Thread: Alternator to Starter or Battery?

I gather it doesn't seem to matter all that much, but it's always worth checking here, (if only for the detailed technical diversions . . . you know who you are!)

I'm fiddling with alternators and batteries on my V8 and now have the main alternator next to the main battery, so it seems easier and simpler to just run a shorter cable to the battery instead of over the oily engine and down the back to the impossible-to-get-at starter motor, as per original. Is there some glaringly obvious reason why I shouldn't?

Originally Posted by Graeme

I'm soon to replace the alternator to starter cable on my D4 with one direct to the battery in order to fit an appropriate fuse/fusible link for the alternator. The fusible link for the starter is 500A which is far too high to be of any use to protect the battery from a short-circuiting alternator. The original alternator cable is only rated at 180A so inappropriate for use with the 500A fusible link anyway.

Hi Graeme and the 500 amp fusible link is fine for a dead short as the cable is 35mm2 so while it only has a CONTINUOS rating of around 190-200 amps, the same cable is rated at well over 500 amps as starter motor cable and as such, if a dead short occurred, then the link would blow.

As for an alternator short, it’s highly unlikely you will get a TOTAL dead short but more likely a high current load.

In my case, when the alternator failed and “shorted” there was a constant 155 amp load placed on the battery.

This constant load did not exceed the cable’s constant maximum current load rating of 200 amps so the cable was never at risk.

Again, in my case, I had to get an auxiliary power supply to allow me to get the ignition on to be able to get the steering lock off and the transmission moved out of PARK, to be able to winch the heap on to a flat top.

I used an Optima 55Ah Yellow Top, connected through a 60 amp auto resetting circuit breaker ( the most assessable point to get power into the RR’s electrics ) to power the electrics in the RR.

The RR’s cranking battery was at 7v and I disconnected the negative terminal off the cranking battery so the Optima wasn’t trying to power the RR’s electrics and charge a dead battery at the same time.

Even though I was powering everything through the 60 amp CB, including the 155 amp draw from the alternator “short”, the circuit breaker still managed to carry the load for over a minute before tripping.

Graeme, the point of all this is, I am not sure what sort of protection you can fit in the set up you are planning?