The battery is only half the circuit
A new group 31 on green, undersize cable still cranks slow. Voltage at the posts is not voltage at the starter. We inspect lugs, the switch, and the run. 33 CFR 183.420 requires that each battery terminal connector not depend on spring tension, and that metallic objects cannot contact ungrounded terminals. A spring clamp from a car jumper set is not a marine terminal.
Size the cable for the job
Starting cables are short and thick because inrush is huge. Trolling and inverter cables are long and still have to carry current without a sag that looks like a dead bank. We do not publish a homemade AWG chart. We look at what is on the boat and whether it heats under load.
Switches and tinned lugs
A battery switch with corroded studs is a fuse you did not intend. Tinned lugs and heat-shrink are ordinary marine practice because salt lives here. Replacing a battery and leaving a chewed lug is a repeat failure on Scout and Everglades consoles that live in spray.
Hold the cable so it cannot chafe
A cable that moves with the engine or rubs a hatch lid will fail after the new battery is installed. We secure the run when we are in the well. That is part of coming to the boat, not a separate shop visit.
What to photograph
Both posts, the switch, and any obvious heat discoloration on cable jackets. Lee County and Tampa Bay salt boats usually need terminals and batteries on the same visit.
What we cut versus what we clean
Green fuzz on a sound lug can be cleaned. A lug that has lost its crimp, or a cable with a cooked jacket, gets cut back or replaced. Salt boats in Lee County usually need both. We carry terminal hardware because 33 CFR 183.420 wants a connector that clamps, and a cleaned post with the old spring clip put back is the same failure in two weeks.
Sources
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.