Alternator Bat Wire - What size fuse?

[gearb0x]

Hey Guys,

I'm redoing the power system in my AE71 to have a dash mounted isolator switch & rear mounted battery. I want the isolator switch to kill the motor as well so I'm bringing the Alternator BAT wire back to the isolator switch so it doesn't keep the motor running once I've cut the power.

Just got a few questions on sizing & fusing that wire. I am thinking of using 4AWG wire from the alt to the isolator switch, I know its plenty but I was just wondering if its over kill (it'll be a decent run, over the top of the motor, into the mudguard at the fuse box (front right of the engine bay) then under the guard, into the cabin to the middle of the dash)

The other question is fusing, its not fused from the factory but since I'm bringing it into the cabin I want to add a fuse. I was just wondering what current draw i would expect on this wire so I can put in a fuse for a little extra protection.

The motor in question is a 4AGE 20v w/ factory alternator. Its a track car so there's no stereo, but eventually it may have some spotlights for night hill climbs, I probably wont worry about that though unless the headlights are completely useless

Any ideas are welcome,

Thanks,
Phill

[RA35GT]

Dood, just get a fuse that's rated higher than the alternator output. should be fine.

No such thing as too big, as long as its practical. (---Edit--- This statement relates to wire diameter, not fuse rating---Edit---)

PS. most alternators wired through a fusible link from factory.

[gearb0x]

does anyone know the size of the 20v alternator? I cant find it on the internets

[TERRA Operative]

It'll be around 80 amps or so. If you use a 100A fuse, you should be fine.

[CrUZida]

If you wire the kill switch to the negative side of the battery you don't need to worry about this.
Plus, it's safer.

[TERRA Operative]

Except that the whole system is still live, so if you need to cut the power to prevent a short or in a crash etc, you haven't actually cut the power..........

[CrUZida]

Umm, might wanna rethink that.

EDIT: Or do you mean it will be running off the alternator, and ignoring the battery?
If so, could well be, its not something I've tested (disconnecting a battery on a running car)

[gearb0x]

Ya even cutting ground to the battery the motor will keep running, the chassis will be still ground as far as power coming from the alternator is concerned.

I ended up getting a nice fuse holder & a 100A fuse today, thanks for the help guys

[CrUZida]

Yeah, wasn't sure if the alternator would lose its reference and shutdown or not.

[TERRA Operative]

If you only cut the ground, the battery will still be supplying voltage to the rest of the car, ie. there will be 12v everywhere up to the isolator. Nothing will work of course, but if there is a problem somewhere and something is shorting out, cutting the negative will do squat. You want to cut the positive before anything else in the circuit, so when the switch is turned off, the power stops at the switch.

[CrUZida]

If the engine isn't running cutting the negative will stop all shorts, regardless of where or what they are.

[timbosaurus]

If you only cut the ground, the battery will still be supplying voltage to the rest of the car, ie. there will be 12v everywhere up to the isolator.

Yeah, but 12V is not 12V without a ground/negative. If you disconnect the ground cable from the battery, then the chassis is no longer ground. So you could even touch the positive terminal of the battery direct to the chassis and you'd see no sparks.

This does not solve the engine running on due to the alternator powering the electronics though. And I think CAMS now require the engine to stop when you isolate the battery.

The best way to do this is with a double pole isolation switch. The entire battery feed (either ground or 12V) goes through one pole, anything else that will stop the car (fuel pump, ECU power, Alternator field, etc) goes through the other.

You could also wire the alternator charge wire direct to the +ve battery post through a fuse, and run the rest of the vehicle electrical system through the isolation switch. Would work, but means the wire from the battery to the alternator is technically not isolated... just fused. It's usually short though.

Cheers,
Timbo

[gearb0x]

I have a 100A circuit breaker at the battery box in the boot for the main power feed. Ive also got a 100A fuse on the alternator wire mounted next to the factory fuse box in the engine bay. The unfused portion of the wire from the alt to the fuse is no longer than the factory setup so I cant see it being a problem

I should have everything crimped etc over the weekend so I will post pics.

These are the switches tims talking about http://www.revolutionracegear.com.au...V=Primary&CDO=

I red somewhere that alternators don't like being disconnected from the battery in this way though, something like a huge spike in power once its disconnected or something, wiring the alt wire to the battery stops it. Still not 100% what it was but i read it on an internet and it was good enough for me (bmsc forums I think)

[gearb0x]

As for the Un isolated alt wire, thats true. Ive also got an unisolated run of cable from the bat in the boot to the isolator switch in the dash. Thats where the circuit breaker comes in if there is a short (hopefully anyway)

Luckily In my series there is no actual requirement for the isolator.

[timbosaurus]

These are the switches tims talking about http://www.revolutionracegear.com.au...V=Primary&CDO=

yup, those are the puppies!

I red somewhere that alternators don't like being disconnected from the battery in this way though, something like a huge spike in power once its disconnected or something,

True, the sudden voltage spike from going full-load to no-load can damage the diodes. Not all the time, but I wouldn't do it daily for fun My assumption is that the worst case is sinking max current into a flat battery, and you pull the terminal

wiring the alt wire to the battery stops it. Still not 100% what it was but i read it on an internet and it was good enough for me (bmsc forums I think)

Yeah, I think you're talking about the two different options above.

One is to have the batt cable go to the isolation switch, then from the switch to both the alternator output and the rest of the vehicle electronics. If you isolate the flat battery, alternator load might drop instantly, giving a potentially damaging voltage spike and/or continuing to run the engine.

The second is to connect the alternator charge wire direct to the battery (like you said), the run the battery lead to the isolation switch, then to the rest of the vehicle. When you isolate the switch, the vehicle electronics (including alternator field) looses power, and it's just like switching the key off.