^^^The RX8 does not require a more complicated grounding structure with a relocated battery. Been running on in the truck for 12 months now with a simple ground to the chassis.

 

Wow really? Was anthing in the engine compartment grounded to the battery terminal? Usually items are grounded to that terminal.

 

I grounded the battery terminal (and everything attached to it) to the chassis and ran a 4 gauge cable from the positive battery terminal to the back of the car. There was something attached to the positive side with its own 60a fuse, so I cut that off and wired in a more compact fuse holder (look at your positive terminal and you'll see what I mean). I'm sure an electrical engineer could provide some kind of military spec that shows a better way, but mine works just fine. No dimming headlights, cranking difficulties, etc.

 

You might appreciate this heads up on your mod. Directly off the + terminal of the battery you have a number of fused connections. From the RX-8 shop manual:

120A Main
60A Electric Power Steering (the "something attached to the positive side")
30A Ignition
15A ETV (Engine Control System)
20A Fuel Pump

245A TOTAL AMPS

PLUS add to this the charge wire from the alternator circuit with battery charge AMPS (this is the battery terminal after all). This I'll not use in the calculations, although it is actually a large but variable AMP loading.

You say you have taken all this current to the trunk with a 4 guage wire. A 4 guage wire has the following recommended capacity from the chart of American Wire Gauge (AWG) wire sizes and rated ampacities :

max amps = 135 (free air) or max amps = 60 (bundled wire)

As a worst case (not including alternator charge current) of 245A at 12V (nominal operating voltage is of course higher), using 10ft of wire to the trunk there is a 1.251V (10.43%) drop in that 10' of 4 guage wire alone.

Using Watts=amps x volts 245A x 1.251V = 306.5 possible watts dissipated in that 10' of wire

You might want to consider resizing that cable. Granted the system does not work at 'fuse capacity' normally, but fuses are there to show potential load worst case. And I didn't include charging current, which will always be there to some extent. And there is no fuse in the car (or shown) for that current.

Your mod may 'work' in everyday driving, but I'd be concerned about the potential overload/overheat danger you've created.

 

^^^See? I was waiting for something like that. Electrical dudes!


How do you calculate voltage drop across 10' of wire? So if I hook my multimeter up to the end of the cable, I should see around 10.5v?

How much power is really flowing from the battery? I suspect the biggest load on it is when cranking the engine. But at that time, most of the car's circuits are open. Power is probably only going to the ignition, fuel pump, computer, and starter. Once the engine starts, the car is getting power from the alternator.

 

Try this (bottom of the page) http://www.powerstream.com/Wire_Size.htm for cable voltage drop. No you won't likely see 10.5 v because that would only occur at the 240A load from the battery itself. I imagine the drop jumps around a lot based on that loading.

Don't forget the 60A fused power steering - it all electric assist ('something attached...')

A final note. The car comes with special ground wires wired directly back to the battery from a number of places on the engine, transmission and chassis - for a reason - they could have just grounded everything to the chassis, but they don't!. The chassis isn't that good a ground (welds, etc.) so you could also likely have resistance between the battery connected to the chassis in the trunk and your neg. connection to the chassis under the hood. This will also cause a voltage drop depending on the resistance that causes and the load from the battery.

How much current is actually going back and forth to the battery? Good question and hard to answer without getting a high-power DC amp meter on your 4 guage line. But surely some of the load is taken by the alternator directly when the car is running, which is a good thing in your case. The alternator does have a max output capacity (maybe 100A or so) which is of course something to consider if you're depending on that to take up all the slack, and it's response/output is based on engine speed. Batteries can produce far more, far quicker. So at idle / slow rpms you're not going to get max Amps out of it.


How much is anyone's guess. That is prop why what you've done works at all.

 

I want one of these kits o_o

I hate the current position of the battery! Not to mention, that looks dead sexy, and will go with my theme of red+black. Less weight to boot, and the weight is now BEHIND the front axle! YAY! :D

 

Well, they summed it up. This is the reason I did not attempt this in this car. There is no way an insurance will pay should you have a fire or something.



Digital! Hook me up man!

 

I would want to do this just to open up the engine bay and move he weight of the battery behind the front axel

 

I didn't forget power steering; it wouldn't be powered when cranking.

I think Mazda grounded everything to one place more for modular installation rather than for any performance gains.

240a is not a realistic measure of power draw. A starter is the most power-demanding component in a car, but most vehicles will start with a 60a jump box. Even at 100a, the voltage drop across 15' of 4g is 0.7v. The same drop across 1/0g is 0.3v. BTW, I just checked my kit and I actually have 2g for a 0.48v drop @ 100a.

In terms of safety, people have been relocating batteries for decades. I've never heard of anyone having a problem with a properly installed it. On a side note, my Miata has the battery in the trunk from the factory and its power power doesn't look any beefier than 4g.

 

BTW, I've seen DD's bracket in person and it looks great!

 

So..when will you be producing them, and how much? :-D

 

Trying to find a part number for the stock pump, anyone know it?

 

Okay Digital, just sell me yours!

 

do you ever know what you are talking about?

dd, i assume you mean the stock washer pump?

beers

 

dd,

check pm.

beers

 

the ? was about a pump. you can read?

sure you know more than i do..

i am going to shoot for you dont know me.. as you have been here for one day.

gfl..

beers

 

Thanks!

 

Oddly enough, using what little bit of knowledge I gained from grade school and common sense, I knew exactly what he was referring to.

 

I don't want to see this die. I can't afford one right now (no matter what the price), but, I would like to see these mass-produced for when I can..the battery is one of the first things I want to get out of the way!

 

i am planning on doing the lightweight battery relocate to the washer position but for 40 bucks you can get a metal bracked suited for the PC680MJT. With a little bit of dremel work maybe some thick lexan and a few metal screws you are set. Thats my plan at least.

 

I suck at fabrication of stuff like this so I would rather buck out. I am currently trying to lighten my car as much as possible just to see if the difference is notable. Come on man, just make a couple and put them on ebay so we can bid!

 

OK, I have figured out the cost. The material is way cheap the problem is the tig welding is not.

I will talk to the shop and see if I could get a group rate, I might have 5 made and see how that goes.

PM me for details.

 

this PC680 battery aluminum holder is only $45 shipped, just got mine

http://www.aircraftspruce.com/catalo...odysseydry.php





I'm going to mount the battery in the trunk with a disconnect switch and a remote set of battery connection posts on a small sheet panel in front of the engine, run the cable from the battery to the post, and then mount the OE battery connections to the posts, the easiest way to do it IMO

http://www.quickcar.net/electric/electric.html

 

^^^^^

That seems like a very good option. I still like this guys better, though.