Also see my Lithium battery page…
Increasing the domestic bank size can be a good idea as it can provide longer intervals between recharging.
New trays neatly welded in with batteries strapped down and terminals covered to comply with BSS standards. It’s sometimes possible to increase the AH capacity of a limited sized tray by fitting a different type of lead acid battery. 4* 6V batteries will generally fit in the same tray as 3* 110AH. For liveaboards it’s better to go for something better than a standard 110AH leisure battery and I tend to recommend Trojan, Crown or US Batteries as they offer a good compromise between price and quality.
Batteries can be balanced so charging currents and loads for each battery are even.
Before thinking about increasing a domestic battery bank you should have means to properly monitor and manage the system with some sort of battery monitor. Also you must have means to recharge a large battery bank properly with suitable alternator.