Yes, thats what it is alright
Either the battery is the problem, as in worn out, or faulty, or a victim of the problem, as in charging system fault

 

All better now after a full charge?

 

thats exactly what it is, I would suspect battery connections since you just washed it pull cables clean and retighten bet it fires up

 

It could also be a stuck starter motor, had the same thing happen on an old sportster,the solinoid will also rattle if the starter sticks however I think the battery is more likley the problem, Check the earth connections too, both ends.

For your info you can sometimes unstick starters by putting the bike in gear and rockng it back and fore with the clutch out, its usualy the return spring that stick's if it is the starter.

Hope you get it sorted soon

Regards

Reddog
Manchester UK