Battery Fuse Size?

[cidsamuth]

I am relocating my battery to the trunk. I know there are lots of threads on that. But, what size fuse should I use on it? Can't find a definitive answer.

I have a PA Performance 215 amp alternator, so I have it fused at 250 amps.

So, should the battery be higher (300 amps?), or should I fuse the battery only enough to cover the starter drawl (150-175)?

 

[manny231988]

I’m doing this also, I’m doing an I line 200 amp back by the battery and an In line 100 right behind the alternator. Still on a stock alternator and my stock fuses was 200 amp starter and 100 alternator.

 

[manny231988]

This is how I did my battery cables

 

[cidsamuth]

I’m doing this also, I’m doing an I line 200 amp back by the battery and an In line 100 right behind the alternator. Still on a stock alternator and my stock fuses was 200 amp starter and 100 alternator.

So, I've been researching a little more since my original post.

The starter is the issue when putting a fuse on the entire positive side of the battery run. The starter will normally pull about 120-140 amps. But, I'm reading it can "surge" much more when you first start to turn the motor over, especially if the motor/oil are cold and the battery is a little weak.

I also read that a good rule of thumb is to wire one amp per cubic inch of engine, and that would leave wiggle room. Perhaps that would be overzealous.

I'd like to stick to 250 amps at that battery, as the distribution block I want to you use is only rated to 250. But, concerned I'll do all this work only to find I need more.

 

[cidsamuth]

Are you concerned that a 200 amp fuse will not be enough for the starter?

Since the OEM starter wire is only 4 gauge, it's hard to believe it would suck up to many amps . . .

 

[manny231988]

So, I've been researching a little more since my original post.

The starter is the issue when putting a fuse on the entire positive side of the battery run. The starter will normally pull about 120-140 amps. But, I'm reading it can "surge" much more when you first start to turn the motor over, especially if the motor/oil are cold and the battery is a little weak.

I also read that a good rule of thumb is to wire one amp per cubic inch of engine, and that would leave wiggle room. Perhaps that would be overzealous.

I'd like to stick to 250 amps at that battery, as the distribution block I want to you use is only rated to 250. But, concerned I'll do all this work only to find I need more.

Ya good thing is if you blow a fuse you probably just put a bigger fuse so your really not gonna redo anything but just change a fuse I’m running 1/0 welding wire for both ground and power

 

[cidsamuth]

1/0 Welding wire for me, too. But, like I said, if I need more than 250, it will mean I also have to change out the distribution block.

 

[manny231988]

I asked my builder Fischer Motorsports and he advised to stick with the size that was on the car from the factory but you have bigger alternator so I’m sure your changed some

 

[cidsamuth]

Well, the alternator fuse I'm comfortable with. It's my understanding that the alternator feeds the battery. So, by going with 250, I have a safety if the alternator goes haywire or grounds out.

But, there was no fuse on the battery itself from Ford, so I'll be adding that. The factory didn't do it because the battery was on the other side of the firewall. But, with the trunk mount, the cable will be in the cabin with the occupants, meaning a fuse is highly desirable.

But, that fuse will need to be able to handle the starter amps. You had an OEM fuse on the battery that fed the starter?

 

[manny231988]

Ya that first picture was my factory fuse that was on my positive cable post at the battery

 

[manny231988]

Well, the alternator fuse I'm comfortable with. It's my understanding that the alternator feeds the battery. So, by going with 250, I have a safety if the alternator goes haywire or grounds out.

But, there was no fuse on the battery itself from Ford, so I'll be adding that. The factory didn't do it because the battery was on the other side of the firewall. But, with the trunk mount, the cable will be in the cabin with the occupants, meaning a fuse is highly desirable.

But, that fuse will need to be able to handle the starter amps. You had an OEM fuse on the battery that fed the starter?

Ya that first picture was my factory fuse that was on my positive cable post at the battery

 

[cidsamuth]

Ya that first picture was my factory fuse that was on my positive cable post at the battery

I've never seen one fuses from the factory .... are you the original owner?

Regardless, it was apparently working for you ... the 100 is alternator only, and the 200 runs everything else, inclusive of the starter?

 

[manny231988]

I've never seen one fuses from the factory .... are you the original owner?

Regardless, it was apparently working for you ... the 100 is alternator only, and the 200 runs everything else, inclusive of the starter?

yup That’s what was on from factory my car is a 2013 so idk of that’s the difference

 

[cidsamuth]

Okay, thanks. I guess that means the 250 amp inline ANS fuse will be fine. Was worried it wouldn't be enough for the starter "surge" that occurs when you first start cranking a cold engine.

 

[cidsamuth]

I've done a bunch of research and talked to several suppliers and manufacturers. Learned a lot.

Here's the deal:

When fusing at the battery, you obviously need the fuse to be able to handle the starter at between 120 and 150 amps, depending on the condition of the battery.

What's tricky, when you first engage the starter, it "surges" for a second while the engine begins to turn over. As a matter of a fact, it actually creates a direct short to the battery for milliseconds. This means the power pull will be much greater than the 150 amps for a split second -- as high as the battery's full cranking amps rating for milliseconds. It starts to come down quickly, but it is still somewhat higher for a second or two while the engine begins to start turning over.

Since you want the fuse to be small enough to protect the line, you want to get the proper rated fuse, but it has to be SLOW BLOW so that it can handle the momentary surge described above.

Commonly used for this are the Littelfuse MEGA fuses. I will use a 200 amp variety, only because I have a 200 amp alternator and want to match the two fuses. It could probably be 160-180 if I wanted.

In theory, based on the listed "blow delay curve," the 200 amp fuse will handle 1200 amps for up to half a second without blowing before coming down dramatically over the first second and continuing to come down at a slower rate between 1 and 15 seconds.

In reality, after the surge and for continuous amperage draw, it won't blow until the amps are at 135% for 15+ seconds. I assume that "cushion" is normal for all fuses.

Hope this helps someone else.