Why is my outboard battery not charging?

Battery and charging problems · Symptom: Battery goes dead, slow cranking, gauges flicker.

Quick answer: Most often: loose battery cables (vibration loosens nuts) — techs say most engine problems are fuel or battery, corroded terminals, battery left connected/discharging between trips. Start here: Wiggle each cable at the terminal — it should not budge; tighten if it does. If that doesn't fix it, work through the checks below, and see a marine mechanic if you're unsure.

Likely causes (most to least common)

Loose battery cables (vibration loosens nuts) — techs say most engine problems are fuel or battery
Corroded terminals
Battery left connected/discharging between trips
Old battery (Discover Boating: expect ~4–5 seasons from a well-kept marine starting battery)
Automotive battery used instead of marine
Charging system (stator/rectifier/regulator) fault

Safe owner checks & fixes

Wiggle each cable at the terminal — it should not budge; tighten if it does.
Clean corrosion (Discover Boating suggests baking soda and water); fit a boot on the positive terminal.
Make sure the battery is secured in a box or tray.
Use a marine maintenance charger between outings; turn battery switch off for storage.
Have the battery load-tested at a parts store.
If a good battery keeps going flat while running, suspect the charging system (expected charging output is model-specific — check your owner's manual).

Parts / tools

Multimeter, terminal brush, baking soda, terminal boot, marine battery maintainer/charger, replacement marine battery (spec per manual).

Call a marine mechanic when

Battery tests good but keeps going flat when running (stator/rectifier/regulator); melted or heat-damaged wiring; any battery that is swollen, cracked or leaking. If you're on the water and unsafe, call TowBoatUS/Sea Tow or the Coast Guard (VHF 16).

Sources

Values not stated here are model-specific; check your owner's manual.