What to do with LiFePO4 batteries when the boat is layed up?

Joerg Esdorn

I have 3x Mastervolt MLI Ultra 5500 batteries on my boat, new this year.  I’m starting to think about what to do with the batteries over the winter when I will not be on the boat.  When I had my gel batteries, I would turn off all loads and put the charger on or leave the solar charger on.  Following some of the discussion about charging LiFePO4 batteries on this forum, I‘m gathering that some people think it‘s not a good idea to leave these batteries on a charger all winter long.  However, the MV manual says just that:

„If the battery will not be used for a period exceeding 3 months, we advise the following:
• If external AC power is available switch off all loads and switch on the charger. Apply a float voltage as specified in the following table.
Model Float voltage setting
12V 13.5V 24V 27.0V
• If no external AC power is available:
- Charge the battery to > 80% of its capacity before storage.
- Set the safety relay knob to “LOCK OFF”, see page 11.
- Make sure MasterBus powering is not set to "Always on" (see Configuration tab in
In this setup the batteries can be kept at least 6 months without maintenance. However, it is highly recommended to charge the battery to > 80% of its capacity every 100 days.“

I hate to second guess MV but I wonder if this group includes any engineers with the relevant specialist knowledge who can explain/critique this advice?  

I have considered just disconnecting them but with the boat in the water over the winter, I don‘t want to be without the bilge pump being operative.  

Thanks in advance!

Joerg Esdorn
A55 #53 Kincsem
Currently in La Rochelle

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