The new batteries arrived on Friday, and Pip installed them.
This simple sentence covers quite a lot of work. We finally found an electrician who looked over our wiring and pronounced it ok (after Maurice) except for one bit of fridge wiring we have reinsulated and will replace when we can find the right grade wire. Grazie Davide. We also spoke at length to the friend of a friend, Ozzie electrician Mark in Port Napoleon, who was very helpful. Ta, Mark.
Buying the batteries was expedited by the wonderful Walter at the nautical store, who spoke English, understood our problems and desire to get moving, and really pulled things together. So thanks to him.
The hard labour was getting the two old batteries (each weighing 70kg) off the boat, and the two new ones(each weighing 73 kg) on the boat. Each time, the battery was strapped with webbing and then a rope bridle lashed on. We used a spinnaker halyard to haul them up through the forehatch. With people on guys - three ropes steering the battery - we moved it carefully over the water and onto the dock. With a lot of effort we got batteries on and off the dock (thanks to Marine del Sole for renting cars at a cheap hourly rate, in which we got the batteries from the shop), and the dead one got lugged away to the battery dump. Thanks to lots of people on the boats around us for handling ropes, helping with winching and providing muscle power.
The new batteries (Mastervolt this time; that's what's available here) are still AGM, as that's what our battery box is now designed and managed to accommodate. In case you're wondering, the top half of the saloon table then fits over the top of them.
They are each a whopping 270 amp-hours (the old ones were 255) and fit beautifully. Incidentally, we did look at what we might buy in Tunisia, but were not comforted by very limited dealer networks. Alternatively we could have gone on with one battery to Malta, and bought there, but couldn't see this would be cheaper or easier. So 48 hours after installation, all is going well, and we will be leaving Cagliari with battery confidence, but a substantial dent in the bank account.