[Bob Lincoln]

Yes. Connections "looking" good and tight do not mean that they ARE clean and tight. Disconnect and clean them with a battery terminal brush, both the posts and the clamps. Inspect for corrosion where the cables meet the clamps.

And have someone monitor the voltage with a voltmeter (preferably digital) while you crank. If it drops below 10V, the battery is bad or the starter wiring is shorting out. Open-circuit voltage doesn't tell the whole story, there is no load when measuring that.

Was this sudden or has it been coming on for awhile?

 

[Bob Lincoln]

Jumper cables have to flow through the post/clamp connection, so if that is dirty, jumper cables and external battery won't help. Seen it happen before.

Try the voltage drop test while cranking, also. If it stays above about 10.5V when cranking, the battery is good, or good enough to start.