Marine Battery Care: Test, Charge and Store for Winter

Turn off all electronics and set the battery switch to OFF. Wait a few hours after charging or running the engine before you test (a "rested" battery gives an honest reading).

Look for cracks, bulges, leaks, melted terminals or loose hold-down straps. A battery must be strapped down and its positive terminal covered with a boot. Check cables for green or white corrosion, cracked insulation and loose lugs.

If there is corrosion, disconnect the negative cable first, then the positive. Scrub terminals and cable ends with a terminal brush. Neutralize acid residue with a baking soda and water paste, rinse lightly, and dry. Keep the cleaning solution out of the cell caps.

Wipe the whole top of the battery clean and dry too. Boating Magazine notes that an oily, dusty film on the case holds moisture and lets a small current leak from the positive post to the negative, which slowly drains a battery that is "off".

Pop the caps and look. Plates should be covered. Top up with distilled water only, after charging (not before, unless the plates are exposed). Do not overfill. AGM, gel and lithium batteries are sealed; never open them.

Set the multimeter to DC volts and touch red to positive, black to negative. Typical readings for a rested 12-volt lead-acid battery (confirm with your battery maker):

• About 12.6 to 12.8 V: fully charged (AGM often reads a bit higher)
• About 12.4 V: roughly three-quarters charged
• About 12.2 V: roughly half charged; charge it soon
• Below about 12.0 V: deeply discharged; charge right away and then retest

Lithium batteries hold a nearly flat voltage until almost empty, so voltage is a poor fuel gauge for them. Use the battery's app or monitor if it has one.

Voltage alone can hide a weak battery. Have it load tested (most auto parts stores do this free) or use a load tester. Yamaha recommends a load test before storage so you know it will crank in spring. A battery that fails a load test after a full charge should be replaced.

Battery keeps going flat? Two quick checks tell you where to look. With the engine running above idle, the voltage at the battery should be clearly higher than your resting reading; if it is not, the engine's charging system is not doing its job (a dealer or marine electrician job). If it charges fine but goes flat while the boat sits, something is drawing power with everything off, such as a bilge pump cycling, a stereo memory or a fish finder left on.

To hunt a small drain yourself: with everything switched off, disconnect the negative cable and connect a multimeter set to amps (DC, on its amp jack) between the cable and the negative post. A reading well above zero means something is drawing power; pull fuses or open breakers one at a time and watch which one makes the reading drop. Never turn the key or run a pump or windlass with the meter connected this way, because the starting current will blow the meter's fuse or damage the meter. If the drain is not on any fuse, leave it to a marine electrician.

Set the charger to the correct chemistry. Connect positive first, then negative, then plug in the charger. Let it complete all stages (bulk, absorption, float). A good rule of thumb from BoatUS: charger amps around 10 to 15 percent of the battery's amp-hour rating.

Charge in a ventilated space, never in a closed cabin with a flooded battery.

• Boat on land in a freezing climate: remove the battery and store it indoors somewhere cool, dry and above freezing. A charged lead-acid battery resists freezing much better than a flat one; a discharged one can freeze and crack.
• Boat stays connected: turn the battery switch off, or disconnect the negative cable, to stop small parasitic drains.
• Boat in the water: your bilge pump needs power. Keep the bilge pump circuit live and the battery charged with a marine charger on shore power, or check it often.

Use a marine smart maintainer that drops to a safe float voltage, or recharge fully about once a month. Avoid basic trickle chargers that never shut off; they can boil a battery dry over a winter.

AGM and gel: follow the maker's float voltage. Lithium: many makers recommend storing at a partial charge (often around 50 to 80 percent) with the battery disconnected, and most LiFePO4 batteries must not be charged below freezing unless they have built-in heating. Follow your battery maker's instructions exactly.

Charge fully, reinstall, connect positive first and negative last, tighten the nuts snugly, and coat the terminals with protector spray or dielectric grease. If your cables are held on with wing nuts, replace them with proper hex or locking nuts: Boating Magazine notes wing nuts no longer meet industry standards because they loosen too easily. Make sure the hold-down is secure and the terminal boots are on.

Record the date, resting voltage and load-test result. A battery that loses voltage faster each year is nearing the end.