Advice needed on defective 6V FLA Battery

[nigel1]

Battery bank consists of 6 x Trojan T145's 6 Volt FLA batteries. All six batteries were purchased in August last year and installed at the end of that month.
About 5 weeks of cruising after installation, and then the boat has been in a marina since then.
The new bank replaced an 8 year old bank of Trojan T125's.


While in the marina, battery charging maintained by 300 Watts solar via a Blue Sky's MPPT charge controller, and then every two weeks when I am on the boat, hook up to shore power and use the Sterling Pro-Ultra 60A charger to make sure batteries are fully charged.


Last week, having charged the batteries via the Sterling charger, (battery monitors showing 13.8V), shore power disconnected. I then decided to make sure the fridge was working OK, but it failed to start. The fridge control module indicated low voltage, and on checking both battery monitors, voltage was 12.3~12.4 V.


Turning on the charger enabled the fridge to start, turning off the charger, voltage dropped to 12.3~12.4.


Not able to spend any more time on the boat, I disconnected all charging sources, and disconnected/isolated all 6 batteries.


I'm back on the boat this Sunday and will check voltage on all 6 batteries, I suspect that I will find one with a voltage of around 4 volts as I suspect one cell is dead.


Fortunatly, the battery dealer seems to agree that this is a warranty issue, but they want the battery returned to them for testing/inspection.


Water levels checked monthly, and topped up using a Trojan Hydrolink system.
SG's tested quarterly, last check 24th January with cell SG's ranging from 1.30 to 1.32.





Questions are
1) What could be the liklely cause


2) Is there a difference between what is described as a shorted cell and a dead cell


3) Is it likely that the dead/shorted cell will have a significantly lower SG than the other cells.


4) I know that accepted good practice is not to mix a new battery in with old batteries, but in this case, with the batteries only have been installed for less than 6 months, I'm hoping I can just change the one battery (assuming the remaining 5 are in good condition)