When I bought my motorhome it came with a pair of 120Ah AGM Batteries which turned out to be knackered and lost power to hab electrics the first night I went away in it. I was planning on updating them but not quite so immediately.
But anyway ... had nothing to lose so on one of the batteries (now out the van), I set my Victron IP22 charger to around 15V and left it connected to the battery for around a week and in terms of amphours, it put in a couple of hundred Ah into this 120AH battery at 15V and the battery got nice and warm.
Idea was to recondition it and just see if it had any effect? Well, that battery became pretty usable after this process. It would hold its charge and provide much more power than it did when I first got it, so there is something to be said for trying this.
I never put this battery back in the van, and instead into a battery box to work as a bench battery and it is not one I would choose to actually rely on, but doing that high voltage recharge did change it from a large paperweight to a usable 12V energy source.
