Thread: Separate Alternator cable for D3

Originally Posted by Ean Austral

I had an 80amp fuse Graeme that lasted ok , , they never had any 80amp C/B in stock so I opted for 70 but I may have stuffed up.

Hi Ean, also note that circuit breakers can be very temperature sensitive.

The higher the temperature, the lower the constant current capacity can become.

Is your CB mounted close to the motor?

If using a name brand, you should be able to check the operating specs for your CB.

My suggestion for the day:

It's not the forward output current you're wanting to protect against but the reverse current. Therefore you could install as pair of diodes in the cable, one forward diode capable of carrying 100% of the output current and one reverse diode plus line fuse set to blow at around 20A or so of reverse current. That way the fuse will only see minimal current until a short occurs inside the alternator. The reverse diode is there to match the voltage drop across the forward diode so the fuse carries very little current under normal conditions.

Originally Posted by drivesafe

Hi Ean, also note that circuit breakers can be very temperature sensitive.

The higher the temperature, the lower the constant current capacity can become.

Is your CB mounted close to the motor?

If using a name brand, you should be able to check the operating specs for your CB.

Its mounted near the main start battery, between the battery box and the header tank. there is just a unused area there and its where I mounted the fuse.


I will have to see what the brand is and look it up, thanks.


Cheers Ean
Cheers Ean

Originally Posted by bee utey

My suggestion for the day:

It's not the forward output current you're wanting to protect against but the reverse current. Therefore you could install as pair of diodes in the cable, one forward diode capable of carrying 100% of the output current and one reverse diode plus line fuse set to blow at around 20A or so of reverse current. That way the fuse will only see minimal current until a short occurs inside the alternator. The reverse diode is there to match the voltage drop across the forward diode so the fuse carries very little current under normal conditions.

I will have to some reading, as I know very little of the diodes you speak of.


I am just a would be if I could be mechanic so these things are off my radar . I am still think a spate cable for the alternator is a better idea than the original design but that's just my simple brain at work.


Thanks and Cheers Ean

Well it seems 70amp is not enough. I put the 80 amp fuse back in and no issues , the 70 amp tripped faster the lower I let the battery voltage drop.




Cheers Ean

Originally Posted by Ean Austral

I will have to some reading, as I know very little of the diodes you speak of.


I am just a would be if I could be mechanic so these things are off my radar . I am still think a spate cable for the alternator is a better idea than the original design but that's just my simple brain at work.


Thanks and Cheers Ean

Well to put it simply, a diode is to electrics what a spring loaded check valve is to a hydraulic circuit. It has a forwards voltage (equiv to opening pressure) and a reverse breakdown voltage that it can hold without conducting in reverse, like a max operating pressure. Symbol:



Current flows in the direction of the arrow (anode to cathode) but not the other way.

Now a handy arrangement of diodes suitable for this task is called a bridge rectifier, used to convert AC current into DC current. Any decent electronics shop should be able to sell you one and a suitable heat sink to bolt it to. Symbol:



If you connect your alternator output to terminals 2 and 4 together to double up the diodes, a battery connected to terminal 1 will get current. Now here's the magic trick: ALSO connect terminal 3, via a 20A fuse, to the battery at terminal 1. Now as you can see power can flow from the battery to the alternator in strictly limited quantities as needed. The rectifier will look something like this:



Ebay has them for under $20 from China. Example:

1200V 100A 1 Phase Rectangle Aluminum Heatsink Bridge Rectifier Diodes

Thanks for that, our youngest daughter is an aircraft avionics tech, looks like a perfect job for her.

Cheers Ean

If you are looking for a means of disconnecting the alternator quickly, these work, but they are not cheap.

https://www.bluesea.com/products/770..._-_12V_DC_500A

A simple switch on the dash, or alternatively, link it to a low voltage alarm or Auber controller set to monitor battery voltage, if voltage is low, then RBS trips.

http://www.auberins.com/index.php'ma...products_id=19