I still think the problem could be a faulty/tired battery.
You say you bought a Battery Capacity tester, what is it exactly?
Have you got a multimeter to measure the battery voltages?
I assume the batteries are lead acid batteries, same as a car battery. They have a limited life cycle...
I’m looking for a good solution to extend my roof out over the gable end.
Although I’m not an experienced builder…. I’m building a small single storey villa in the south of France. Most of the roofs in the area stop flush with the gable end, meaning that when it rains the water runs or drips...