Battery Isolator Alternator Protection...

[450Nick]

Hi all,

Just getting around to wiring up the battery isolator and I have a question. I'm running an ETA 921 and it has both remote disconnect and aux circuit functions. I've asked ETA about the AUX function and it is essentially a switch that runs parallel to the main master switch. ETA say that this is intended in race cars to control a very large relay which sits between the alternator output and the rest of the car; in the event of someone throwing the kill switch, it should immediately disconnect the alternator from the rest of the car, whilst the main switch disconnects the battery.

First question; is this correct? Second question; is this necessary? Third question; if it is necessary, where on earth do you get a relay that big?

[Chassis 33]

IIRC Mr Holden uses a 921 so he might be the best man to advise, however...

If you use a traditional kill switch with the relay and resistor combination underneath the relay allows the resistor to dump the current produced by the alternator in between you cutting the supply to the battery and the alternator stopping spinning. The reason behind this is so you don't risk blowing the rectifier diodes in the alternator. I'd imagine the AUX output is to allow you to use a standard 5pin crossover relay to replicate this function.

Getting back to your other question, you're probably getting into solenoid territory if you are planning on switching the whole of the alternator output via a "relay", in which case I can recommend the Albright range of solenoids eg the SW80-358 is rated to 125A continuous however I'm told by a colleague who's fitted many to forklift trucks and the like that they only really start to struggle approaching 800A. My setup is based around one of them and a crossover latching relay. Let me know if you're at Rockingham and I'll bring it all over for a look-see

Regards
Iain

[450Nick]

Chassis 33 said:

IIRC Mr Holden uses a 921 so he might be the best man to advise, however...

If you use a traditional kill switch with the relay and resistor combination underneath the relay allows the resistor to dump the current produced by the alternator in between you cutting the supply to the battery and the alternator stopping spinning. The reason behind this is so you don't risk blowing the rectifier diodes in the alternator. I'd imagine the AUX output is to allow you to use a standard 5pin crossover relay to replicate this function.

Getting back to your other question, you're probably getting into solenoid territory if you are planning on switching the whole of the alternator output via a "relay", in which case I can recommend the Albright range of solenoids eg the SW80-358 is rated to 125A continuous however I'm told by a colleague who's fitted many to forklift trucks and the like that they only really start to struggle approaching 800A. My setup is based around one of them and a crossover latching relay. Let me know if you're at Rockingham and I'll bring it all over for a look-see

Regards
Iain

Thanks for that Iain, I'm not really very experienced with alternators so I'd appreciated it if you could bring your setup to Rock and talk me through how you're wiring it up and why. I don't have the alt in the car yet but I've got just about all of the wiring loom in and tidied up now so I'd like to get this bit thought out and installed so I can bolt down the dash once and for all.

I understand that you want to allow the alternator to discharge somewhere while it spins down, but I'm not clear on where its being sent and how at the moment.

[Chassis 33]

No worries Nick, if you bring your breaker too I'll attempt to help you out with that too!!

Ultimately when you switch off you're discharging the alternator to earth, but if you just shorted it out with a length of wire you'd get a massive current spike. The resistor essentially throttles back the flow of electrickery so it doesn't cause things to get hot, melt and upset people.
Regards
Iain

[450Nick]

Unfortunately the breaker is now firmly bolted into the car beneath a cunning bracket supporting the datalogger so it would be a bit of a mission to remove. It's pretty simple though, here is a picture:



Essentially, all it is is a switch between the two big terminals, with the two little spades you see being a disconnect loop (momentary switch to disconnect), and then mine has two more spades next to those, which are connected/disconnected along with the main breaker for use with the aux circuit.

I'm guessing then from what you've said that you want a contactor like you've shown in line with the alternator output, controlled by this aux connection on the breaker, with the NC line going to ground via a resistor of some size, and the the NO channel supplying the car. Does that sound right?

[Chassis 33]

Pretty much, the "alternator side" doesn't have to be anything special, a standard 40amp crossover relay and a "high" power resistor, the one supplied with most of the twisty FIA breakers is an 11w 3.3ohm resistor, 'Tweeks charge about £3 for it, RS charge about 75p, RS# 206-1287

Regards
Iain

[450Nick]

Chassis 33 said:

Pretty much, the "alternator side" doesn't have to be anything special, a standard 40amp crossover relay and a "high" power resistor, the one supplied with most of the twisty FIA breakers is an 11w 3.3ohm resistor, 'Tweeks charge about £3 for it, RS charge about 75p, RS# 206-1287

Regards
Iain

Why only a 40amp relay? Surely if its running in line with the output of the alternator then it's likely to see more than 40amps... Or have I missed the point? I have one of the resistors - got one of the manual kill switches with it and then changed my mind and bought the ETA one!

[Chassis 33]

Nick, it doesn't run inline in normal operation so never sees the full charging current. I've sketched a quick wiring diagram for you, it will be in your snotmail inbox soon.
Regards
Iain