Odyssey PC680 Battery

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Any help/suggestions from the electrical gurus would be gratefully appreciated.

I have a Griffith with 2 Odyssey PC680 batteries wired in parallel, they are connected to an onboard Optimate battery charger.
The charger shows two green light strips which I take to mean that both batteries are charged. In between the green lights there is
an orange strip light that I think shows when lit that the batteries are being charged. The issue I have is sometimes the green lights are steady,
sometimes one green light strip is lit and the orange strip is lit and sometimes the unlit green strip turns red, so one green one red.
My suspicion is that one battery is breaking down, anyone able to suggest what is going on?