The question is more about how big (amp rating) a circuit breaker I would need in the line (I'm using #2 cable). I don't want the current from just starting the car to trip the breaker but I also don't want a breaker too much larger than I really need since it would not provide any real protection in that case.
I'm planning to put the battery under the pass side headlamp (just to be different!) but since it's pretty inaccessible under there I want some sort of protection in place just to be safe (these things catch fire you know !)
I'm thinking of using a stinger 100 or 150 amp breaker.
Edit.
After searching the web it looks like the starter can draw upwards of 250 amps under load so I don't think putting a CB in the line at the battery end is gonna work. Looks like my best bet is to put a battery cutoff switch up front by the battery and maybe a regular circuit breaker up back for the rest of the cars electrics.
I have seen numbers as high as 240 amps. The current draw will depend on the starter motor, and the friction on the engine it has to turn over. Also, the peak current was at initial start. Why do you want to add a circuit breaker/fuse anyway?
My battery is up front and I used 0AWG cable from battery to starter and I have not needed a fuse. Also, If the fuse blows, it will act the same as removing the front mount battery from the car.
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10:50 AM
ltlfrari Member
Posts: 5356 From: Wake Forest,NC,USA Registered: Jan 2002
Just trying to be super safe but I think it's not going to work with the starter in the circuit and I don't want to run two cables to the back (one for starter, one for rest of car) so I may just put an isolation switch up front somewhere so I can disconenct the batters without removing everything (since it will be inaccessible under the headlight),
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12:26 PM
Formula88 Member
Posts: 53788 From: Raleigh NC Registered: Jan 2001
Just trying to be super safe but I think it's not going to work with the starter in the circuit and I don't want to run two cables to the back (one for starter, one for rest of car) so I may just put an isolation switch up front somewhere so I can disconenct the batters without removing everything (since it will be inaccessible under the headlight),
My first thought would be to put a relay at the battery for the positive wire to the starter, but that would necessitate running another wire for the rest of the system.
Either way, it sounds like the isolation switch is a good idea.
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01:11 PM
darkhorizon Member
Posts: 12279 From: Flint Michigan Registered: Jan 2006
In real world terms, I would estimate about 80-100 amps sustained for the rotation, you would most likely see a 200 amp spike when the solenoid first kicks in.
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01:52 PM
Whuffo Member
Posts: 3000 From: San Jose, CA Registered: Jul 2003
The starter current draw depends heavily on how hard it is to crank the engine; a cold engine with heavy oil will cause a higher draw than the same engine warm or with lighter oil in it. The cranking current probably won't exceed 250 amps in most cases, though.
But be aware that electric motors (like the starter) produce maximum torque and their maximum current draw at 0 RPM. The initial current surge as the starter gets the parts turning can exceed 500 amps - or more. This "locked rotor" draw is limited in most cars by the battery cables; their size is chosen to limit the amount of current that can be drawn. This initial surge only lasts half a second or so - the cables don't have time to get hot.
If you want to protect that feed to the starter the most workable plan is to armor the cable. Fuses aren't very useful here; if the fuse is large enough to not blow during the initial surge, it's big enough to carry enough current to burn up the wiring. A circuit breaker would be more appropriate, but at these current levels they're not much good either. Every electrical device has some internal resistance; if that breaker is super-high-quality and only has 0.01 ohm of internal resistance - when that 500 amp surge comes along 5 of the 12 volts will be lost heating up the breaker contacts. The 7 volts that makes it to the starter won't be enough to start the engine.
If you want to see what switches and breakers that can safely handle this kind of current look like, check your local electic power substation.
For your car, run your battery cables up along the side of the gas tank; they shouldn't be where they can be damaged even if you run over a rock or whatever. Running them through a length of garden hose will provide protection against rocks / gravel that get kicked up along the road. And if you run the cables through garden hose or some other pipe / covering, increase the wire size one grade.
If it was me, I'd be running 2 gauge stranded cable through a protective sleeve (probably garden hose)/
Go to your nearest Autozone or Advance, they should be able to check your starting amps for free, with a starter test using the digital battery tester they have. The amp draw has a lot to do with the application, The only time I have seen amp loads in excess of 100 amps when cranking an average size engine was during a test on a bad starter where amps went up to about 140 during starting and you could smell the starter burning. For most average size cars under normal conditions I would be surprised if cranking amperage levels reached higher than 100 amps. On V8 engines there would likely be higher draws of course.
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02:04 PM
ltlfrari Member
Posts: 5356 From: Wake Forest,NC,USA Registered: Jan 2002
In the past I did run the wires up past the tank, protecting the +ve with some shielding and it was fine. This time round I'm planning on running the wires through the passenger side rocker (grommets in the holes of course) and around the fender liners so there will be very little if any wiring actually 'outside' the car.
Now whilst that protects the cable it is also in a place where any damage could (probably would) go unnoticed. I will also probably still protect the wire with additional covering such as a hose as suggested, but all these things aside, I'd still like to avoid the possibility of a short in that +ve having any potential to cause a problem.
However it looks like the current draw will be so great as to make it just not possible to use an automated circuit breaker, and running two wires, one for the starter and one for the rest of the car just defeats the purpose.
So it looks like the solution will be a manual isolation switch up by the battery and PLENTY of protection for the +ve wire. Just gotta take the car apart and do it now !
