Many modern vehicles have "smart alternators" that drop the charge voltage fairly quickly which is fine for the start battery but leads to 2nd batteries being undercharged and failing very quickly.
Dc-dc chargers have been around for a while and are mainly used to over come the voltage drop when charging batteries in caravans etc. These are generally very expensive like anything that is 4x4 related.
What alternatives are out there? Has anyone used one of these
Are there slightly more expensive, quality ones?