Welcome to the Forum & posting Charlie. :2thumbs:
Batteries really don't like sitting around not being used; or worse, being used just a little but not enough to fully re-charge it after the drain of starting - and they need to run at least 30 minutes or so at highway speeds to do that properly, or you're just slowly killing the battery! Plus, if you leave them sitting on a charger that's not a multi-stage battery maintainer/conditioner all the time except when riding, it'll eventually mean that you might be able to start the bike at home, cos it just came off the charger, and then
just as a trap for the unwary, only to have it fail once you're well away from home cos the charger was simply providing a surface charge but never actually loading the battery up/maintaining the battery's Cold Cranking Amps &/or A/h load capacity so that it could barely manage that initial starting load to get the engine going, and then it crapped itself!! Anything about all that sound familiar yet?? :dontknow:
Regardless, that 18 month old battery
might have 13.6 volts of surface charge left in it, but if it's dead &/or clagged up inside, it's likely that it was pushing the friendship simply to provide enough grunt to drive the multi-meter display before :shocked: even tho it might have
juuust been able to make the 13.6 volts that it showed in order to kid you into thinking it was good when it really wasn't!! And now that you've done a start or two and not run it hard enough/ling enough it's just given up entirely in terms of it's ability to crank those sorta volts under load.

What you've described above is classic '
Dead Battery' symptoms with the addition of being caught out by the '
but it started OK after charging'
trap for the unwary!! :lecturef_smilie:
So from what you've described, it
REALLY sounds like that battery is
DEAD, meaning that you can either confirm that for yourself with a load test (do a rudimentary test by measuring the voltage with a multi-meter
AS you crank it over & try to start, anything less than 12v while actually cranking is effectively 'flat' on these power hungry Spyders;
or a 'proper' load test with the correct tool - many battery suppliers/auto part/workshops provide those tests free of charge!

) or you can
just buy a new battery, put it on a charger/maintainer for AT LEAST 8 hours before installing, then fit it & most likely ride away a happy chappy for probably a good few years to come!

hyea:
Ps: No matter what the battery re-seller may tell you, unless you know and trust them implicitly, or you've actually witnessed it, the chances they've done that pre-installation charge are pretty tiny, and if you
DON'T do that, you'll probably be looking at buying yet another battery pretty soon, certainly very likely within a few years, if not sooner!! :lecturef_smilie: They
NEED that pre-installation charge to stand any real chance of working well for too long - fail to do it, you might as well just burn the $$ you were going to spend on buying a new battery, cos at least that way you'll know who ****** up and won't have any doubts about why your Spyder dash is dead & it won't start/run anymore! :banghead:
Get a New Battery; charge it properly before installing it; and then check the charging system once the bike is running again! :thumbup:
Pps: If you just purchased a 2012 Spyder that has only 8000 miles on it, then it's done
a LOT of sitting around over the last 11 years or so, meaning that unless they've recently been changed, there's a good chance the tires are also dead and quite possibly dangerous to ride on! And again, unless this has all recently been done, there's almost as much of a good chance that it's in
DIRE NEED of an oil & filter change; a coolant change; a brake fluid change; a gas tank flush & refill with fresh fuel; and a thorough inspection if not replacement of all the perishable radiator & vacuum hoses, the belt, & any other perishable over time filters/items etc in order to make sure that something else doesn't fail on you once you get out there & away from home - cos as it sounds like you may've already found out,
Tow trucks are really expensive....... :gaah: