Current Load Calculations - Much larger alternator needed

[ETI4K]

Also, another issue to deal with is output at idle.  I can use a smaller alt drive pulley, but the rpm ratings for ones I've looked at are typically 6k.  I plan to be able to run to 7k.  So if I use a smaller alt drive pulley, then the real rpm max will be less than 6k.

So you end up having to use a much larger alt (output) just to get the current you need from idle up to the rpm limit of the alt.

Edited April 24, 2020 by ETI4K

[Zed Head]

22 minutes ago, ETI4K said:

So you end up having to use a much larger alt (output) just to get the current you need from idle up to the rpm limit of the alt.

Not necessarily  That's why I posted the plot.  If you want a big number get a big number.  Read that boat article though.  Don't forget that all of the charging current has to pass through that single white wire.  If you use it all.

Here's a 10si plot, with some discussion.

Get a Mazda alternator - https://www.roadandtrack.com/new-cars/future-cars/a21719317/mazda-confirms-2019-miata-gets-more-power-higher-redline/

http://www.earlycj5.com/xf_cj5/index.php?threads/powermaster-alternator.49477/page-2

[ETI4K]

Zed, Just finished reading the articles.  The boat article is a good overview of case sizes and styles as well as illustrating your point.  I've been on the MAD site before (very good information there), but I must be missing something on the Mazda article, except of course that a new MX-5 would be even more fun than my wife's current one.  As an aside, I've owned a lot of different cars, Zs, Porches, Jag, Hondas, Ford, etc.  On a dollar per smile basis, I swear that Miata PHT wins.  The PHT is key!

Back to alternators.  In summary, perhaps we can say at idle, if your load is very high, as in my table at the top of this thread, you'll be pulling on your battery.  Coming up on RPM, if the alternator output is greater than the load, the battery gets recharged and the alternator takes over supplying the load.  If the alternator doesn't have it, the condition of your battery will be determined by how long you use circuits that extend the load past the alt's output, and are you driving long enough to "refill" your battery.  It's a multivariable equation.  The easiest path to follow then is probably to rely on anecdotal evidence or use the ever popular overkill method. 

As for me, my problem may have been solved.  My brother offered to give me a spare (12si) 140A alternator he has on hand for his Cadillac.  It's free and free wins.  If it won't keep up, there's plenty of options in the 12si family that will.

This is another good bit of info - taken from the Mazda 6 Club site and has to do with installing a 260A alternator: https://www.mazda6club.com/threads/new-high-output-alternator-solution-for-us.235352/

Quote

He also included some general instructions. Per the instructions, I had to run a 1/0 gauge wire from the positive post on the alternator to the positive terminal on the battery, a 1/0 gauge wire from the bottom-back mounting position of the alternator to the actual frame, a 1/0 gauge wire from that same place on the frame to the negative terminal on the battery, a 2 gauge wire from the negative terminal of the battery to the sheet-metal body of the car, and a 4 gauge wire from the positive terminal of the battery to the positive post of the fuse box [btw, this is all IN ADDITION TO the stock wiring, not in place of]. I'm sure others that have used his alternators have said this, but I'm going to call it the 'big 5' upgrade. It was a pain in the arse cause 1/0 gauge wire does not like to bend easily.

At 140A, I might get by on #4, but I plan to run #2.  Later in the thread someone recommends Knu Flex wire as being very easy to route, which is exactly what I was looking for.

[wheee!]

You will need to shorten your mount for the 12si. My thread has a lot info on this.

Sent from my iPhone using Tapatalk

Edited April 27, 2020 by wheee!

[Zed Head]

On 4/25/2020 at 4:10 AM, ETI4K said:

 I must be missing something on the Mazda article

You said that your higher RPM would be a problem.  Mazda has it covered, and has high output alternators.

[ETI4K]

Ahh yes, of course, how could I have missed that.  I know, it's because I was imagining myself driving that thing on a warm summer morning, early before everyone is awake, on a great winding road going nowhere, and with no traffic lights!  I'm easily distracted.

 

[AK260]

At the risk of stating the obvious, don’t forget all those charts are showing alternator rpm and if you are using stock crank pulley at 150mm and stock alt pulley are 75ish, your alternator is spinning twice your rev counter speed.

One other consideration which I’m sure you’ve thought about is to upgrade the wiring to cope with 140amps. In the real world your alt will not be pumping out 140amps constantly unless you have a flat battery and revving hard. But your 40+ year old wiring rated for a 40A alternator with headroom won’t be man enough - the last thing you want is a car-b-q! Although your fusible link “should” blow before that happens.

I had my 280zx alt rewound for 90A. Rather than replacing the wiring, I doubled it up by running a parallel cable back to the battery with its own fuse. That way you avoid running silly large cables under the bonnet.

[ETI4K]

Wheee! - Thanks for the details needed for modifying the bracket.  That'll make the job much easier!

                  btw: Very nice pics above.  I'm up to page 73 of your thread!

AK - Yep, good advice.  Planning to make dramatic changes to the entire harness while it's out.  First will be wire gauge upgrades where needed.  I'm actually working right now on a wiring diagram to add features (low oil level, low coolant level, additional sensor wiring for MegaSquirt, electric power steering, carjack shutdown, taking all high loads off stock switches and running them through relays, etc., etc.  I'm adding fuses for the loads as well as the relay coils, and replacing the fusible links with circuit breakers.  I figure if something shuts down on some cold night while it's pouring down rain, all I have to do is find which discrete circuit is the problem.  It means I'm adding a lot of fuses, which I plan to locate under the passenger seat.  That of course means I need to be able to easily move the fuse panels or seat so I can service them.  Haven't decided to make them slide out from under the seat or hinge the seat out of the way.   Either way, I'm fabbing new parts

 

[AK260]

Nice work ET[emoji106][emoji106] I really like the low oil and coolant warnings!

That sounds like a big job but a VERY worthwhile thing to do. I have been (for 5years) planning - but not for round to it yet - to put a combined relay and fuse box in the passenger side footwell close to or above the blower motor so that it’s accessible and hidden. Would that work for your fuse box? You wouldn’t need to modify the seat then.

[ETI4K]

It might.  I will need four boxes that are about 3 x 4 each plus one box for the relays that will be about 4 x 9.  It's a lot of real estate.  I started off planning to buy the fuse panels and making the relay box using typical Bosch style relays bc they are ubiquitous and cheap.  Then I found the mini(?) or micro(?) relays made to plug into fuse boxes - they are a lot smaller.  Searching for these parts could be a full-time job for the hours needed to find the right bits.  I couldn't imagine doing the searches if we weren't in lockdown.  Every night and weekend would be fully consumed.

Since I will be removing some old relays from under the dash, there may be room in that area, but I am sort of expecting there won't.  The problem is my car is 100% disassembled so I can't fit check anything, but I know the space under the seat works.  I kinda like the idea of a pull-out panel, not so keen on flipping the seat up. 

[AK260]

Yes that does make it more difficult. I will try to take a photo for you with a tape measure in it.

IIRC there is quite a lot of space up there. The only thing to think about is how to get it far enough up the well to not get kicked. Also as your car is already apart you are able to easily weld brackets or mounting points. One of the things that slowed me down was the fact that I will not add holes into my car by drilling the bulkhead. There is already a mounting point for the blower motor, which I intend to piggy back onto with a home made bracket.

If you’re clever about it, you can design a bracket that soars high up in the footwell but can be unclipped to point down / come down to be easier to deal with on a dark night. You could even install a low wattage light / led over it to come on when needed, negating the need for a torch.

You are absolutely right about the mini relays. The other thing to possibly consider are the fused relays which means each relay has it’s own fuse and negates the need for a separate box and associated wiring, see this ...


https://www.12voltplanet.co.uk/make-and-break-relay-normally-open-contacts-12-volt-30-amp-integral-blade-fuse.html

[ETI4K]

Measured pics would be great - thank you for offering to do that.

A hinged drop-down assembly with lighting is also interesting.  I'd probably implement that so the LED lighted automatically whenever the assembly was in the down position. ? Lots of fun options. 

The more I get into this, the more I am leaning toward the smaller footprint solutions.  I am up to 25 additional fuses and 13 relays (some are replacements for OEM relays).  Thanks for the link.

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