How many batteries do I actually need?

[helotaxi 5,000+ posts]

So your batteries are not @ 14V with the car running....thats not correct....You know this.

The voltage measured across the terminals of the batteries is 14V or whatever the alt is putting out, however, as soon as you exceed the capacity of the alt, and the reserve of the battery is needed it does not discharge at 14V because the battery is only 12V and that is the voltage that it discharges at. The system voltage

is going to be the lowest voltage of whatever sources are supplying current to the system.

Ok so you think a battery has no capacitance?

Sure it does, but so does multistrand wire. The capacitance of a battery will affect its charge and discharge but it is not sufficient to hold a reserve of current and is not relevant to this at all.

What do you think fills a capacitor ?

It is filled by creating a potential difference across the internal plates. Thsi intial charge for our purposes is usually added using the battery and an inline resistor to limit the initial spike in current that would occur if you were to just hook an uncharged cap up to a battery.

When you first charge one - is the car usually on ...... No. A battery and cap are similar - one holds a reserve and by adding a second you doubled the reserve (assuming both batteries are equal) Adding a cap just adds capacity to hold power that is already in the system. Its like an air tank for airbags - it did not make the air but will hold it and release upon demand.

They are similar but nowhere near the same. The relevant difference for the purposes of the discussion here are their behaviors when it comes to holding a charge. Assume that we have a car with two batteries and a cap installed. If we checked the voltage across the terminals of each of these with teh car running, we would get whatever the voltage coming off the alternator is. If we were to disconnect one of the batteries and the cap from everything else and then measured the voltage across them the battery would read something in the mid 12V range and the cap would still read whatever it did before. As we place a higher demand on the car's electrical system and begin to exceed the current output of the alt, the system voltage begins to drop. Between 14.4V and and whatever the battery's discharge voltage is the battery is contributing absolutely nothing to the situation. The battery is not going to provide usable current until it's working voltage is reached. Because a cap is truly charged to the working voltage of the system, it will begin to immediately discharge once the system voltage drops below the highest voltage. If enough power is drawn that the alt and the cap can't fill the gap and the system drops to the battery's working voltage then the battery will kick in.

A battery will certainly stop any disco light or slow fade. You have to look at the whole picture, not just one part. Everything works as a system. With the car on the entire system operates at 14V. You cant say that this burst came from the ALT or this low note was powered by the battery (unles car is off, we know that) All parts are daisy chained together and therefore react together.

How can a battery stop any disco effect if the cause of the disco effect is the difference between the alts working voltage (which has to be higher than the battery's for the battery to be charged by it) and the battery's working voltage?

Again, second battery - the ALT does not need to catch up. You see the ALT and batteries are directly connected, so while the ALT creates the 14V operating voltage, large spikes come from a battery and the ALT adjusts to reload for the next burst. More reserve means less to reload.

More reserve only means that you can dip into it for longer before the thing falls completely on its face. If I discharge 20Ah from one battery or from 200 batteries, I still have to put back 20Ah. The amount I have to reload has only decreased as a percentage of the total reserve. The alt just "sees"s that it has to replace 20Ah and if a bunch of batteries are wired in there then it will, as a result of the much decreased ESR compared to one battery, have to put it back much faster.

I meant it is harder on the ALT but it will add a load to the engine, if you do over drive your ALT it could cause premature failure. If in a car that is taken to redline often, you could possibly exceed the bearings rpm tolerances (this is not likely, but in extreme cases like we seem to be talking about - could really happen) Bearings go on units all the time just from time, excessive heat could break the grease down premature.

If a person is regularly running the car IVO redline then they aren't really a candidate for an overdrive pulley systme because not only will they trash the alt but they won't even be getting a benefit from it. If someone was so inclined, they could devise a system that would drive the alt at a constant (or relatively so) RPM independant of engine RPM.

 

[johnecon2001 5,000+ posts]

so in the end.. whose e-penis will be bigger?

You both have valid points, but you're also ignoring the OP's question and are discussing a topic probably over his head and over his OP.

To the OP:

If it were me, and I had that alt and the big three done, I would just upgrade the underhood battery to something a little stouter. A Kinetic battery or an Exide Orbital get my vote for now. Look for something that will replace the stock one.

 

[helotaxi 5,000+ posts]

Funny, I thought I said that on my first post...

 

[jujumantb 10+ year member]

so in the end.. whose e-penis will be bigger?
You both have valid points, but you're also ignoring the OP's question and are discussing a topic probably over his head and over his OP.

The problem with this forum is that we dont have ENOUGH in depth debates about subjects... the reason alot of the more knowledgable guys dont stick around is because this is more or less a "n00b" forum where the simplest answer is just repeated over and over...

 

[knukonceptz 10+ year member]

so in the end.. whose e-penis will be bigger?
You both have valid points, but you're also ignoring the OP's question and are discussing a topic probably over his head and over his OP.

To the OP:

If it were me, and I had that alt and the big three done, I would just upgrade the underhood battery to something a little stouter. A Kinetic battery or an Exide Orbital get my vote for now. Look for something that will replace the stock one.

I answered that back on post #11 - for him to get this - CHECK to see if he even needed one - I know scary. Someone suggest use hard facts on the application at hand. I see a lot of assuming and generalization on this board. If you teach people the right way to do their own thinking they can put that information to use in their own installation. That also helps keep stupid comments/urban legends to a minimum. One reason I do visit this board, to help promote member's skills/knowledge.

PS - to answer the OP question - I would check your voltage with car on and stereo running. That info was never actually established. At most any normal install would need is a second battery.

Back to our debate - I will say this - in the system the cap / battery / ALT all operate at the same voltage. If you separate them you get different numbers. With the cap being disconnected and holding the 14V - thats not relevant IN the car - because that cap will level off with battery voltage once the car is off. Car on the battery voltage is at 14V, you cant say "where" power comed from once again because everything is connected. You can't say that because the voltage is above 12.5V that power is only coming from the ALT when they are directly connected.

Seriously try the jumper cable test - its free for anyone to try with a voltmeter and a friend. Guys in SPL dont add 20 batteries with out a reason and I bet some of them BOOST voltage above 15V - does that still mean the batteries are doing nothing till it gets to 12.5V - Not possible because no single ALT could handle an extreme car by itself. The batteries take the brunt of it.

 

[johnecon2001 5,000+ posts]

The problem with this forum is that we dont have ENOUGH in depth debates about subjects... the reason alot of the more knowledgable guys dont stick around is because this is more or less a "n00b" forum where the simplest answer is just repeated over and over...

oh sure I totally agree with you. But the fact that the simple question doesn't get answered, bugs me.

 

[jrz126 10+ year member]

Back to our debate - I will say this - in the system the cap / battery / ALT all operate at the same voltage. If you separate them you get different numbers. With the cap being disconnected and holding the 14V - thats not relevant IN the car - because that cap will level off with battery voltage once the car is off. Car on the battery voltage is at 14V, you cant say "where" power comed from once again because everything is connected. You can't say that because the voltage is above 12.5V that power is only coming from the ALT when they are directly connected.
Seriously try the jumper cable test - its free for anyone to try with a voltmeter and a friend. Guys in SPL dont add 20 batteries with out a reason and I bet some of them BOOST voltage above 15V - does that still mean the batteries are doing nothing till it gets to 12.5V - Not possible because no single ALT could handle an extreme car by itself. The batteries take the brunt of it.

The alt is providing the power until the voltage drops to the battery level (say 12.5V). Once the voltage hits 12.5V, then current will stop flowing into the battery and start flowing out to support the system.

The voltage between the alt and battery want to be equal. If you were to connect a 6V battery in parallel with a 3V, the 6V would try to bring the 3V up, draining the 6V battery. (thats why some battery operated devices say not to mix old batteries with new ones).

I would carry on some more here but I have to go work on my friends car.