Charging batteries by running engine

[pvb]

An Adverc would replace 95% of full charge, from 50% discharge, in about 4 hours of running (alternator size relatively immaterial)...

That's rather a sweeping statement! It might be nearly right for a single battery, but most people these days have several batteries in a domestic bank and therefore alternator size starts to be an issue. Although I'm a big fan of Adverc boosters, even I'm aware of the limitations.

The above refer to flooded lead acid batteries, gel and maintenance-free are far less efficient.

In fact, maintenance-free batteries are much better at accepting increased charging voltages. AGM batteries in particular have very high charging efficiency.