How to winterise a boat battery and electric outboard: what charge level? What the manufacturers require

Lithium batteries for electric outboards should go into winter storage neither full nor empty. Depending on the battery, Torqeedo and ePropulsion require a charge of 20 to 60%, a cool, dry place and a check every 3 to 6 months. Most batteries may only be charged above 0 °C, Victron's only from 5 °C. House batteries are the exception: ECTIVE wants at least 80%.

What charge level should a battery be stored at?

It depends on the manufacturer and even on the battery. The figures differ even within one brand, so the manual for your battery always takes precedence:

Sources: manuals Torqeedo Travel/Travel XP (ch. 8.3), Torqeedo Power 48-5000 (ch. 9), ePropulsion Spirit 1.0 Evo, ePropulsion E-Series, Victron Lithium Smart with datasheet, Victron SuperPack NG, ECTIVE LC Marine LT and ECTIVE white paper.

The differences are striking: for the Cruise, Torqeedo gives 30 to 40% in the motor manual and 20 to 40% in the battery manual. ECTIVE says "at least 80%" in its manual and "60 to 80% optimal" in its white paper. Victron relates its maximum storage time of one year at 25 °C to a fully charged battery. Which house batteries exist and what they cost is covered in Lithium boat battery: Victron and ECTIVE compared.

Why not store it empty?

An empty battery keeps discharging in storage and can be permanently damaged. Torqeedo explicitly warns that storing below 20% can lead to deep discharge (Power 48-5000, ch. 9). Only Victron and ECTIVE give a figure for how fast this happens: under 3% per month when switched off (Victron Lithium NG, ECTIVE). For the Cruise battery Torqeedo only speaks of "very low self-discharge"; the battery switches off after 48 hours without use.

Too full is not wanted for some batteries either. ePropulsion's Spirit battery discharges itself: if it sits above 60% for more than 10 days, the electronics lower the charge to 60% (manual Spirit 1.0 Evo).

When does the warranty end?

Victron is the clearest: deep discharge and charging below 5 °C void the warranty (manual Lithium Smart). For its NG batteries Victron states the exact limit: below 10 V (12 V battery) or below 2.5 V per cell the battery is permanently damaged (manual Lithium NG).

ePropulsion may refuse warranty on the Navy 3.0 Evo for improper storage and over-discharging (manual Navy 3.0 Evo), and its general warranty terms list incorrect storage as an exclusion (ePropulsion). Torqeedo and ECTIVE do not explicitly exclude deep discharge, but refuse warranty if you have not followed the care instructions (Torqeedo warranty conditions, ECTIVE warranty). In practice: if you ignore the storage rules in the manual, you risk the warranty with all four manufacturers.

What do I need to do to the motor?

There is little to do on the motor itself; there is no oil change and no spark plugs. The manufacturers list:

• Rinse with fresh water if you used it in salt or brackish water (Torqeedo, ePropulsion).
• Check the sacrificial anode and replace it if necessary before the Cruise goes into storage (Torqeedo Cruise).
• Look after the connectors: Torqeedo recommends silicone grease on the Travel plug and contact spray; ePropulsion recommends contact spray on the connectors and, for the E-Series, protective caps and contact gel (Travel, Spirit, E-Series).
• Lay it down correctly: ePropulsion wants the Spirit and Navy stored horizontally on a padded surface with no pressure on the propeller, ideally in the original packaging. Never tilt the Power 48-5000 battery by more than 20 degrees.
• Charge the remote: the Navy's Evo remote control needs charging every 6 months during long storage.

If an ePropulsion motor is not used for more than 2 months, the manufacturer recommends cleaning and an inspection, for the Navy by the dealer.