Modern Vespa

In October I broke my collarbone (ouch!) and since then the LX150 has just been sitting. Didn't touch it. So yesterday I decided to see if it would start. I put it on a battery tender overnight but when I tried to start I just got clicking.

I jump started it from a battery pack and the engine fired right up.
The battery tender said the battery was fully charged so I put my car battery charger on the battery….5 minutes later that charger also said that the battery was fully charged.
The battery reads 12.8 volts with the key off, but it clearly doesn't have enough power to start the bike.

Can a battery go bad just sitting in the garage? No extreme weather. It's a normal (lead acid?) battery, 2.5 years old. The cables are tight.

Any thoughts or should I just replace it?

Yes, after an extended time your battery can read full volts but not enough amps to start. See that all the time. A load tester will tell you the truth. Sorry to here about the collarbone.

Once a lead-acid battery has sat discharged for more than a day or two (and with absolute certainty after a month or more) it'll be toast, with eff-all capacity left. Even the 'clever' chargers with de-sulphation cycles won't revive it into a trustworthy device again.

Replace it - and my current recommendation is to get a LiFePO4 battery - the cost is equivalent to a quality lead-acid, they keep their charge for months (even years), under load they'll hold their voltage (13V +) right up until every bit of energy has been used up. No more fussing with battery maintainers.

Thanks guys. I'll replace it.

I should have followed up - you guys were right. I replaced the battery, now the bike starts great!