Battery choices

[Jaxonalden]

Originally Posted by JackG

A small correction... the battery is used to start the car. Once running, the alternator supplies *all* the current to run the car, and it also recharges the battery. The battery is simply a storage device used for starting and for supplying power when the alternator isn't spooled up.

Having a smaller battery will not put a bigger strain on the alternator. After all, it still takes the same amount of energy to start the car, so the alternator has to replace the same amount of energy back in the battery after the start. And it's already sized to handle all those electrical items you listed.

If you add big amps for your stereo, and you run them at loud, thumping levels all the time, you may exceed the alternator's output capability. The fix there is to upgrade the alternator. A bigger battery is a patch at best. If you're exceeding the alternator's output capability on a nearly constant basis, the battery will eventually be drained, but that's an extreme case.

Jack,

I totally disagree with everything you wrote and stand behind my thread. The car does not run off of the alternator, it pulls power from the battery. The battery is a constant, large capacity source of ~12 volts of electricity. The alternator does nothing more than recharge the battery at 13.5-14.5v. Anything less than 13.5v and the battery will not recharge ("Alt" idiot light). The alternator produces A/C power thats converted to D/C through diodes and the voltage regulator does just that, it regulates the flow of current to the battery. Flow starts when battery volts fall below 13.5v and shuts it off at 14.5v.

So as I said;

"It's all about electrical capacity. Don't forget your putting a strain on the alternator trying to keep up...if you have a small battery." That's because the alternator is not meant to constantly supply power and will eventually heat up (which is the limiting factor) and fail.

[JackG]

Originally Posted by Jaxonalden

Jack,

I totally disagree with everything you wrote and stand behind my thread. The car does not run off of the alternator, it pulls power from the battery. The battery is a constant, large capacity source of ~12 volts of electricity. The alternator does nothing more than recharge the battery at 13.5-14.5v. Anything less than 13.5v and the battery will not recharge ("Alt" idiot light). The alternator produces A/C power thats converted to D/C through diodes and the voltage regulator does just that, it regulates the flow of current to the battery. Flow starts when battery volts fall below 13.5v and shuts it off at 14.5v.

I'm a Electrical Engineer, and I understand that you believe that's how things work, but it's simply not the case. The battery is in parallel with the alternator, and the car loads see them as a unified source of power. There is no mechanism that prevents the flow of current from the alternator to the car, limiting it to supplying only the battery. Current does not flow into the battery, and then out to the car.

As you point out yourself, the battery is ~12V, while the alternator produces ~13.6V. An accurate analogy of voltage is water pressure in a pipe. If you have two pipes connected together in a "Y" that then go to a sprinkler, and one pipe has a higher pressure than the other, which pipe will be feeding the sprinkler? The one with the highest pressure, of course! The lower pressure pipe will in fact reverse it's flow until the pressure equalizes, which is how the battery charges!

Since the battery's built-in voltage (pressure) is 12v, even when it fully charges it will not take over the sourcing of current. If the alternator were turned off, the battery would return very quickly to it's 12v potential, which would signal the alternator to immediately turn back on.

The "Alt" idiot light actually lights up when the battery is sourcing the current instead of the alternator. Remember the real current gauges in cars? The current gauge had a "0" in the middle, and it swung both positive and negative. It measured current flow in/out of the battery. Positive showed when the battery was charging, and negative showed when the battery was sourcing power. At idle, the gauge would go to 0 or just slightly negative, as the alternator was not spinning fast enough to provide 13.6V. When you revved the engine or were travelling at a steady speed, it swung positive. A big negative swing of the meter was your signal that your alternator had quit. The idiot light shows the same thing, that something is wrong, and your battery is providing the car's power.

One last thing, the voltage regulator does exactly what it says... it regulates voltage. The alternator will produce voltages far in excess of ~13.6v, so it has to be regulated. The battery is self regulated, in that as it comes to a full charge, it will accept less and less current. The voltage regulator keeps the voltage at safe levels that will not overcharge the battery or damage any electronic equipment in the car. There is no current regulation going to the battery, except for the max voltage potential applied to it by the alternator as it runs the car.

[edevlin]

"As you point out yourself, the battery is ~12V, while the alternator produces ~13.6V. An accurate analogy of voltage is water pressure in a pipe. If you have two pipes connected together in a "Y" that then go to a sprinkler, and one pipe has a higher pressure than the other, which pipe will be feeding the sprinkler? The one with the highest pressure, of course! The lower pressure pipe will in fact reverse it's flow until the pressure equalizes, which is how the battery charges!"


So, based on your discussion, if I wanted to provide more current to my outboard audio amps which are wired directly to the to the battery) the best strategy would be:


1. Get a higher output alternator

2. Get a larger battery

3. Stick a large cap in the circuit from the battery to the amps


I am especially curious about the third case. My current setup seems to be working fine, but when I had a smaller battery (11 lb Braille) in the car, when I was driving with the headlights on and the music playing loud, you could see the dash lights dim in rhythm to the music at high (very) high volume levels. With my current 15lb Braille battery everything seems to be working fine, but I am still curious about a large cap in the circuit.

Ed

:dance:

[JackG]

Originally Posted by edevlin

"As you point out yourself, the battery is ~12V, while the alternator produces ~13.6V. An accurate analogy of voltage is water pressure in a pipe. If you have two pipes connected together in a "Y" that then go to a sprinkler, and one pipe has a higher pressure than the other, which pipe will be feeding the sprinkler? The one with the highest pressure, of course! The lower pressure pipe will in fact reverse it's flow until the pressure equalizes, which is how the battery charges!"


So, based on your discussion, if I wanted to provide more current to my outboard audio amps which are wired directly to the to the battery) the best strategy would be:


1. Get a higher output alternator

2. Get a larger battery

3. Stick a large cap in the circuit from the battery to the amps


I am especially curious about the third case. My current setup seems to be working fine, but when I had a smaller battery (11 lb Braille) in the car, when I was driving with the headlights on and the music playing loud, you could see the dash lights dim in rhythm to the music at high (very) high volume levels. With my current 15lb Braille battery everything seems to be working fine, but I am still curious about a large cap in the circuit.

Ed

:dance:

What you were seeing is voltage drop across the resistance in the car's wiring, and also internal resistance inside the battery. The more current you pull, the larger the voltage drop.

Since you are wired directly to the battery, you're seeing the drop in the wiring from the alternator to the battery. You old, smaller battery also likely had a higher internal resistance, so it was less able to keep up with the dynamic current demands of your audio setup. You new larger battery probably has less resistance, and while replacing it you may have created better connections at the battery as well, which would help.

The big capacitors are placed as close to the amp as possible, and are there to provide a low resistance path for providing those short peaks of high current that the amps need for musical peaks. Capacitors are basically energy batteries that can release and recharge their energy very quickly and easily. The end results is you will have less distortion during peak power demands. Whether you'll hear the difference depends on your equipment, musical taste, and ears.

[Jaxonalden]

Unbelievable...have you ever owned a recreational vehicle that didn't have a battery? At idle the headlight is dim, rev the motor and the light brightens up. That's what happens in a system with no battery (remember constant voltage?).

So in your scenario the same thing would happen, quote "the alternator supplies *all* the current to run the car". The alternator is not run on a constant speed. Explain that one.

[JackG]

Originally Posted by Jaxonalden

Unbelievable...have you ever owned a recreational vehicle that didn't have a battery? At idle the headlight is dim, rev the motor and the light brightens up. That's what happens in a system with no battery (constant voltage).

So in your scenario the same thing would happen, quote "the alternator supplies *all* the current to run the car". The alternator is not run on a constant speed. Explain that one.

Your first one... apples and oranges. And a system with no battery is NOT "constant voltage". That's why the headlights dim.

Your second one... That's what the voltage regulator is for. I already explained it as well.

Take a look here: http://www.bcae1.com/charging.htm

Look at the second paragraph: "As long as the engine is running, all of the power for the accessories is delivered by the alternator. The battery is actually a load on the charging system. The only time that the battery would supply power with the engine running is when the current capacity of the alternator is exceeded or when engine is at a very low idle. "

I really don't have time to educate you on this. Please do some searches, and some reading.

[blue2000s]

JackG doesn't really need my help, but he's 100% right.

[Jaxonalden]

Then why does the OEM use the battery it does? Being the sports cars these are they should have a lawnmower battery under the hood.

[JackG]

Originally Posted by Jaxonalden

Then why does the OEM use the battery it does? Being the sports cars these are they should have a lawnmower battery under the hood.

Because Porsche designs the cars to suit 90% of the buyers. Most people buying Boxsters don't want to eek out the last 1% of performance that a lightweight battery would provide. They'd rather have a battery that will last through a week or two of non-use and still start the car; a battery that will start the car when it is negative 18 degrees out; a battery that will last more than 15 minutes of leaving the lights on before it's dead.

If you replace the OEM battery with a lawnmower battery, those are the kind of issues you'll have to look forward to. And Porsche, along with the vast majority of the buyers, just don't want that.

[Lil bastard]

Well, it seems that in terms of losing weight for ax, it's not all or nothing.

Dedicated ax'ers regularly spend time prepping their cars for an ax or track event, swapping wheels/tires, brake pads, some even pulling the passenger seat. Why not add a battery swap to this regimen if the results warrant it?

Instead of removing the battery terminals, you could just as easily remove the cables at their other ends (still a single nut) and install a pair with terminals dedicated to the smaller battery. Even a lawn mower battery would be sufficient for the day and could then be removed and kept on a tender until needed again. It's not a lot of $ or effort, if you're dedicated enough.

[edevlin]

Just as a point of information, my 1.5 year old Braille 15 lb battery popped the engine right over this morning no problem. It was 18 degrees when I started the car. Battery did great, but boy the suspension felt hard, dont know if it is the tires or coilovers, but I could really feel the road. Hey the other thing I noticed for the first time this year is the "innercooler" effect of cold weather. Feels like I have picked up 10 hp or so when its cold outside. This thing really scoots when its cold out.

Ed