On our test vessel we have six rather old and weak Marathon M12V90FT AGM batteries from the same batch. They have a 12 year design life and are about 11 years old. 

Looking at a day in the voltage and current graphs  (August 2017) I noticed the nightly quiescent current of about -600mA drop down to -1.3A.

Note: A negative current means the battery is being discharged.

Just what had happened? At the time there were no consumers that would kick in sporadically to cause this. (Please look at the gallery further down for higher resolution pictures)

Comparing this graph with the other batteries turned out that the additional current was sucked out from the windlass battery into the bilge1 battery! Hence that battery must have had an internal short which then gradually disappeared over the next 30 minutes.

This rather unusual behaviour can now be used to make a decision which battery to replace first.

As always, please leave your comments below.


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