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Correct physical location for battery+ fuse when relocating to trunk

Practical Motorsport Wiring - Club Level

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I have the battery moved to the trunk on my Sunday driver. I would like to add an inline fuse in the trunk area before the positive cable makes the long run to the front of the car and the 100 amp main fuse. In my application, I'm considering an enclosed ANL fuse holder mounted to the chassis, exactly inline on where the positive cable is (see yellow rectangle, courtesy of MS Paint). It'd be very close to the negative chassis grounding point. Is there any issue with that? I suppose I should probably move the fire extinguisher then, but... it's not like that get's a lot of use anyways, right? ;)

I do only have the extinguisher in the car due to the fuel system being modified, it's not there to meet any track requirements.



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From the advice I have seen is don't fuse the cable to the starter.

My perspective is as close to the battery as is possible, keeping in mind the sudden inrush of the starter on top of the balance of the system load. ECU, fans, headlights, ac etc that may all require power on startup.

Normal practice is to place the fuse as close to the power source as practical, as the fuse will only protect against short circuits or overloading from that point.

You cannot use a fuse on the main cable to the starter because, depending on the specific engine/starter the current can be in the hundreds, and the fuse that had the current rating for that draw would give negligible protection under normal operation. If you have a concern about the main cable being a potential short risk, an option is to connect the starter solinoid terminal to the main power terminal so any power there will operate the starter, and use a solinoid in the boot to energise the cable.

100A may be a little on the light side, depending on the vehicle and equipment used, and may also limit the protection. You may wish to consider using two (or more) fused circuits, such as one for engine related power, and one (or more) for everything else.

I would suggest fitting some form of battery cover as, while rather unlikely, it's possible for crap stuff in the boot to move around and short out the terminals.

I would also suggest using a mount for the extinguisher that allows it to be fitted within easy reach inside the vehicle, as not only will the boot storage slow down access, assuming you can actually access it, but if there is a fire actually started in the boot you won't be able to access it.

Thank you for replying.

I do have a battery cover that comes with the box, it just happens to be off at the moment.

The 100 amp main fuse in the engine bay runs the EFI system and the body harness functions like lights, ac, etc. I do have the thermofan and intercooler pump on separate circuits with separate fuses. I used 100 amp since that's what the manufacturer used out of the factory.

My only reason for wanting to add in a fuse in the boot is concern of the positive cable shorting somewhere between the battery and the fusebox at the front. I did take sheathing precautions when running the wire, but I would really like to not find out the hard way that I had a chafing issue after 5-10 years :).

I DO have 300 amp fuses on hand that I was hoping would be enough, but I suppose there is no way to know without testing. Could you please provide me with an example of the type of solenoid you are speaking to? I haven't seen something like that before.

thank you

This sort of solenoid is commonly used as a remote mounted starter solenoid.

Cole Hersee 200A

That would certainly work, but it seems to be marine spec' and designed more for continuous use as a switch and is quite expensive for those reasons.

What I was thinking of were the solinoids used for inertia starter motors, anything meeting the current draw should work well and they should be carried by any auto' electrical shop and should be much cheaper. Or you can buy them through AMAZON - https://www.amazon.com/Starter-Solenoid/s?k=Starter+Solenoid - and help finance Blue Origin's next test mission.

I used a 100A breaker on my main battery positive from the trunk to the starter/fusebox.

Had no issues with it. I measured the starter inrush current at 500A, and the continuous cranking draw was 150-175A. Never popped the breaker once, not even in 90+degree weather.

The inrush is so short that it shouldn't ever blow any fuse or breaker. I suppose the continuous draw could have popped it, but it never did. It wasn't high enough over the rated limit for a long enough period of time.

Having said that, be aware that the larger the displacement and/or higher the compression, the higher the inrush and continuous currents will be. So 100A might not work for everyone. But 150 or 200A breakers should be good enough for almost anyone.

hi Wedge - what type of engine were you seeing 500A with? I'm just pushing a little 4 cyl here :)

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