Could be a LOT of things i am afraid and not easy to tell from distance.
Bad connections, corrosion
. To check connections touch them (under load) if they get hot it means (too much) resistance.
An odd phenomenon that I am into myself is our batteries are apparently approaching the end of service
life.... And me, the clever trouble shooter, suspected faulty charge controller, solar panels
Fact is, when the batteries can't accept a charge any longer there will be (almost)
. I read 2-3 Amos solar instead of 7-9. Yesterday I noticed that the windgenerator was spinning in a 10 knot
breeze, normally about 5 Amp into the battery, but read 1 on the analogue Ampmeter....that's when it finally dawned on me that the batteries are shot!
Slow processes can go unnoticed a long time without reaction from operator
Just dig out the DMM and start at the source and check everything. Guess we better do it every couple of years, but then diving
and socializing is more fun!
P.S. Could still crank the engine, though starter spun slower. D.S.