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Starter and Alternator wire sizing gauge

Practical Motorsport Wiring - Club Level

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Discussion and questions related to the course Practical Motorsport Wiring - Club Level

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So have a CRX though I'd like to ignore that for the actual question.

Backstory: The lead acid Walmart battery finally kicked the bucket after 5 years. Wanting to save weight and size (and self discharge benefits) I picked up a LiFe battery. The new battery uses M8 studs to secure the ground and "12V". The current setup is POS to alternator. POS to distribution box (fused). NEG to chassis to engine. All factory looks like maybe 8ga? possibly 6 to the starter. Since I won't have lugs I'm going to have to install ring terminals. Since the wire is old and somewhat chaffed might as well replace it while I'm messing around but what gauge to use.

Am I correct in thinking I should be able to calculate resistances and figure out what size wire to use by that? Since the new battery isn't a standard "group" battery I'm thinking I can shorten the POS-Starter wire by 6 inches (its already probably only 18" long). Can I also drop the wire gauge to something like an ETFE 10 gauge? Presumably with the distance so close to the starter the voltage drop due to the wire will be very low (less than the terminations?) Would you be able to sum up the internal resistance/starter resistance calculate a standard termination resistance and gauge how much starting power you'd have? Since starting is "burst" I presume even the 10 gauge would handle the hundred or whatever amps and you could calculate even how much power would be lost in the wire? Am I thinking of this the right way? Any additional insight?

Personally, because I loath voltage drops and sluggish starters, I'd be using at least the OEM gauge, probably bigger - the first time you hear that starter struggling you may agree.

However, in your case if you want to save weight or money - negligible in either case - you're going to have to first confirm what gauge you have before making a change. Then you can check what the voltage drop will be for the current you expect to draw - if you can, check that, if you can't you can check the voltage drop over the cable you have as a datum.

Oh, have you compared the stated cranking currents for each battery, to compare?

https://prowireusa.com/p-96-4-awg-motorsport-battery-cable-black.html

https://prowireusa.com/p-870-6-awg-motorsport-battery-cable-red.html

Motorsport 4awg is listed as 0.228000 (presumably lbs per foot) compared to 6awg at 0.200000 lbs per foot so given your short runs I don't know that the weight difference is going to net many gains. So as Gord has suggested having good cranking for the small additional weight might be a better approach.

As for costs you would save around $0.36 per foot but again not sure I would be tackling this from a pure financial point of view either since performance of the car starting will be better having a larger cable with little or no voltage drop in your application.

I realize that these are not the gauges you are talking about, this is just for a comparison.

I owned a ducati so I'm more than familiar with struggling starters. Unfortunately where I don't have many of the numbers I'd like is with the battery output and starter resistance, going from a group size 51 wet lead acid advertised as having "500 cranking amps" to a LiFe battery advertised as "480 cranking amps" and can start engines up to 3500cc... I don't particularly trust advertised specs and that last one feels like a huge shot in the dark given starter compression and other differences.

With the old lead acid battery new it cranked quickly without issue. With the lead acid battery dead (charged but dead) the lights were dim and it just clicked.

Looking just at wire resistance and the existing wiring (13.5" battery to chassis and12.5" battery to starter) if it's 4ga/6ga/8ga(need to count/measure strands to know for sure) that should be .00061/.00096/.001519 ohms respectively and weigh .33/.2/.13 (etfe, presumably higher for whatever normal battery cable is)

Cutting those lengths in half for 4/6/8/10 you get .00030/.00048/.00076/.00137 ohms and .16/.10/.07/.04 lb (obviously)

Terminations add resistance (what was there before was downright ugly poorly crimped ring terminal to wing nut lug to battery terminal) I'd guess it's safe to figure 1miliohm per termination? So removing one termination and shortening the length I could probably even downsize from a 4ga to a 10ga if the battery output stayed the same (but will it?)

Which also raises the question in my mind of what the biggest factor in the system to tackle. Seems to me (after buying the 480 cranking) maybe even a smaller battery would do the job right now

Swapped the battery to starter wire with 10 ga. Starts fast

https://www.instagram.com/p/CNDR_vcHv88/?igshid=q89p8jvdpf35

Do you notice any heat in the cable after a start? just curious.

I ended up running the math earlier and noodling some more because I was curious. I assumed 200 amps but have no real idea how much current the starter takes and didn't come up with anything during a quick Google.

Knowing the resistance and current you can calculate power.

1 watt second = 1 joule

So if you know how long you hold the starter for (and ignore resistance change over temperature for copper to make it easier)

And know how much copper is in your wire cross sectional vs gauge and length = volume

Volume to grams

And know the specific heat of copper J/g °c

You can calculate it ignoring heat loss radiation and all that to get an approximate temp rise.

Assuming ambient and all the other things (it would probably be a worse case if the engine bay was already hot and engine hot since the wire would be hotter increasing resistance though oil would be thinner and easier to pump but you'd have to also look at alot of other factors if you really wanted to nitpick i don't have a clue what the factors or changes are you're also in a battery going from a chem reaction so the warmer the faster those usually happen etc etc) i think i came up with 7c? Feel free to check that logic/math

The mag wire in the starter windings is probably lower gauge than 10 but also has insulation for a higher temp compared to household THHN lol

But yeah don't wail on the starter and it should be fine based on the math I ran. If I held the starter for 30 sec i might get into the danger zone for the insulation rating

With a smaller engine and, presumably, geared starter the current draw will be less than a direct acting starter on a large engine - I would assume something like a nominal 1kW starter operating at nominal 10V, so around 100A?

As you pointed out, the engine starts readily, so potential heat build-up shouldn't be an issue - my concern was about the voltage drop as you have several pictures of the vehicle in rather cold weather and, as this will put a lot more stress on the starter, you may experience problems then.

Depending on from where, and how, you are powering and grounding the ECU, you may also run into problems there when cranking under higher load, as they normally have a minimum operating voltage.

Looks as though you have a good handle on the math side of it.

Also worth noting at the start you were looking at changing this up for a D16y as they are not a high compression motor which would also contribute to the load on the electrical system.

But good on you for working it thru and making an informed decision.

I've been spending some time lately trying to do the same, nobody on the net has said exactly what the current draw of a B or K series starter is but a few people have said that the starters pull less than you think. I'm trying to source a DC amp clamp that will give me the exact value so I can use the smallest possible gauge wires for some race cars I'm doing a bit of wiring on. The OEM starter cable on my B16B is 6AWG, I used the wire barn calculator and done some maths, I ended up at an 8AWG cable (the run is less than 2 foot) and its starts perfectly hot and cold for the last couple of months. I would still like to know the exact current draw though, for peace of mind.