Rover

Look at the output voltage of the alternator. Usually about 14.4V. If you look at the charge characteristic curve for a lead acid battery at that voltage, it rises steeply to about 80% and then flattens and the last 20% can take many hours to achieve. In practice, just relying on the alternator you are unlikely to do much better than 80% charged. In addition to charging the batteries, the alternator will also be supplying current to power the connected loads; fridge, freezer etc. The optimum solution with present technology is a battery to battery charger from Victron or Sterling or similar. These have a multi stage charging profile and will charge the domestic battery many times quicker than relying on the alternator.

Here is a scan from the 1983 Blakes brochure.

Here is a scan from the 1973 Hoseasons brochure.