How Does a Multi-Stage Charger Handle Battery Charging With Parallel DC Loads?

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?

I would think that a 3 stage charger should have the battery disconnected from the boat load when charging. I found this document on "battery charging":http://bit.ly/vlPFHW pretty informative.

I suspect a multistage charger is not needed. A current-limited, constant voltage source is all that is required to charge and float a healthy lead-acid battery.

A heavily discharged lead-acid cell will accept a very charging current hence the need for a current limited charger. When fully charged and supplied with a constant voltage the float current will be 1/500 the cell amp-hour rating.

I believe the de-sulphation mode provides a voltage higher than 14.2 volts. But if you take care of your batteries (don't discharge more than 50%, keep them on a float charge) sulphation should not occur and there is no need for this mode. I have charged sulphated 12 volt batteries and they required a 17 volt, current limited charge for a couple of days.

The charge and float modes are taken care of with a current limited 14.2 volt charger.

Now to answer your questions. I did some research and the de-sulphating mode used these days consists of short, high current pulses. These may push the battery terminal voltage above 14.2 volts.

If the charger is indeed a current- limited constant-voltage source, having loads connected will not harm anything. Let's say the charger is supplying 14.2 volts at 40 amps. And let's say the load is 25 amps. The battery charging current is then 15 amps. As the battery reaches full charge the load current will remain at 25 amps while the battery current drops to a low level (of perhaps 1 amp for a 500 A-H battery bank).

But, as you said "heavily discharged lead-acid cell will accept a very charging current..." so, how dose a current limited charger deal with this inrush current(in addition of load current) at the beginning and keeps fuses without breaking?

With a current-limited battery charger there it will only output so much current. No damage is done to the charger. If the current that the circuit draws is less than the current limit it will supply 14.2 volts. If the current that the circuit draws is more than the current limit it will supply that current and the batteries will supply the remainder.