Charging a 12 v flooded cell battery to 100% of it's capacity takes a long time. From 50% capacity to 80% might take several hours. To get it to 100% might well take a further 6 or more hours. As someone said earlier volt drop in the cable from alternator to battery will have an impact but even with no drop it's still a lengthy process.
If anyone really wants to get their head around the issue can I suggest they look on a site such as
www.canalworld.net you can go on as a guest. Use the search facility and you will find enough information to keep you reading until doomsday. Like most forums you will find a range of opinions but also many absolute facts
Frank (Boatie) Narrowboat owner with a 700 ah battery bank.