REALLY Dim Lights!

So it's been a minute since I last rode my '78 KZ650B, but Spring is here and I decided to get her going once again. Charged up the battery (12.53.volts), hooked 'er up, and Nothing...!  Wouldn't even crank. The headlight and turn signals are VERY dim - like, 'almost dead battery' dim, - but I don't know why. The Hi beam on the headlight won't work (blown?? I don't know), the Lo beam is 'fine', but VERY dim, and the bike's fuses are intact. The battery itself is three (?) years old, but it still registers at a decent voltage...but what would make the lights so dim? Where should I start looking? I checked the ground off the battery, but still No Joy...I checked the voltage when I tried to start her and it registered 11.72, but then went back up to 12.45 volts after it rested for a bit...I am mystified; any ideas...? 

Battery volts are just the carrier. Amps are the doer. Your battery can show 12+ volts but be way down on amps.
Try jumping it from a non running car battery. If it's good then it's new battery time. 
This is typical of a gel battery that's been drained. After charging it'll deliver volts but not amps. 

Thanks Warren, I'll try the car battery thing; but it IS a gel battery, so probably IS time for a new battery.
 

Hi, also a really good idea to keep your batteries on a charger such as Optimate, which charges and maintains. I have used them for over 25 years and not had to buy a replacement battery since. I have even recovered a dead battery as a test, and my friend was that impressed, he now keeps his bikes hooked up to them.
Rich.

I'll give it a shot.
Grab your meter
DC V
key on, engine not running, headlight on
we'll do it quick to keep your coils from overheating

black probe on negative battery post (post is the metal part of the battery)
red probe on positive battery post
what is your voltage to two decimal places (ex 12.45)

move your black probe to the fuse block
probe both sides of each fuse
voltage should be as close to 0.0V as possible
any higher than 0.5V and you have high resistance on that circuit

move your black probe back to the negative battery post
move your red probe to a bare metal part of the engine block.
voltage should be as close to 0.0V as possible.

turn off bike