Battery Ch and DC supply

[concentrik]

This is my first and possibly only post - I asked this question on a boaty forum, didn't get a clear answer and rather than make a pest of myself among my boaty peers I thought I'd try the experts.... here goes:

Three 12V 110Ah sealed lead acid batteries in parallel supply a number of 12V systems on a boat. The average dc load is about 20A but can increase to 35-40 depending on who is using laptops, TV, DVD, fridge etc. The batteries supply all this current until they expire unless the engine is running, in which case the alternator (40A) is happy to supply the current and charge the batteries at a bog-standard 14.2v (approx).

A lot of the time the boat will be in port overnight so has access to shore power. Current wisdom insists that boat batteries should be charged using an 'intelligent' multi stage charger with 'de-sulphation', 'absorbtion' and 'float' stages.

Here's the question. How does the charger 'intelligently' discriminate between battery load and parallel DC load from the boat systems? For example, the boat berths with drained batteries and the shorepower is connected. Everyone immediately wants to use lots of boat systems calling down 25A. How does the charger deal with this? It has been suggested elsewhere that such chargers 'disconnect' to assess the terminal voltage for a short period - how can this happen without cutting DC to boat systems? What routine must the charger follow to ensure that varying parallel loads are supplied (also these should be ideally supplied at no more than 13.8v as they don't have internal regs) and the battery bank gets its clever charge regime?