Dual battery / Dual alternator wiring

Well just ordered a dual alternator bracket off Pressbrake. Now onto the wiring for the 2nd alternator. I've never actually fitted dual alternators and dual batteries to a car before but this is an explanation of my thought process including the current setup.

I'm hoping for feedback/suggestions


This is the standard setup for a defender:



Basically the ground goes to the body and onto the battery negative whilst the positive goes to positive and the 3rd connection (excite) goes through a diode to a bulb with it's other side connected to ignition live. The resistor is there to make sure that the alternator doesn't stop charging the battery just because the bulb has blown.



So onto my current setup which uses two batteries:



This is basically the same as the standard one BUT a second battery is installed with it's ground connected to the body as per the original battery and the positive connected via a 200amp relay and fuses to the 1st battery. The relay is switched by a wire coming from ignition live via a switch. So that with the switch off OR ignition off both batteries are disconnected and you can draw current from the 2nd battery without affecting the first. The two batteries are connected together when the switch is on and the ignition is on.



Now for my proposed dual battery dual alternator setup:



This is the same as the single battery setup BUT I connect the main live output to the 2nd battery. The switch for the 200 amp relay is normally off. There is a 2nd charge light but instead of connecting this to the ignition live it is connected to a new "2nd battery ignition live" this is because it is the sense wire and need to sense the correct batteries state.


or alternatively I could remove the connection switch?





So does this make sense?

Am I over complicating matters?

From a fault tolerance perspective I would leave the 'connection switch' in. Without it if your main alternator packs up you will be left just running off the battery, with the switch in you will still have battery charging via the second alt.

Kwakerman said:

From a fault tolerance perspective I would leave the 'connection switch' in. Without it if your main alternator packs up you will be left just running off the battery, with the switch in you will still have battery charging via the second alt.

Click to expand...

I will do, the 2nd relay i.e. "the aux ignition live" relay is already fitted because I use it to power the stereo, CB etc at the moment.

The only changes will be the addition of the alternator, diode, resistor and charge light. I've got some seriously decent welding wire and bought a battery cable crimper so if should be easy to sort out the alternator to battery cable plus the location of the 2nd alternator is already grounded to the chassis with welding cable so they are sorted.