Thread: Separate Alternator cable for D3

Tim,


What about a voltage indicator direct off the batteries , on my D2 I had a in-cab voltage reader that when the battery got to 11.5V it activated a beeper in the unit. I bought it for when we did the Canning Stock Route to keep an eye on the battery voltage in case we had an issue's with 2 fridge's running.


Is something like that the answer to this problem.


Cheers Ean

For a warning to be of use firstly you would need to be able to stop quickly without endangering yourself or others and secondly disconnect the alternator whilst its drawing 150-200A all within 1 or 2 minutes, although a heavy duty switch might do the trick. I still think the 50% fuse is worth having.

Hi Ean and sorry I missed your question.

A voltage warning is what I am looking at as a substitute for a Current Direction Indicator.

While Graeme is partly correct, even with a low voltage warning, you are still going to have to act quickly.

But if you set the Voltage Warning for say 12.2v or even 12.0v, you will still have plenty of time to stop safely.

Then you can check to see if the voltage at the battery is still dropping once the motor is turned off and then be able to disconnect the battery or batteries.

If needed, you can do a quick disconnect by removing the NEGATIVES ( - ) off all batteries and then you can take your time to disconnect the alternator itself.

Hi Graeme, test it and see what you come up with.

The point is, to allow for normal operation of the alternator, the fuse still has to be larger than what is required to protect the battery from the a failed alternator overload.

For instance, it you fitted a 100 amp Manual Reset type circuit breaker ( a fuse is not really feasible in this type of situation ), it would tolerate 155 amps for about 1 to 2 minutes.

Not much time to draw high currents off the battery!

Add to the problem is that all of this is presuming the battery was in a fully charged state when the alternator failed.

There is a VERY good possibility that alternators are not charging properly, prior to their failure, so your battery is probably going to be in anything but a fully charged state at the time of the alternator failure.

There would be no problem with using such a low current CB providing you didn't have any extras drawing of your normal electrics.

But add an auxiliary battery and a set of driving lights and on a D3 you are not likely to trip a 100 amp circuit breaker because their alternator is flat-out at 140 amps.

However, in a D4, with these sorts of loads, you are likely to trip the CB in about 5 to 10 minutes. So you have to go to a higher rated CB and this then defeats the protection.

As posted above, a voltage monitoring system incorporating a low voltage warning, is a far more realistic means of protection and if you wanted to give yourself a quicker disconnect time, fit either battery switch or a 200 amp Manual Reset type circuit breaker, and use it as a switch.

I'll replace my 200A fuse with a 100A manually resettable CB that incorporates a switch but then try 120A, 150A if the lower rated CBs trip in normal use. A friend had a few warnings of his D3 alternator's pending failure when some systems started to shut down but it still left him stranded in the middle of the road in busy traffic when it failed totally. I don't know how early the warnings occurred but even 30 seconds may allow a head-start on getting to the switch if one was fitted.

Even if the circuit breaker or manual switch doesn't save the battery then at least with a dedicated alternator cable the alternator can readily be disconnected and then switch over to the auxiliary battery or a fit temporary battery. This would allow the vehicle to be driven even if the battery has to be recharged via external means after quite short intervals. Disconnecting the original alternator cable is not a roadside task that I'd like to try, but would probably cut the cable from the starter cable at the starter motor if a suitable tool was on hand.

Originally Posted by Graeme

Disconnecting the original alternator cable is not a roadside task

Hence the suggestion of fitting a battery switch!

But for the alternator, not the starter cable.

I am currently running an 80amp fuse running a freezer and other stuff whilst out offroad and so far so good. I font have a winch so cant comment on how that will affect things. I have had the start battery at 11.8 volts and the other at 12 volts and started the car and charged up both batteries ok.

Will see how it continues to perform.

Cheers Ean