2005 Prius went totally dead, but came back

A loose clamp could have caused it, but no one knows what other reasons might have caused it.

 

Was the connector all the way down on the post? I don't recall if this is the case for the Prius 12V, but at least some battery terminals are not cylindrical, instead they are slightly conical (base part of the cone, the pointy part is missing). If the connector is not shoved down all the way to the deck of the battery on one of those the connector "loop" may close fully without getting a very good grip on the post and then it could work loose.

Also, I'm not sure if this is what you meant, but I never put dielectric grease between the post and the connector. The grease is there to prevent corrosion, it goes over the top of everything once assembled to keep water and air away from the metal. If there is grease between the post and the connector it is lubing a junction which is not supposed to move, which is counterproductive.

 

All automotive battery terminals are conical as you describe. That is why it is important to open up the clamp before reattaching it and ensuring the clamp is firmly seated all the way to the bottom of the post.

Remembering to first loosen the clamp and force it open a bit before attaching and tightening.

I 100% agree. I do not use any corrosion countermeasure and the battery posts are 100% corrosion-free, unlike the (good) old days of unsealed batteries.

 

There is a paste that's supposed to prevent corrosion in battery terminals.

 

Not that I have ever heard of, but since it would just be a little piece of lead, probably?

 

I'd be more inclined to use a battery with the proper terminal size.

 

If you can't turn all that palaver on the positive terminal by hand, I'd leave it alone. You've found your answer.

Yes, but those shown two posts back are designed to shim a JIS (Japanese Industrial Standard) terminal post to an SAE (Society of Automotive Enginers) standard Terminal post. It would only be useful if you were using the OEM battery (or equivalent) with a JIS post and someone had converted the post clamps to an SAE clamp. If you can get it tight without, I'd leave well enough alone.