Matts2k87

Hey guys, just a quick question. So I'm about to relocate my battery to the boot. I don't mind buying new cables but I have a spool of 4gauge wire that's just been sitting round for a few years now. I really wouldn't mind putting it to use but have my doubts wether it will be heavy enough gauge for what I want. What does everyone recommend as a minimum size gauge for the power and earth cables? Thanks in advance

windhund116

4 gauge is enough for under hood stuff. You may need to double (parallel run) two 4 gauge wires to starter & alternator branch point, if you are going to put battery in trunk. Short, good grounding points are important, too. Here's a quote from wiring site:

Alternators and starters are usually the two things with the highest current. An alternator might be required to output 100-150 amperes (which is always more than 2-3 horsepower load on the drive belt) and a starter might require as much as 200-300 amperes. High current means we have to be very careful about wire length and connection quality.

An operating vehicle normally gets 100% of its electrical energy from the alternator. The battery primarily supplies cranking power, but also provides an increasing share of electrical power as alternator output falls below 13 volts or so. Excessive voltage drop in alternator leads and connections, a slow spinning alternator, inadequately sized, or defective alternators make vehicles battery dependent. At the same time excessive alternator lead resistance makes the vehicle battery dependent, excessive alternator lead resistance slows battery charging. The alternator should turn at maximum permissible RPM at maximum engine RPM.

The alternator should feed the power distribution point of the electrical system. This is why factory alternator leads almost always run to the common point where electrical feeds branch, and NOT to the battery post.

Long battery leads can cause slow cranking when hot. Long battery leads can also ruin dynamic regulation of alternator voltage, and slow battery charging. When battery leads are long, they should not be sized for current. Long leads should be sized for voltage drop. Long battery leads almost always have to be significantly oversized. Number 4 might be fine for a six-foot starter lead, but it is almost always too small for a trunk mounted battery.

Running a ground lead from the engine back to a trunk mounted battery is a common battery wiring mistakes. This adds unnecessary resistance. The same is true for running the alternator output wire back to a trunk mounted battery.

Voltage drop for each 10 feet of wire (ONE direction) at normal starter currents:

Gauge Resistance --- Voltage drop @200 amps --- Percentage loss (of 12.6V) --- Percentage loss (of 10V/) --- for distance**

0 ga .00099 ohm --- 0.2 volts --- 1.6% --- 2% --- 12ft
1* .00124 --- 0.248 --- 2% --- 2.5% --- 10ft
2 .00156 --- 0.312 --- 2.5% --- 3.1% --- 8.5ft
4 .00249 --- 0.5 --- 4% --- 5% --- 6 ft
6 .00395 --- 0.79 --- 6.25% --- 8% --- fused link only


*also typical maximum resistance from trunk pan to inner fender, with clean tightly bolted connections. Most of the chassis resistance loss is in the sheet metal near the bolted connections, so front to back connection distance makes only very small changes in resistance


Good luck!

Slowcrash_101

I tested my alternator after I replaced the rectifier, max output was 93 amps, the starter drew ~140 amps during cranking.

rrounds

If this is for track use, you move the battery to the trunk to get a better balance(weight wise) and then you add weight because twin 4 gauge cable is not light. I'd would take a look at a light weight battery like this 540-CCA Lithium Iron battery and all at less than 5 pounds. Its not cheap but they work very good. I have three in my motrocycles and they have been the best battery I've ever had in any of my bikes in the last 45 years. This battery would need a float charger if your car has a draw on the battery when the key is off if you let it sit for weeks on end.
http://www.amazon.com/Shorai-LFX36A3.../dp/B005FIUMFU

ROD

Chuck S

The only function of the battery is to start the car. Putting it in the trunk requires either a completely sealed battery like an AGM or a battery box vented outside the car. If you're cutting weight there are several small batteries including the Odyssey PC680-POdyssey PC680-P that mount in the OEM position and save several pounds.

-- Chuck

Slowcrash_101

The battery does more than start the car, if you think your alternator runs all the electrical aspects of the car, you'd be incorrect. The alternator charges the battery, but the battery is what runs the show. Even rectified, alternator output is too "noisy" to properly run the car's DC system.

windhund116

↑↑↑ 100% correct. Prolly why ppl put banks of capacitors onto some batteries. Not only to help with current storage --- also caps help smooth out the AC-ripples in rectified circuits.

Matts2k87

Thanks for info guys. Really helpful! As much as I don't want to add more weight to my torque pig, it's not a big deal adding a few pounds of cable. I've previously just done a full wire and hose tuck under the hood and just wanted to continue cleaning it up by relocating the battery as I already have cable and a battery box laying around. Still not 100% sure about running dual 4gauge cables tho. Might go out and buy some 2 gauge yet. Sounds like the most sensible option

flanders

Parallel cables would be safer, if you get a short somewhere on the 2 gauge cable there would probably be a small fire before the fuse would blow.

windhund116

I agree. In addition, it's hard enough routing 4-gauge wire. Two-gauge maybe like working with a solid garden hose.