2008 Chevy Silverado Electrical Load

Tiny
JKBEAUMONT1
  • MEMBER
  • 2008 CHEVROLET SILVERADO
  • V8
  • 4WD
  • AUTOMATIC
  • 12,000 MILES
My '08 Silverado X-Cab has a OEM 145 Amp alternator and an AC Delco 7641 battery. I only have the single battery although there is space for an additional battery. How do I determine the maximum load with this OEM setup in watts? Is it simply by taking the max voltage (I believe it to be 15.5V @ 90% commanded duty cycle) and multiply it by the rated alternator max which is 145 Amps? That would give me 2,248 watts. Is that correct?

The reason I ask is I have 1000 watts RMS car audio in the truck and am looking at a Kinetik car audio battery to replace the AC Delco battery. Based on my calculations, I would need to take into account the 2,248 watts + 1,000 watts which means I need a battery that can handle 3,248 watts. Does that sound right?

Thanks!


http://www.2carpros.com/forum/automotive_pictures/322095_Battery_1.jpg

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Sunday, June 7th, 2009 AT 6:18 PM

11 Replies

Tiny
PEAR69
  • MEMBER
In order to be safe you should measure the running amperage of the truck to be safe. Also measure the running voltage of the truck at the battery with a high impedance volt meter. A simple formula to use is P=I X E -- P=watts, I=amperage, E=voltage.

Also, just adding another battery brings you to another issue that MUST be considered. The alternator. A larger (amperage wise) battery will require a larger (amperage wise) alternator.
Always subtract 5% off the value of the alternators capacity for temperature differences. The higher the temp. Under the hood, the harder an alternator must work. This means that you have in actuallity an alternator of 137.75amp capacity. Without knowing the actual voltage/amperage readings, I would say increasing the alternator size to 225 amps will be where you will get the best performance without any issues.
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Sunday, June 7th, 2009 AT 11:37 PM
Tiny
JKBEAUMONT1
  • MEMBER
I'm not 100% sure you answered my questions.

I'm not looking at adding a second battery, I am simply looking to determine if a larger battery to replace the current AC Delco OEM battery would make sense.

How do I perform a load test to determine if the present alternator is adequate? Is this something that I can do or do I need to bring the truck to my mechanic? I am assuming that a 145 Amp alternator should be sufficient considering I only have a 1000 watt RMS car audio system? Or is this alternator matched only for the vehicles OEM electrical system?

I'm trying to figure out if the current 145 Amp alternator is up to the task. Voltage at the battery varies depending on the load, but it is typically around 14.5 to 15 volts. Charging does not appear to be an issue at present. However, I believe that a volatage measurement either at the battery or at the alternator isn't going to tell me the whole story. The only issue I notice is some dimming of the headlights when the audio is cranked up and big bass notes hit. Again, my subwoofer amp is 600w RMS X 1, so it's not huge.

Some have suggested a larger battery as the next step, while others suggest a high output alternator. I'm just trying to determine which next step may be the best choice.

Thanks.
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Monday, June 8th, 2009 AT 7:04 AM
Tiny
PEAR69
  • MEMBER
Add a "bigger" battery or adding another battery is just about the same thing. I understand that you just want a bigger battery.
The alternators' job is to charge the battery - not to run the electrical system of the truck. You are correct in thinking that you need a larger battery. But realize that you are dealing with a very small voltage. A.5 voltage drop can mean a larger voltage demand. If you get 14.5 volts with everything running - at the battery that is good.
Use the formula I gave you to figure out the amperage needed to run your system
Assume that the 2248w + 1000w = 3248w figure is correct. If you are looking for the amperage (or I in the formula) you will divide 3248 by 14.5 = 224 amps. The formula P=I times E is what is known as OHMs Law of Power. In other words you need an alternator capable of handling 224 amps. And a battery capable of handling around 200 amps or so -- the bigger the battery the better -- Adding another battery may be cheaper that getting a giant one. You can manipulate the formula how ever you want, it is an Algebra formula.

BTW - If you decide to add a battery remember to hook them up in Paralell NOT series! If you do you will get 30 volts.D/C voltage in series adds and in parallel it stays the same AND D/C amps is parallel adds and in series is the same. This is NOT difficult and I am sure you can do it. Purchase a digital voltage/amperage meter capable of measuring D/C voltage and you'll be set. Let me know if this helps.
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Monday, June 8th, 2009 AT 1:44 PM
Tiny
JKBEAUMONT1
  • MEMBER
OK, I understand about wiring a second battery in parallel. I do the same for my subs so I understand the concept.

However, are you saying that if I add another battery my 145 AMP alternator is adequate or I should also upgrade to a high output alternator, either 200 AMP or 250 AMP? And if I do need a HO alternator, do I use the same pulley and such or do I need a different pulley?
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Monday, June 8th, 2009 AT 4:15 PM
Tiny
PEAR69
  • MEMBER
Yes. You need a 250 amp alternator and a bigger battery -- or -- one or two more batterys.
Remember -- the alternator charges the battery (no matter how big the battery is - amperage wise). The alternator must be capable of supplying enough amperage to charge the battery. The battery runs the electrical systems in your truck. The battery must be big enough to run everything - especially an awsome sound system! If the battery is not charged (or fed) it will die.

There are all kinds of 250 amp alternators out there for trucks and cars. Most of them are direct bolt ons and come with a matching pulley for your truck -- if not don't buy it - find one that has a warrenty and bolts directly up with no hassles. DB Electric Alternators is a place to start looking. ROCK ON
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Monday, June 8th, 2009 AT 7:06 PM
Tiny
JKBEAUMONT1
  • MEMBER
Any idea what the wattage rating is on the AC Delco 7641? I cannot find specs on this battery anywhere. If I upgrade to a car audio battery (Kinetik, Optima, Batcap, etc.) These batteries are all rated in watts. I know I can "calculate" what the wattage rating should be for the 7641 (2,135W) but I'd like to know what AC Delco specifically says it is rated for. Is there somewhere where I can get a full set of specs on this battery?
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Wednesday, June 10th, 2009 AT 8:17 AM
Tiny
PEAR69
  • MEMBER
That battery is a 70 amp battery. If you use the formula I gave you twice - you will come up with 1015 watts
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Wednesday, June 10th, 2009 AT 1:10 PM
Tiny
JKBEAUMONT1
  • MEMBER
So, will my 145 Amp alternator charge any battery up to 145 Amps or is there a calculation to determine how big of a battery (nnn Ah) my stock alternator can handle?
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Wednesday, June 10th, 2009 AT 1:12 PM
Tiny
PEAR69
  • MEMBER
In theory yes but I do not suggest that. If you make the alternator work at full capacity all the time if will fail prematurely. I would not go over a 125 amp battery on a 145 amp alternator--here is why -- The temperature under the hood is always variable. It can get over 300 degrees. The higher the temperature under the hood - the harder all the electrical components have to work--especialy the alternator.
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Wednesday, June 10th, 2009 AT 1:38 PM
Tiny
JKBEAUMONT1
  • MEMBER
Awesome, thanks. I have definitely learned a lot. I appreciate all of your help! Great forum! :-)
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Wednesday, June 10th, 2009 AT 1:42 PM
Tiny
PEAR69
  • MEMBER
Just remember P = I x E -- (or)-- watts = amperage X voltage.
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Wednesday, June 10th, 2009 AT 2:34 PM

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