Someone may find this useful!
Whilst performing open heart/wallet surgery on my van recently, new leisure battery, xs50 b2b, Cerbo GX, I had both starter and leisure batteries disconnected for the sake of electrickery safety. It took me a total of 4 days with work in between. The leisure battery was healthy but I knew the starter battery was going to be low. It was at 11 volts.
Everything installed and reassembled I thought my Ablemail AMT12-2 would spring into action and trickle the starter battery, nope.
I was on the verge of buying another AMT, thinking I had somehow goosed mine.
Fortunately the van started and after a good run, when I got back home I was able to watch the Ablemail 'do its thing' and keep the starter at 12.3 volts, basically if you let your SB go under 11.5v it won't work. Default setting 4, out of the box, works perfectly.
Cerbo is a wonderful thing, easy enough to fit (must be if I did it) and you can remotely keep your eye on all things electrical in the van, love it!