Join Date: Oct 2013
Location: Chilliwack, BC, Canada
If the voltage is dropping to zero or near zero when a load is applied to the battery (key turned on), you either have a very high resistance between the battery and load point where the voltage is being measured; or, you have a very high resistance in the battery.
I'm not certain whether you are measuring voltage directly from the battery posts (not the bolts)?
If you are measuring the voltage from the bolts or cables, there may be a bad connection. If you are measuring the voltage directly from the battery posts and voltage drops to near zero when the key is switched on, the battery is junk.
You can duplicate the bad connection phenomenon by connecting a test light between the battery post and battery cable of a properly functioning bike. Hook the voltmeter to the battery post and note the meter shows full voltage. Now, switch on the key. The bulb will glow and the meter will show battery voltage. Now, switch key off and move the meter to the cable side of the test light (side away from the battery post). The meter will show battery voltage. Switch the key on again and notice that the voltage drops to near zero because of the resistance of the test light.
Another way to approach this is to use another battery such as jumper cables to a car battery. If it behaves normally, chances are the bike battery is at fault.
If the problem isn't the battery:
You mentioned that the cables are tight but have you removed them and cleaned the surfaces. Shiny black is not good, silver is good. Black oxide is not a good conductor.
I think someone mentioned that this is a common type bike battery failure. You are likely experienced with automotive batteries but bike batteries are strange creatures which have very different failure characteristics. They often break one of the internal buss bars which makes temporary or poor contact. When a load is applied, they may carry a small one but open the circuit for larger ones such as starting. In other cases, they will show battery voltage but fail to carry even a test light.
The other posts have offered very useful advice. If the above doesn't address the problem, follow what has been said.