Winter storage of AGM batteries

Sep 22, 2019
118
Chrysler 26 Pymatuning Lake
Hi,
I am finishing a series of projects on our new to us 1986 Catalina 30. One of things I did was replace the batteries with three 100ah AGM batteries. The question is:

During the next off season we will be storing the boat in a shipyard in sandusky ohio. It will be stored there from october until about a late 2024 season launch in july. There is no way to keep the batteries maintained on a charger during storage unless I remove the batteries which i do not want to do if i do not have to since they are heavy. i have read that agm batteries discharge 1-3% per month. That means over 6 months that would be about a 17% total discharge, Would i be ok leaving them on the boat disconnceted for the duration of the six months?
 
Jan 11, 2014
13,012
Sabre 362 113 Fair Haven, NY
Yes. Discharge rates go down with temperature. Make certain they are fully charged before disconnecting.
 
May 17, 2004
5,704
Beneteau Oceanis 37 Havre de Grace
I would not be at all concerned about October to springtime. Once it starts to warm up the discharge rate might increase a bit.

I just checked on my AGM’s after 60 days in temperatures averaging 40 degrees. They absorbed less than 0.5% of their capacity before going back to float. Do be sure they’re actually disconnected - don’t rely on switches as sometimes circuits bypass those.
 
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