[External regulator Alpha Pro III wiring for Volvo Penta D2 40: how to avoid low battery alarm?]

@CarpeDiem thanks for your thoughtful and detailed answer, I realise reading you that this part of my electrical refit is more complex than anticipated and that I haven't done enough research.

It seems like there is no way around a "hack" to install the Mastervolt external regulator, which feels a bit silly.

I did not know about the Nordkyn product which seems to be purposely designed around the constraints we face on a boat set up, wish I knew about it before.

I guess I have modified my alternator already, so backing off now would be a rather costly exercise involving buying a new alternator and a new regulator, but maybe I should consider it?

[External regulator Alpha Pro III wiring for Volvo Penta D2 40: how to avoid low battery alarm?]

Hi everyone,

I am currently installing a Mastervolt Alpha Pro III external regulator to the stock 115A Mitsubishi alternator on my Volvo Penta D2 40. The alternator charges my starter battery (Endurant 680CCA).

(For context: the rest of my set up includes 2 x 120Ah Relion lithium batteries that are charged from the starter battery via 2 x 30A Victron DC to DC charger. I decided against sending the alternator charge directly to the lithium batteries as the BMS does not have a way to warn the regulator before shutting down, and I don't wanna run the risk of losing my alternator)

I believe I understand the basic wiring to make this work (see attached diagram, courtesy of James in England):

• I got my alternator modified by a professional auto electrician and I have the two field wires pocking out of the casing
• I must add a relay powered from the MDI (when the ignition comes on, wire nr7) between the battery and the RegOn terminal on the regulator (I understand this is where the voltage sense happens too)
• I also purchased the alternator temperature sensor so that the regulator can monitor the alternator temp
• I will be very conservative in the setting of the regulator to avoid frying the alternator (probably cap the output at 65% of the alternator capacity and lower the output based on alternator temperature thresholds)

So I feel like I am ready to get started, but I have a question based on my exchange with a fellow boater James who has a very similar set up.

Since I am losing the D+ connection on the alternator, I believe the Volvo Penta MDI will be unhappy and trigger low voltage alarms (again relying on James' experience here with a similar set up). Doers anyone know of a way to deal with the alarm? Whether to shut it down completely, or to provide the MDI what it needs?

And also is there anything wrong you can see in that wiring diagram? Would you do anything differently?

I have heard of a solution with an extra oil pressure switch and two relays, but I am getting out of my comfort zone here, so any help would be really appreciated  

note: small differences with the diagram attached

• my alternator modification got rid of the connection between the positive brush and the D terminal, so I would connect the Batt and Field terminals of the regulator directly to the two field wires pocking out
• I do not have a battery isolator in my setup, so I will directly go to the starter battery