AGM " Shelf Life "

[callistov42]

I have had a group 27 AGM battery onboard for about two years that has never been in service. It has been charged two or three times over that period of time. I now need to put it into service as an isolated starting battery. Can I expect a normal 4 to 5 year life span from this point forward or has some of the battery's life passed.

[Rick]

High quality AGM batteries have a very low self-discharge and can stand for years (like 5) at relatively low temperatures (below 65 deg F). Test your battery internal resistance at a heavy load (about 50-75A noting that the terminal voltage drops then settles out to a constant value for several seconds before dropping again if the test runs too long). Remove the load and observe the voltage rise. The resistance is the change in voltage (from load to no-load) divided by the load current.

For a group 27 the internal resistance should be about 12 milli-Ohms. If the battery charge accepts a good current after the test (apply more thasn 14.4 V and note that the acceptance current is at least 20 A) then the battery is good and you should get 5 more years as long as you keep it on a good float voltage (13.9V) when using shore power.

[deckofficer]

I couldn't have answered any better than Rick. I am a fan of AGM and came to this thread to offer advise but with Rick's reply, none needed from me.