Safely wire a rear-mount battery with kill switch

[Jehannum]

I balked some years ago at the cost of replacing the stamped metal tray for my battery and instead put a container in for moving it to the hatch.  In doing so, I became subject to a number of rules regarding how the car must be prepared to run on the track, the most onerous of which is a kill switch on "the rear-most part" of the car.

For reference, here's the ever-popular internally regulated 60/70A Hitachi from a 280ZX:

I've had a Moroso 74102 for some time acting as a kill switch (mounted to the rear panel, through the license plate), installed per the directions but I'm getting ready to run a 100A alternator (to support some other modifications), and realized that I'd been dangerously wrong on the install for some time.  See, the 74102 is limited to 20A charging capacity, which runs afoul of even the most modest of 240Z alternators' peak output (I say peak, because certainly they aren't running at their rated output, when the battery is charged).  I noticed this when I melted my first 74102 after getting a jump on a dead battery (the 10 gauge wire running back certainly got hot as well).

So, after rethinking my options, I decided to go with the Longacre instructions (below) and use the switch to kill the field wire.  This works fine, if you've got a race car, and you use the kill switch to shut the car down.  Otherwise, it'll run the battery dead overnight.

After rethinking my options again, I happened on a simple diagram that helped me out.

Basically, the field wire should only receive ignition switched power.  The problem, then, is figuring out how to get ignition switched power that won't also get fed by the alternator.  So I kept the leg of the Moroso 74102 that their instructions intended for charging the battery, but used it as the source for a relay that is triggered by the ignition switch, like so (though adding the 1N5402 relay to keep the car from running on after the key is turned off):

Now, you can run a (fused) wire from the alternator to the starter post at whatever you think is a safe gauge, which hopefully already has a big 2 gauge cable running to the back (either with its own breaker, or relying on the switch to melt-safe).  I went with 4 gauge, and an inline 100A fuse (though not visible):

I'll be switching to a GM 3-wire alternator soon, but this wiring should support the new one just fine.

Edited March 28, 2017 by Jehannum

[Zed Head]

The topic isn't really location dependent, is it?  Wires is wires.  Amps is amps.

And after that it's really "don't run over 20 amps through a device that can only handle 20 amps".  And 1N5402 is a diode, not a relay.  Not to wizz on your post, new views are appreciated.  The big gauge wires are generally used for the starter draw.  As noted in a recent post., the starter motor draws a lot of current.  Beyond that, a dead battery would probably be the biggest draw.

Jumping dead batteries causes many problems.  My radio in my other car reminds me often.

[Jehannum]

8 hours ago, Zed Head said:

The topic isn't really location dependent, is it?  Wires is wires.  Amps is amps.

And after that it's really "don't run over 20 amps through a device that can only handle 20 amps".  And 1N5402 is a diode, not a relay.  Not to wizz on your post, new views are appreciated.  The big gauge wires are generally used for the starter draw.  As noted in a recent post., the starter motor draws a lot of current.  Beyond that, a dead battery would probably be the biggest draw.

Jumping dead batteries causes many problems.  My radio in my other car reminds me often.

Did you miss the SPST relay in front of the 1N5402?  Because it's like 4x as big in the diagram.

And I think it is relevant to location, because there aren't any kill switches with alternator included that will handle more than 20A.  I looked.

Edited March 29, 2017 by Jehannum

[SteveJ]

Something is not right here. If you have the jump a properly wired car with the jumper cables in the correct location (in the back at the battery), you would not have much current going over the 20A contacts of the Moroso switch. It should be going to the solenoid. As I understand it, the kill switch is designed to isolate the battery from the rest of the electrical system. As a matter of fact, that is how the Moroso switch is described on Jegs.com.

As for having a 100A alternator, you still should not have much current going back to the battery. If you are pulling 20 A on that connection, you have a dying or dead battery. 

If you are trying to use the battery kill switch as an overall kill switch, you could wire up something like the second picture. It would isolate the battery and kill the ignition. With the ignition cut, the engine stops, and the alternator stops with it.

 

[Jehannum]

The "not right with this" was the fact that the battery was completely drained, so the alternator was running at capacity for a period of time sufficient to overload the switch.

[SteveJ]

10 minutes ago, Jehannum said:

The "not right with this" was the fact that the battery was completely drained, so the alternator was running at capacity for a period of time sufficient to overload the switch.

So the problem was operator error.   I missed that in the first read.

[Jehannum]

5 minutes ago, SteveJ said:

So the problem was operator error.   I missed that in the first read.

The guy who jumped me couldn't stick around, so I didn't really have that many options.

I did learn something, though: that the kill switch can't be counted on to fail safe when you overload it

Edited March 29, 2017 by Jehannum

[SteveJ]

Yeah, if you put too much heat through it, you could weld the contacts.

It is an interesting issue, though. The battery kill switch is designed for race applications where you wouldn't be expecting to start out with a nearly dead battery. You might want to opt for a tow the next time.

[Jehannum]

3 hours ago, SteveJ said:

Yeah, if you put too much heat through it, you could weld the contacts.

It is an interesting issue, though. The battery kill switch is designed for race applications where you wouldn't be expecting to start out with a nearly dead battery. You might want to opt for a tow the next time.

I don't have to worry about it anymore.  My setup will work just fine with a jump now.

[gnosez]

I had a dead battery on the race car and just plugged in the jump battery when I needed to start it. Not sure why the kill switch would inhibit the ability to jump start an engine.

[SteveJ]

The kill switch doesn't affect jumping the battery. The problem is that a dead battery is a heavy load on the alternator. In the case of the OP, the current draw was greater than the 20A rating of the alternator circuit of the switch. You might be able to double up by placing the alternator in the solenoid terminal if the stud is long enough.