How should I maintain lithium boat batteries over the winter?
My boat will be laid up ashore from November to April with lithium domestic batteries aboard. Should I leave the shore-power charger connected, and what should I do when temperatures drop below freezing?
Answer
For winter storage, bring your lithium boat batteries to the manufacturer's specified state of charge, isolate unnecessary loads and check them periodically. Leave a charger connected only if the battery manual permits it and the system prevents charging below the battery's minimum charging temperature.
Check your battery's storage instructions
Read the battery label, then find the storage instructions for that exact model. Most lithium domestic banks are lithium iron phosphate, or LiFePO4, but storage charge levels, temperature limits and shutdown procedures differ between manufacturers and generations.
Advice for a portable lithium battery may not apply to an integrated yacht installation. Some batteries need storing partly charged, commonly around 40 to 60%; other marine systems require more than 80% charge or an approved continuous float supply.
Record these details before laying up:
- Battery make, model and capacity in amp-hours.
- Recommended storage charge and permitted storage temperature.
- Minimum charging temperature and whether automatic cold-charge protection is fitted.
- The shutdown procedure for the battery management system, or BMS.
Check the charge using the battery monitor or manufacturer's app. LiFePO4 voltage changes little through much of its working range, so a voltmeter alone gives a poor estimate of remaining capacity.
Storage and shore power
Without shore power: charge to the specified storage level, then isolate loads and charging sources using the manufacturer's shutdown sequence. Switching off the domestic master switch may leave the inverter, battery monitor, alarm, router or communications network connected.
For the MasterBus-equipped installation with a safety relay covered here, storage exceeding three months requires charging above 80%, putting the safety relay knob in LOCK OFF and ensuring MasterBus powering is not set to Always on in MasterAdjust. Recharge above 80% every 100 days. These settings apply to that installation; they are not universal lithium storage settings.
With shore power: leave the charger running only if the battery manufacturer approves. For the same installation, the specified float voltage is 13.5V for a nominal 12V system or 27.0V for a nominal 24V system, with all loads switched off.
Avoid lead-acid equalisation, desulphation or reconditioning modes. Check the charger's actual settings rather than trusting a profile labelled simply "lithium".
Prevent charging cold cells
Cold storage and cold charging are different matters. Many LiFePO4 batteries tolerate storage below freezing within their published limits, but charging cold cells can permanently damage them.