# batteries or caps?

sedate
Silver - Posts: 1,173
Joined: July 03, 2004
Location: Colorado, United States
Posted: March 25, 2008 at 7:47 PM / IP Logged
KP wrote:
 But, as you draw current away from the batteries at a rate faster then the alternator can charge you'll stress the alternator.  Heaven forbid you leave the lights on for 30 mintues with the car off (or listen to the radio with the car off).Now, your alternator has 3x the load to charge back up, which will make it work 3x harder.  It's simple math if you have a 100 amp alternator and a 150A load eventually you're going to have problems.

WHAT?!?!

None of this makes ANY sense.

From this logic a 40watt bulb put to three seperate batteries draws the same amount of current from each battery that it would require from a single battery!?!?!

Like you say - simple math - now I'm REALLY confused. . . . .

"I'm finished!" - Daniel Plainview
KPierson
Platinum - Posts: 3,527
Joined: April 14, 2005
Location: Ohio, United States
Posted: March 25, 2008 at 7:49 PM / IP Logged

Hey!  When did I get an "I"????

When explaining theories you always try to assume everything is identical.  The drain (or bleed) characteristics of the batteries may be different which may make a small battery that can't hold a charge for very long appear to be leeching off a large battery with a long storage capacity.  If you have to identical batteries you can assume they will discharge at the same rate and you can assume that the voltage will drop from each battery at the same rate, so no leeching effect will be present.

The ESR of the system will be reduced by wiring the batteries in parallel, so you might actually see an advantage in that department.  The ESR deals with resistance of components in series - well when you double the capacity of the storage devices the resistance will drop (what happens if you wire two 8 ohm sums in parallel - you get 4 ohms).

Let me try to explain what I was getting at a little better above.  If you have a battery at 14vdc and another battery at 10vdc and you wire them in parallel you will have a 4vdc difference in potential.  The 14vdc battery will power the 10vdc battery.  As the 10vdc battery receives current from the 14vdc battery its voltage level will rise, while the voltage level of the 14vdc battery drops.

Eventually they will settle at ~12vdc, at which point there is no more potential (12vdc - 12vdc = 0vdc), and with out potential current can't flow, so you'll have the equivalent of an open circuit.

Kevin Pierson
KPierson
Platinum - Posts: 3,527
Joined: April 14, 2005
Location: Ohio, United States
Posted: March 25, 2008 at 7:59 PM / IP Logged
sedate wrote:
KP wrote:
 But, as you draw current away from the batteries at a rate faster then the alternator can charge you'll stress the alternator.  Heaven forbid you leave the lights on for 30 mintues with the car off (or listen to the radio with the car off). Now, your alternator has 3x the load to charge back up, which will make it work 3x harder.  It's simple math if you have a 100 amp alternator and a 150A load eventually you're going to have problems.

WHAT?!?!

None of this makes ANY sense.

From this logic a 40watt bulb put to three seperate batteries draws the same amount of current from each battery that it would require from a single battery!?!?!

Like you say - simple math - now I'm REALLY confused. . . . .

Not at all.  The lightbult will draw the same amount of current regardless of the device feeding it.

Say you have one battery with a storage capacity of 50A (not even the correct way to rate a battery, but it makes it much easier).  Say you have an alternator thats rated at 150A.  Now, assume the car and the radio together, at max load is going to draw 175A.  At first, you will be fine, as you have the 150A the alternator is providing and you have the reserve capacity of the battery (50A).  But, eventually you'll drain the battery, and you'll be left with only the 150A the alternator can provide.  At that point the car starts hesitating, the radio starts cutting off, etc. so you turn the radio off, dropping the load on the system to 100A.  However, in addition to the load of the car the battery, being drained, is now a load and will consume 50A to charge back up, giving you a 150A load on the alternator.

Now run the same scenario but increase your storage capacity of the battery by 3 (to 150A).  Sure, the system will play longer as it will take longer to drain the electrical system, but once you do drain it you now have to charge up a 150A load (three batteries in parallel) PLUS power the car.  So, at this point, even with the radio off you have the 100A that the car needs to function and you have the addional 150A that the batteries require to charge.

The point is that a battery that has less potential then the output of the alternator will be a load on the alternator.  The bigger the battery (ie the more storage capacity) the more power its going to take to charge it.

I'm trying my best here to explain it!

Kevin Pierson
sedate
Silver - Posts: 1,173
Joined: July 03, 2004
Location: Colorado, United States
Posted: March 25, 2008 at 8:03 PM / IP Logged

KP wrote:
 Eventually they will settle at ~12vdc, at which point there is no more potential (12vdc - 12vdc = 0vdc), and with out potential current can't flow, so you'll have the equivalent of an open circuit.

Okay - so by ur logic the idea of identical batteries is irrelevant - b/c they always 'equalize' - so why did you parentheically state "if they are idenctical" . . . ?

KP wrote:
 I'm trying my best to explain it!

You'll be digging deep before I'm done with ya. . . . .

"I'm finished!" - Daniel Plainview
KPierson
Platinum - Posts: 3,527
Joined: April 14, 2005
Location: Ohio, United States
Posted: March 25, 2008 at 8:37 PM / IP Logged

To cover my butt.  If someone mixes a Red top with a Yellow top I have no idea what would happen, as they use different cell technology.  Manufacturers are always saying that they don't recomend mixing battery types in an installation, and that is all I was trying to do.  Whenever using multiple batteries it is good practice to use identical batteries.

That, and who would ever buy two identical brand new batteries and try to prove me wrong?  :)

Kevin Pierson
jvillefinest
Copper - Posts: 255
Joined: December 29, 2003
Location: Florida, United States
Posted: March 26, 2008 at 2:33 PM / IP Logged
Either i have been lucky or have proven alot of people wrong. I have bought all three batteries at different times and yes mixed Starting cell and deep cell technologies and have yet to have a problem. All ran off of a STOCK ALT from 2001.
And as far as leaving a light on it would be no problem...just jump the front battery for about 20 sec and it starts right up. the PAC200 keeps them all separate and happy!
So my vote again Batteries. Unless you drive a car with an 80amp alt. Then your in a bigger problem. But in no Serious car audio system should thier be caps! not even for lighting or effect...if you want to do that get some LEDs off ebay for \$20 and wire them in! ;)
2007 Acura TSX
SQ setup in the works
KPierson
Platinum - Posts: 3,527
Joined: April 14, 2005
Location: Ohio, United States
Posted: March 26, 2008 at 3:28 PM / IP Logged

So you do have an isolator?  The isolator will prevent the alternator from seeing the entire load at any given time, which will reduce the loading effect, but require longer time to fully charge all batteries.

Do you have a single PAC200 isolating your main battery from your two additional batteries?

Kevin Pierson
atl530i
Member - Posts: 2
Joined: December 14, 2007
Location: Georgia, United States
Posted: March 26, 2008 at 4:18 PM / IP Logged
Hi,
Interesting thread. I've been doing high end stereos on boats for about 2.5 years now and we have been using deep cycle and starting batteries on boats without any problems. We have a PAC200 in there to keep them separate when the boat is in a cove (and off) or something so the boat starts without any problems. The amps are ran off the deep cycle battery while the rest of the boat remains on the regular battery. My understanding is that if two batteries are used that the battery with the lower voltage will make the battery with the higher voltage drop a little. We have never had problems with batteries killing each other though because they are of different cell types.
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