Bad onboard charger... or what?
Here is my situation:
I have a 3 bank onboard battery charger. Batteries only a couple of years old.
When I plug in the charger, I read that my cranking battery is "green" and my trolling batteries need some juice... so I leave it plugged in. In morning, all indicator lights on charger are "green", or indicating fully charged. I go to start the engine, and nothing... no horn, cannot raise/lower engine, just dead. I put a 12 volt test light on battery and it says dead...but trolling motors all light up the test light, so I figure I have a dead battery and replace it.
New battery lights up test light, and when I put it in the boat, boat starts great and the charger shows that it is "green" (fully charged)
Question 1: If cranking battery was dead, why was charger indicating full charge?
Then, for giggles, I take old battery and put a stand alone battery charger on it...and it takes the charge, and when I put a test light on this old re-charged battery, it lights up test light. Now I am thinking that maybe I bought a new battery when really, I have a charger problem, and not a battery problem.
Question 2: How can I test the charger to see if in fact it is putting out a charge on my cranking battery? I am not an electrical engineer...so complicated answers with lots of testing equipment is not an option.
I have to figure out if I need to buy a new charger or not...