Once you've figured this out, take a look at the proper way to charge marine deep cycle batteries. I use them too, but I think we are killing them by using them in a car (as a primary).
The start sequence is hard on them. It takes a while to take its toll, but it is hard on them. Also, they have a particular way of being charged that they like, and our alternators are no where near smart enough to charge them properly.
I started suspecting this a year or two ago. But then I got a dedicated smart charger for deep cycle batteries and was surprised to see that although I was daily driving the car, the battery was often only at 50-70%. I'd then charge it overnight with the smart charger, and it would stay at about 98-100% for a few days, but then start loosing its capacity. This is not bad for the battery when it is used slowly, but the shock of starting, then having a 'bad' charge after, actually drains the battery in quick spurts that are never fully fixed by an alternator. This has the effect of ruining the battery over the course of a year or so.
Anyway, I'm not an expert, but I've had issues in several of my Rovers that I've tried to read about and come to some consensus on.