VZ800 Charging issue

Morning Folks
I purchased a 1999 VZ800 as a project bike with issues which I'm working through. One of the issues is with the charging system on this bike. With the bike running I'm only reading 12V across the battery terminals and this does not increase when I rev the bike.
This looked like a typical regulator rectifier failure so new reg was purchased and fitted but the output voltage remained the same. I sent this back to the vendor as a faulty item and they replaced it but eve after this was fitted the same issue remained.
What seems to be happening is that when I start the bike the regulator output is around 13.5V but this quickly reduces to 12-12.5V. All three phases from the stator seem to be emitting a good voltage and all the wiring has tested good but I'm stumped. Am I missing something? Thanks Steve.

Have you tried using another battery as even if it seems ok it can be breaking down ok.

Well after much investigation and unnecessarily replacing the regulator for a genuine Suzuki part at a cost of £210 I managed to get to the bottom of the charging issue on my Marauder. On the positive circuit between the regulator and the battery it passes through a 30 amp fuse which is integrated into the starter solenoid. The connector which plugs into the starter solenoid which also contains this positive charging circuit was burnt out. So I had to cut these wires out of the connector and replace the 30 amp fuse with an inline one. Job done.