Battery Replacement - AGM

Jul 8, 2005
Jeanneau 389 Grosse Pointe Farms, MI
We put in "Group 27" AGM batteries in place of our "Group 24" batteries about 8 years ago. The AGM batteries show no signs of weakness. Removed them from the terminals over the winter and kept them on the boat thru the Michigan winter. The batteries were fully charged this spring without any assistance.

I am worried that the batteries will deteriorate quickly and likely when I am on the lake.
What is the best way to test the batteries?
Feb 6, 1998
Canadian Sailcraft 36T Casco Bay, ME
Really the only true test of state of health, for cycling batteries, is to test them by doing a 20 hour capacity test.

How to conduct a 20 hour capacity test:

#1 Fully charge battery then allow it to rest, disconnected, for 24 hours

#2 Make certain battery temperature is between 75F & 80F

#3 Create a DC load = to Ah Capacity ÷ 20 (small light bulbs and resistors can work)

#4 Start DC load and a stop watch at the same time

#5 As the voltage drops during discharge adjust the DC load to maintain as close to C/20 rate as is humanly possible.

#6 Stop the discharge test when battery terminal voltage hits 10.499V

#7 Note the hours and minutes of run time and figure your percentage of 20 hours that it ran. This is your batteries Ah capacity or state of health as a percentage of as new rated capacity. For example if a 100Ah battery ran for 16 hours it is at 80% of its original rated capacity. Lead acid batteries are generally considered “end of life” when they can no longer deliver 80% or more of their rating but can sometimes continue to "work" down into the 70% range before failures start to occur..

#8 Recharge the battery immediately.. Follow this up with equalization level voltage and measure specific gravity until all cells match. (not for non-Lifeline AGM's or GEL's). A long slow recharge can have a slight reforming effect on flooded batteries and can actually recover some lost capacity.

Is 10.5V bad for my batteries?

A once yearly discharge test, done correctly, is actually less damaging than taking a battery to 50% SOC and leaving it in that range for a day or two, or the PSOC cycling (partial state of charge) the average boater thinks nothing of. Regular PSOC cycling is more damaging than a once or twice yearly Ah capacity test. A capacity test, done correctly, simply counts as another deep cycle.

WARNING: The only time your batteries should regularly be taken below 12.1V is during a capacity test. For regular house use, at your average house loads, your deepest loaded voltage should ideally not dip below 12.1V or better yet 12.15V - 12.2V. Unless you are running short duration high load device such as an inverter, windlass, electric winches, thruster, water maker etc. don't let your bank voltage dip below 12.1V....
Jul 8, 2005
Jeanneau 389 Grosse Pointe Farms, MI
Thanks for the quick reply! I will check out this process for sure.