Batteries fail in 3 ways
High Resistance (Will have no effect on the converter)
Low capacity (likewise)
Shorted cell (Problem may appear).
Monitor charging current ... Normally if the batteries have been run down a bit you will have fairly high charge current when you first plug in.
But as the batteries fill the charge current drops. Less than 1 amp when full.
A Clamp on Multi-Meter with DC capability (not all can do DC) will indicate the amount of current flowing.. SOME may even do direction but mine does not.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times