Storing a Boat Battery Over Winter: Lithium vs AGM Care

Store a boat battery clean, disconnected and out of the cold if you can, then follow a charge routine that matches its chemistry. AGM batteries should not be left to go flat and are usually given a periodic top-up. Lithium (LiFePO4) batteries have their own rules, and the makers' published guidance does not agree on the ideal storage charge, so your battery's manual decides. If you are still choosing between the two, start with AGM vs lithium marine batteries.

Disconnect it, and take it off the boat if you can

A boat battery in winter loses charge for two reasons: slow self-discharge, and small loads that never fully switch off. A lithium maker notes that turning off a main disconnect may not stop small drains such as a CO detector or a monitor, and suggests removing the cables completely where practical. Another lithium maker says to isolate the bank from loads and chargers and to protect the terminals. If you suspect a drain, our guide to finding a parasitic battery drain shows how to track it down.

Where the battery lives matters too. One AGM maker recommends a cool, dry place above freezing and not directly on concrete. Lithium makers publish storage temperature ranges that vary by battery and by how long it will sit, so use the range on your battery's data sheet. Batteries are heavy and can be hazardous, so follow the maker's handling instructions, and use a box or tray in transit.

AGM care: do not let it sit flat

AGM and other lead-acid batteries self-discharge over the winter. One AGM maker's support article puts typical loss at about 2 percent of charge per month at 77°F, slower when cool and faster when hot, and recommends a refresher charge every three to four months. It adds that a discharged battery can freeze in very cold weather and that damage from long storage without charging may fall outside warranty.

A flat lead-acid battery may also be harder to bring back, which is why a quick spring recharge is not a cure for a winter of neglect. If a battery was left connected to a small load all winter, treat it with suspicion: check it with a meter or have it load tested before you trust it on the water.

That is the practical reason AGM owners use a maintainer or set a calendar reminder to charge. Check your battery maker's instructions for the charge to start from and the voltage settings for any refresher charge, and use a charger with a profile for your battery type.

Lithium (LiFePO4) care: the advice differs

Lithium care is different, and the guidance varies by maker. One maker advises fully charging before storage, topping up about every six months, and not charging below freezing (32°F). It says a trickle charger is not needed, because those are made for lead-acid and AGM, and that leaving a charger connected and unattended for long periods is not recommended. A second maker advises storing at roughly half charge for long periods and checking at least every six months.

Cold also changes the picture. At least one lithium maker warns against charging below freezing, so a lithium battery left on the boat with a charger running in a cold shed is a different situation from one kept indoors. If your boat lives somewhere that drops below freezing, bringing the lithium battery inside removes the question.

Because makers differ, do not copy a forum number. Read the manual for your battery, since the built-in protection and the recommended storage charge may be specific to it. If you cannot find storage guidance, ask the maker before the first cold snap.

Maintainers: how they differ by chemistry

When you shop for a charger, the question is not how many amps it has but whether it has the right profile and whether the maker of the battery allows it to stay connected. See best marine battery chargers and maintainers for the buying side, and LiFePO4 vs AGM in 2026 for chemistry basics.

A simple storage routine

1. Switch off and disconnect. Turn off loads, then disconnect the cables in the order your manual gives.
2. Clean and inspect. Wipe the case and check for cracks, bulges or leaks. Do not store a damaged battery.
3. Label it. Write the date, chemistry and the date of the next check on the case.
4. Store it. Choose a dry, cool location suited to the battery type, on a stable surface, away from heat sources.
5. Set reminders. Mark the refresher-charge date for your chemistry on a calendar.
6. Recharge before spring. Bring it back to full charge as the maker directs before it goes back in the boat.

In spring, reconnect in the order the manual gives, check that terminals are tight and clean, and look over the battery again for swelling or leaks. If you have a battery monitor or voltmeter, note the reading after a full charge so you can compare it next winter.

Our winterizing guide covers the rest of the boat.