Domestic- starter charging query

[VicS]

The battery combining relay might be something like this:
Volvo Relay 838518 12volt 75 amp relay

Thanks

75 amp is a bit on the light side with 2 x 90Ah domestic batteries and a Red Flash

Furneaux Riddall offer a 100 amp one
Heavy Duty Relays - Electrical Equipment for Boat, MotorHome & Caravan Owners

12 volt planet offer a 200 amp one and a 300 amp one
High Performance Heavy Duty Relays | 12 Volt Planet

 

[BabaYaga]

75 amp is a bit on the light side with 2 x 90Ah domestic batteries and a Red Flash

Furneaux Riddall offer a 100 amp one
Heavy Duty Relays - Electrical Equipment for Boat, MotorHome & Caravan Owners

12 volt planet offer a 200 amp one and a 300 amp one
High Performance Heavy Duty Relays | 12 Volt Planet

Maybe so, depending on how it is wired.
In my set up it is only the charging current for the 38 Ah Red Flash that ever passes through the relay.

 

[VicS]

Maybe so, depending on how it is wired.
In my set up it is only the charging current for the 38 Ah Red Flash that ever passes through the relay.

But what is the maximum current that could flow if something unexpected happened.?
Perhaps the relay should be protected by a fuse in one of its main connections in the same way that a VSR would be. 75 amps in your case.

 

[BabaYaga]

But what is the maximum current that could flow if something unexpected happened.?
Perhaps the relay should be protected by a fuse in one of its main connections in the same way that a VSR would be. 75 amps in your case.

I agree about the need for an additional fuse (my diagram is a bit simplified for clarity). In reality there is a 80 amp fuse in the charging wire.
In the diagram I would place this fuse by the positive busbar. It could also protect the wiring to the relay and to the fuse box top left.
Maximum charging current through the relay is difficult to predict, maybe something like 100 percent of the battery's Ah rating, around 40 A for a short while, if the Red Flash was totally depleted?

To add: I have a shunt in the engine battery charging wire, after a normal engine start I see a charge current of about 4 amps for a short while. After a minute or so 2 amps.