failing battery in a bank of four

[mahendra]

After doing my occasional Checks on my system.I noticed a weird phenomenon,One of my battery shows a lower voltage that the rest

Three out of the four 8D Deka solar battery(245ah ) reads 12.7 at rest while the other reads 12.56 at rest. While charging all other batteries shows a higher voltage reading than the lower voltage in the bank.
I have accepted it is a failing battery but i have never experienced a situation as such.
There was no sign of  failure nor is there no over heating of the failing battery.
Can someone elaborate on this.

[Westbranch]

What are the specific gravity readings on each cell?

[mahendra]

these are agm not flooded

[mahendra]

i should add they were lightly used occasionally in the night mostly during  the daylight,occasional heavy load. and they are just about below three years.
So that urged me to investigate .

[Westbranch]

does the manufacturer recommend an EQ or 'refreshing charge' value?

[mahendra]

they do have eq charge voltage  but nothing on refresh charge

[Resthome]

It appears you are running these 4 batteries in series for 48V. It only take one bad cell out of the 24 cell to render a battery low. Just out of curiosity where in the string was the low battery located? Are you measuring the voltage after resting with no charge or loads?

[mahendra]

weird it was on the negative side of the string.
I measured both resting and on load .
My minimum measure  on the bad battery is 12.54 while on the others 12.69-while resting
on load it will go to 12.26 on the bad one while the others is 12.56 -moderate load(no more than 300w)
heavy it will sag to 12 on the bad one while the other is 12.2 -12.3(800w )