Originally Posted by Downhilldai
4-cyl Merc 50 needs hardly any CCA to fire it up. Local motor factors will sell you a dead basic car battery for around £25, which will be fine, unless, as jyasaki says, you're running lots of electrical gear while the motor isn't running/charging. If you are, then consider a 2nd battery (leisure type), to run your toys, when the motor's switched off.
When the battery on the 5.4 went dead I replaced it with the cheapest one Halfords (because they're open on Sundays) could supply. Worked a treat with my 90hp 2 stroke.