 over and over with out dying. I also have a GYZ20H in my FXRDG and it spins that EVO even after sitting for 2 weeks!

 

Buy a new one and take better care of it.

taking a battery down to where they dont have enough charge to start a vehicle is NOT good for them and does the most harm. They tend not to hold a charge very well after that.

 

check your wires...the battery is old but not that old and those voltages are not out of tolerance and are not telling anything. You need to see how far the voltage drops when you hit the starter. If it hangs around 10 volts, then your battery is good.

Or you can take it to your local Auto Zone and they will load test it por nada.

 

When I hit the start, it drop to 9.5v and back to 14.2v around 2800rpm

 

If it drops to 9.5 volts its probably shot. I work at an Interstate Batteries and will agree with skinman13. Typically you want it to sit at 10 volts minimum when its got a load put on it, if its dropping to 9.5 volts its time to replace it. Unfortunately newer batteries just don't seem to hold up as well. Its not uncommon for a 10 year old vehicle to come in with the original battery, however its also not uncommon for a 3 year old vehicle to come in and need a new battery. It wouldn't hurt to have it tested just to be sure, but make sure its charged up before you take it in, but I would guess its time for a new one.

 

https://www.revzilla.com/common-tread/how-to-test-a-motorcycle-battery

time to go shopping.

 

I was going to say unless something has changed over the Decades... 9.6v has always been the magic minimum during a Load Test. Since the link was provide above, I checked...Yep, 9.6v