Buy Nigel Calder's book 'Boatowners Mechanical and Electrical
Manual'--you may not always have an internet
connection when you have a problem, and he does a good job of the procedure for testing the charging system, as well as most other systems on the boat.
The short answer is "what is the voltage across the battery
bank(s) when the engine
is/is not running? If the voltage does not rise after you start the engine
, you have a problem with the 24v charging system--it should be 26-28v with the engine going.