Long term AGM battery storage

[BrianSmith]

My main engine has a 24v starter, and 2 Group 31 12v start batteries. One of them seems to be failing, but the other seems solid. (They're 2 1/2 years old.) I am going to replace them both with new, but I'm wondering about keeping the good one in storage as a spare. From what I can learn on Google, the best way to keep an AGM healty in long-term storage is with a smart charger with a "maintain" cycle, and that is based on the specific battery type.

Has anyone done something like this? What has been your experience with storing a battery, unused, for a year, or two, or more? (I've searched TF and haven't found much.)

Thanks!

 

[DDW]

AGMs will keep pretty well as long as they are properly and fully charged every 6 months or so. If you use a maintainer type charger, you either need a temperature controlled storage facility or a temperature compensated maintainer, as the correct float voltage changes with temperature.

The Lifeline AGMs on my sailboat were purchased in 2006 and are still in use in 2018. They were stored (but periodically charged) from 2006 - 2008 when the boat was finally launched, then stored for an average of 8 continuous months each year since. A the beginning and end of each storage period they were charged with a 3 stage temp compensated charger.