Battery types in the field

• Starting: thin plates, high cranking amps, hates deep discharge.
• Deep cycle: thick plates for trolling motors, house loads.
• Dual purpose: the compromise most runabouts carry.
• AGM/lithium: sealed, mount-flexible, need charging profiles that match — check the charger's settings before swapping chemistry, and treat lithium retrofits as a system design job, not a battery swap.

Always load-test; resting voltage lies. A battery can read fine and still collapse under cranking.

Marine wiring rules that matter dockside

1. Marine-grade tinned wire only. Automotive wire wicks water up the strands and rots from inside.
2. No wire nuts, no household connectors, no solder-only joints in vibration zones. Crimped connections with adhesive-lined heat shrink are the standard.
3. Every positive circuit fused or breakered at the source of power, sized to protect the wire. Follow ABYC guidance and the device manuals for sizing.
4. Wire gauge sized for load and length — voltage drop matters at 12 V; check an ampacity/voltage-drop table rather than guessing.
5. Battery terminals: no more than a few terminals per stud (ABYC limits stack-ups — check current guidance), wing nuts replaced with nyloc nuts on ring terminals.
6. Battery secured in a tray or box with the positive terminal protected — an unsecured battery with a wrench-across-the-posts is a fire.

Battery switches and charging

• A 1/2/BOTH/OFF switch should let the house run without draining the start battery. Verify the alternator output path isn't broken when switching (many switches are make-before-break for this reason — confirm before advising customers to switch underway).
• Bilge pump float circuits typically wire ahead of the switch (always hot, separately fused) so the boat can save itself with the switch off.
• Add or verify an ACR/VSR or DC-DC charger on two-battery setups instead of trusting the owner to manage the switch.

Common mistakes

• Replacing batteries without finding the parasitic drain or the failed charging output that killed them
• Undersized ground returns causing gauges to wander and pumps to whine
• Mixing old and new batteries in the same bank

When to walk away

A melted harness, backyard AC shore-power wiring, or battery cables green to the core is a rewire quote — patching one symptom of a rotten system puts your name on the next failure.

Safety: disconnect negative first, reconnect it last; batteries vent hydrogen — no sparks over the battery; remove rings and metal watch bands.