Well, yesterday I had some time to check dead battery, using a densimeter. Starting from positive pole, I measured specific gravity of each cell. Cells 1 to 5 were just fine (close to 1.30), with good electrolyte level so I was a bit worried... until cell #6 (Murphy at its best
!) showed no indication, which means it was completely discharged (it should have an internal short, as I suspected). So, well, battery is gone.
Then I took out the battery plastic tray (with some difficulty, as you need to partially dismantle battery support) and found a lot of acid-induced corrossion
. Good thing is that, despite its terrible looks, it was just affecting surface and, once neutralized with baking soda (a great hint!) and rinsed with clear water, results were good enough. I then sprayed some oil to stop a possible oxidation.
I also replaced broken battery connector (it was broken when getting out bad battery, because former owner did not use any grease and was literally soldered to the battery!). I used a non-original one because local Renault agent was closed yesterday (but they told me the day before they will be open!!!)
I had no time to put new battery and will do today... with just the time to do that and take the car for a short trip, full of family people... let's hope all works fine!