Been working on upgrading the domestic battery system on Olive Oils big brother , Popeye ( mine and Ellens new boat )
This is the battery bank we get our power from for the electrical stuff we use like bilge pumps , lighting , stere ereo , hair straighteners and the like . Sick of run down batteries and no leccy we decided to do a proper job on it . First we got some fat new deep cycle batteries . Found them on ebay . Used to be back up power for the landing lights at Manchester airport . Never used , but replaced routinely every few years , a bargain at £ 100 for 2 6v to make one 12v 185 ah . We got 4 giving us 370 ah . GGrrrr . Should do the job .
We had a Sterling battery to battery charger laying around from bike power generating project , so we stuck that in . Simply put , this changes the voltage coming from the alternator to one that the leisure batteries like a bit more . Charges them quicker and more efficiently . Was easy to put in with only three 3 wires to connect . 
To make sense of whats going on with it all , I splashed out and bought a NASA battery monitor . Until now ive always used a multimeter across the battery terminals to check the voltage . This gives some kind of indication of the state of charge of the batteries , isnt too accurate . The new monitor tells the batteries voltage , amps being drawn , percentage of capacity and time left till the next charge . Hopefully all this accuracy will prolong the batteries life , keeping them working better for longer. 
Hunting around on ebay for a decent , cheap solar panel to add in to the system . Free sunshine power when its there and engine / alternator back up when its not .