12 volt battery seems dead

Could be loose connection. Check voltage with a digital multimeter directly at the battery posts.

 

yes, you need a new battery. how did the terminal become loose? probably never tightened properly, and vibration did its job well

 

Check and tighten both of the connections at the 12v battery, also check for corrosion where the ground strap is bolted to the car body.

How old is the 12v battery, and how many hours per week are you driving the vehicle?

 

Are you REALLY saying that you don't believe that a screw/bolt will never work it's way loose just from vibrations and heat and cold ??
It happens often.

But your battery voltage readings are a bit low too.
First clean and tighten both battery cable connections.
Then drive it for a week or so, let it sit overnight and check the voltage again.
I suspect you will be needing a new battery SOON.

P.S. It is also possible that there is a bad connection INSIDE the battery near the positive post.......but that is a rare thing to happen.

 

If it's seeing very little use better put it on a smart charger (owner's manual recommends 3.5 amp max). And do take it for a good drive periodically, say once a week.

 

First clean up and tighten the connections, including the negative ground strap attached to the body.

Sounds like you're not using the vehicle enough, let it recharge in 'ready' mode for more than 10 mins every 2 days and see if things improve.

You can check the 'date code' on the top of the 12v battery, if the age is "unknown" it may need replacing, or you can try to recharge it.

Your terminals/posts might differ, but this thread may be useful : Fitting mobility 12volt AGM battery. | PriusChat