Battery choices

[Jaxonalden]

The small batteries are nice if your racing and looking for quicker times. But is the trade off worth it? Can you quantify the benefits? Our cars didn't come the size battery it has for no reason.

Think of your battery as a bucket of water and the alternator as a small facet. Now all of electrical components all take some "water" out of that bucket and the facet tries to keep it full. I don't know about your car but I have heated seats, Bose amp, electric windows/mirrors and seats, deployable spoiler, all the electronic hardware (ECU, ABS, PSM, fuel pump) and the convertible top. I'm sure I missed a few things but thats allot of drain on a battery. 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.

I feel if you want to save some weight in your Boxster, go on a diet. Don't sacrifice you car.

[threpwood]

Originally Posted by Jaxonalden

The small batteries are nice if your racing and looking for quicker times. But is the trade off worth it? Can you quantify the benefits? Our cars didn't come the size battery it has for no reason.

Think of your battery as a bucket of water and the alternator as a small facet. Now all of electrical components all take some "water" out of that bucket and the facet tries to keep it full. I don't know about your car but I have heated seats, Bose amp, electric windows/mirrors and seats, deployable spoiler, all the electronic hardware (ECU, ABS, PSM, fuel pump) and the convertible top. I'm sure I missed a few things but thats allot of drain on a battery. 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.

I feel if you want to save some weight in your Boxster, go on a diet. Don't sacrifice you car.

No need to go on diet...I bought midtronics PBT-300 to test the battery just to avoid having to go through a diet. That way you'll know for sure how much electricity that your system eats and how much power that the battery has.

Porsche Tequipment charger also seem to do its job everytime I leave the car in the garage for more than 3 days. It gives me a peace of mind and I don't think I can live without it!

[vath2001]

Amazing! People want to install a light weight battery, then they install 200 pounds of stereo upgrades: amps, speakers, head units. Probably drive with the golf clubs in the trunk too.

Hey, go ultra light weight and Sears will sell you a lawn mower battery. It will start the car, but will be fried in 6 months... but you'll save weight.

[stephen wilson]

I agree, you'll never notice the weight difference. I didn't notice any change in lap times in my Formula Ford racer after losing 10 pounds. This car only weighs 1100# with driver, so that's equivilent to a 25# change on a Boxster. (And yes, I run competitive lap times, so it's not because I'm slow!)
Steve

[JackG]

Originally Posted by Jaxonalden

Think of your battery as a bucket of water and the alternator as a small facet. Now all of electrical components all take some "water" out of that bucket and the facet tries to keep it full. I don't know about your car but I have heated seats, Bose amp, electric windows/mirrors and seats, deployable spoiler, all the electronic hardware (ECU, ABS, PSM, fuel pump) and the convertible top. I'm sure I missed a few things but thats allot of drain on a battery. 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.

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.

[edevlin]

"Amazing! People want to install a light weight battery, then they install 200 pounds of stereo upgrades: amps, speakers, head units. Probably drive with the golf clubs in the trunk too."


To each his own. I like my stereo and a light car. My outboard amps, subwoofer, head unit and lightweight battery weigh less than the factory battery. That way I save a few lbs and still have the audio rig I want.

Saving weight does make our cars quicker and more responsive, but how much weight is necessary to lose, and what are each of us willing to go through to save it? I dont know, seems like I can notice 40-50 lbs weight savings on how the car handles, and I can easily feel 100 lbs. Try putting a 50 lb bag of dogfood in the passengers seat a see if you can notice a difference in performance.

If someone is autocrossing and has an older car, to put a lightweight battery in and if they lose their spare tire, thats about 50 lbs savings that most could feel. Is it worth the bother and expense, depends on the owner. Owning these cars is a hobby/passion for many of us, and messing around with them is part of the game.

Is it cost effective to try to increase performance by losing weight? Mostly it is not, a battery is probably the cheapest way to lose a fair amount of weight. Most of the ways available to save weight are quite expensive and would not make sense to many:

Lose spare-if you have one 0K 25 lbs

Light weight flywheel 1K l5 lbs

Full exhaust system 4K 25 lbs

Lightweight seats 2.5K 48 lbs

Lightweight battery <0.2k 20+ lbs


Looks like you could pretty much put a larger engine in the car for the cost of all of those mods. But hey, if thats what floats your boat, so be it.....


Ed

[vath2001]

If I was ever to race / autocross, I would make the car loose weight:

As small of a battery I could install - yes, a lawnmower battery
Remove the easily pulled carpet - including engine covers carpet (no tools req'd)
No spare or tools
1/4 tank of fuel - max
Only a drivers license and insurance card
One key
Pull the wind deflectors (un-clip)
Nothing in the pockets, doors or trunks
Remove the radio display

And I feel like wrenching: no Pass seat (4 bolts, need a star socket set)

The battery would take the most time to set up, everything else could be stripped in an hour. When you race, weight is everything.... if you don't race, set the car up to be comfortable. You have decide on which side of the track you want to live.

[stephen wilson]

True, as part of a greater weight loss program it would make sense. 50-100# is worthwile.

[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.