"I've just read some things from the computer based training 737 NG and they don't state an automatic discharge function."
Generally, just be aware that with automatic discharge on some aircraft types, it may be airplane specific. In our 747-400 fleet there is one aircraft which does not have this function, the rest do. I would advise asking the advice of your engineering technical specialists if you have any doubts.
Also be aware that a particular auto-extinguishing system may be operative on the ground, but not in the air.
Regards.
Q.