In the past year I have went through 3 Optima red tops. I always thought it was something draining the battery overnight but according to the haltechs readings im getting average of 12.0v @ idle(1000rpms) and 13.0v while driving(3000rpms), usually @ idle the voltage will drop to 11.8v. My battery has been relocated using welding cable(50c-6000v is all I could see on the cable). I run my fans wide open after warm up. My p/s and a/c have been removed. As far as pulleys go, Im using a Greddy pulley kit(water pump/ Alt) and a Pettit power pulley(main drive pulley).So far I have had my altenator rebuilt, swapped altenators from another fd, added an engine ground, and replace the battery. It seems like after a few months of driving the car the battery goes to sh*t (needing a jump off to start). What I don't understand is a "underdrive pulley", does the word "underdrive" mean less rotation=smaller pulley? Im all confused about pulley size and the gains/losses and performance if any, could someone explain?

 

more than likely it is a voltage drop problem from relocating the battery, mine was doing the same untill i upgraded to larger cables (voltage decreases as wire length increases) its a common problem when relocating batteries to the boot

 

you can work it out by using ohms law triangles voltage=resistance x amps.and amps =voltage divided by resistance you know the starter is 12v measure the resistance through the starter and divide voltage by the resistance of the starter, this will give you how many amps it should draw +/- 10% for the effect temperature has on resistance now get a clamp meter hold it over the cable when cranking and see how many amps it draws, if its more than your equation you have a voltage drop problem

 

What size cable are you using for the relocated battery?

Does your alternator have an internal or external voltage regulator?

If all else fails, maybe you should go back to the stock pulley on the alternator.

 

sorry to say this, but I'm in the same boat.

but with a small gel cell battery and 4AWG wire relocated. Underdriven everything...and Haltech. If the car sits for weeks the battery gets weak.

Are you using an MSD 6A?


UD main crank = smaller = everything attached will not rotate as much

UD ALT/WP = larger = less rotations again, respectively

less rotations = slower moving water pump speed and ALT won't deliver as much voltage...i.e. why you are seeing 11.8V at idle (I'm seeing just over 12V at idle w/ no electrical load ~900rpms)

 

As far as voltage reg, I guess its internal(100amp fd3s oem alt). Im planning on throwing the A/C, P/s and the msd box back on soon, I really dont think the charging system will support the 3. Im going replace the welding cable(I think is 4ga) with a low guage audio cable. I just talk to a friend at a local sound shop and he said that 0(alt) gauge would be kind of an overkill, he recommended 4gauge but I think thats too small (looking at the size of it).
What size cable are most you using?

 

Its not the cable. 4AWG cable will support 135 amps continuous current draw without overheating... you would ahve some SERIOUS issues if you were pulling 135 amps while the car was running.
Switching to audio cable won't help either... copper is copper. 4 AWG welding cable will contain exactly the same CMA of copper as the audio cable... only difference will be the insulation.
Again, you will see no change with audio cable. Its got the EXACT same amount of conductor in it.

Now, lets look at the voltage drop on your battery cables.
The Circular Mil Area (CMA) of 4 AWG must be a minimum of 41, 740.
R = (K x L) /CMA

where
K= constant for the material, 10.4 for copper
R= resistance
L= the length of the conductor, in feet

So lets say you ran 20 feet of wire... the resistance of the cable would be:
R = (10.4 x 20) / 41,740
= 0.004983 Ohms.

We'll double that value to account for the ground path, even though the chassis would have an even lower resistance.
so, even at our peak amperage of 100 amps, our total voltage loss due to the cable is:
V= I R
100 amps x 0.009966= 0.9966 Volts.

One volt... and you would only see this while cranking.
You should have 13.2 at the battery to begin with, which would put you at 12.2V minimum measured in the engine bay with the engine off... though I REALLY doubt you'll be losing that much. Its easy to test. Measure the voltage at the battery with a DMM, then measure the voltage at the fuse box in the engine bay... it'll likely be a difference of 0.1-0.2 V.



But, who cares? This is not your problem. Your car starts. You care about the voltage while running, which is not affected by battery cables.

 

So why do we know its not your battery cables?
While... the alternator grounds at its mount, and supplies the fuse box from its terminal.
With the engine running, the alternator is responsible for charging the battery from its discharge during cranking and for providing the current to start the car. The battery cables have no affect on the total system voltage anymore... only the alternator.

So we know its something alternator related.
So... what would cause the low voltage?

Well... with the car running and the acessories on, there is a certain amount of resistance connected to the circuit. If the alternator cannot provide enough current to supply all of these resistive loads, then the battery must supply the remainder of the current.

But the battery is supposed to be charging right now.

So if we're drawing current from the battery, when its supposed to be receiving current, the battery will run down.

Run down a non-deep cycle battery 5 times and its garbage. lead sponge dissolves and sinks to the bottom.

So if its not the battery, not the battery cables, not the alternator connections, and not the alternator cable.

Must be the alternator.

If you say its not the alternator itself, then its not being driven fast enough to power your load.

You need to spin the thing faster. Change the pulleys.

 

I have main, alternator and waterpump underdrive pulleys with a Miata battery relocated to the rear through the fender (FD) using 2 AWG welding cable and no problems for over a year.

 

Its not the cables. Disconnect the battary completely with the car running. It'll keep running. The voltage of the car should remain above 13.7 volts while idling. Ideally, you should be one volt above the battery voltage of 13.2V. If its below this... you need more amperage output. Spin the alt faster, or upgrade the alt.

 

Well scathcart, My alt cable is a little fried at the end due to a bad connection from and old alt. . So a way to check this problem would be to run an additional cable from the fusebox to the alt (Im guessing)? So more and likely its not the pulleys.

 

That would help mildly... but I still don't think its your problem.
Do you have a DMM? Throw it on the DC volt setting and measure the voltage at the alternator terminal with the engine on and all your accessories on and probe the negative to the alternator case. Then grab the throttle and increase the revs to 3000 rpm and check the voltage again.
Report back with the voltages... I'll bet they're around 12.5... which means the alt is not providing enough current and the battery is under constant discharge.

 

Scarthcart, I will do so tomorrow. I have been paying alot of attention to my voltage while driving. Sometime I will show 13.2 voltz @ idle(1000rpms) with fans running but never 14 while driving. I think this is an altenator problem also.

 

[QUOTE=scathcart]So why do we know its not your battery cables?
While... the alternator grounds at its mount, and supplies the fuse box from its terminal.
With the engine running, the alternator is responsible for charging the battery from its discharge during cranking and for providing the current to start the car. The battery cables have no affect on the total system voltage anymore... only the alternator.

how do you think current produced by the alternator charges the battery its passed through those cables

 

I am using a tiny Oddesy PC536 meant for Harleys weight 5kg, its in the stock location with a custom positive wire that spreads to 4 different wires and locates onto two positive clamp on points.

@ idle I have 14.1 volts with no fans
after warm up adn cruising I will have 13.8 to 13.9 volts
and with both fans going at 91 deg C water temp for brief periods I will go down as low as 13.6 volts, does not matter if I have stereo on, headlights etc etc...

Using HKS CDI too
Stock pulleys
PS, Air pump, and AC all connected

Maybe check your wiring and then pulleys?

 

I ran underdrive pullies for years with no battery issues. I only had issues with wiring which I very heavily upgraded as I worked at a stereo shop at the time. There's no such thing as overkill. I had a 4 gauge wire running from the alternator to the battery. A 4 gauge wire running to the starter, a 4 gauge ground wire from the battery to the drivers side fender, a 1 gauge ground wire from the battery straight to the chassis, a 4 gauge wire from the battery to the engine, and an 8 gauge wire running from the firewall to the engine. I also had a 1000 cca dry cell battery. Never had another power issue ever again. Before that I'd go through a couple of batteries and alternators a year.

 

Great thread. I've been using the GReddy pulley kit for about 6 years and haven't experience any problems, but when I relocated my battery to the rear bin I needed to replace my alternator and battery within one year. Now with the new battery and alternator I still can't barely get 13.1v while driving. I'm definitely going to use these suggestions you guys have posted, I don't feel like wasting my money on another battery and alternator.

 

rotarygod, did you pull apart your drivers side engine harness and replace the cables with good cables or did you just use extra cables?

 

Ok, i used the DMM, threw it on DC-Volts, with everything on @idle(950rpms) the multi-meter showed 12.1-12.3 @ the alt, I had the same readings at the battery. Btw, the haltechs was off by -0.4 volts. Im going to throw my old alt back on and see what can get out of it.

 

Being a car audio installer... I would 2 guage for a battery relocation. If you want to play it safe.