HO alternator vs. Second Battery
07-17-2008, 08:58 PM
#1
50 Watt CAFz'r
Thread Starter
Join Date: May 2007
Posts: 384
HO alternator vs. Second Battery
I have a 93 civic with a 70 amp alternator. Right now the alternator is producing too much voltage( up to 20 volts) I suspect that my 900 watts rms stereo system(mtx tc 4001 front chan. @ 4 ohms rear chan. @ 2 ohms, mtx mxa6001 A 2ohms) is to blame. So I decided to get a HO alternator, but the closest guy that does it is in brampton a few hours drive. After doing some homework I found out that added a second isolated battery may provide better protection. Does anybody have any advice, pros or cons and what would you do? HO aternator, Second battery or both? I have already upgraded my battery and wireing for those who will ask.
08-12-2008, 02:15 AM
#6
Think about it like this...
Power is ONLY drawn from the batteries when the system voltage (normally 14.4v) drops below the battery voltage (12.8v).
Your alternator puts out UP TO 70A.
At idle your alt may be putting out about half of that, so let's say 40A.
Your only chance of getting anywhere near 70A is when driving (1500+ engine RPMs).
Most cars are setup to load the alternator at about 50% with all accessories running (A/C, factory stereo, headlights, rear defroster, etc.).
Most amp outputs are rated with a 14.4 volt input. A fully charged battery can only deliver about 88% of that.
Class AB amps are up to 75% efficient. Class D amps are up to 95% efficient.
Here's a scenario for you...
So you start up your car, and your alternator kicks in to start replacing the power that was just ripped from it from the starter. The car's burning hot inside, so you flip on the AC to cool it down while you sit fiddling with stuff on your passenger seat. Since you're idling you've only got 40A to play with, and your AC, battery and engine are pulling in, let's say, 30A of juice, so your alternator is running about 75% load.
You shift into gear and take off, so now you're up to 70A, and the alternator drops down to around 45%. You realize you have no music playing, so you flip on the deck, and start cranking it up. Being a class D amp, at 600W (full power), it starts to draw 55A, because you decide to play some Ludacris track with a constant rumble in the back.
Suddenly, you're pulling over 85A, and your alternator doesn't have the power to supply it, and the voltage is starting to dip below 12.7 volts. At this point, power starts to flow OUT of the battery, instead of IN from the alternator. 15A is now draining out of the battery every second, and charging has stopped. Not that big of a deal just yet, because the battery has alot of juice in it, but since the voltage has dropped down so low, your sub output also suffers.
528W is about the most your amp can put out at 12.7 volts, so you're already suffering thump at this point. If you went with a second battery, this is the point it would finally start kicking in, splitting the 15A overload across two batteries. Not really helping anything, because once you're not overloaded, they BOTH are going to draw juice to recharge.
So you pull up to a red light, and stop, so you're back to idle. Now that your alternator RPMs have dropped, you're only putting out 40A once again, and you're now 45A over what your alternator can put out, so once again, it starts coming out of the battery, and the voltage continues to drop.
Depending on the condition of your battery, it determines how long it would take for the battery to be drained below what it takes to start the car, but if you sit in a traffic jam long enough and the car stalled from getting overheated, you have to wonder... Will the battery have anything left to turn the car back on?
Light turns green, you take off, the RPMs jump back up, but now since your battery is discharged even more, it's got even less to push out to your amp, and your A/C and amp are still pulling in all the juice from the alternator. You go back to pulling 15A from your battery every second.
You decide, enough of the AC, it's not too bad outside, so you flip it off, and down the windows. That frees up 20A. You're now drawing 65A, and have 5A free to recharge the battery. Since you've been drawing over 15A per second for the past 5 minutes, it will take over 15 minutes for the power to be put back in at this rate.
...Do you see where I'm going with this?
Power is ONLY drawn from the batteries when the system voltage (normally 14.4v) drops below the battery voltage (12.8v).
Your alternator puts out UP TO 70A.
At idle your alt may be putting out about half of that, so let's say 40A.
Your only chance of getting anywhere near 70A is when driving (1500+ engine RPMs).
Most cars are setup to load the alternator at about 50% with all accessories running (A/C, factory stereo, headlights, rear defroster, etc.).
Most amp outputs are rated with a 14.4 volt input. A fully charged battery can only deliver about 88% of that.
Class AB amps are up to 75% efficient. Class D amps are up to 95% efficient.
Here's a scenario for you...
So you start up your car, and your alternator kicks in to start replacing the power that was just ripped from it from the starter. The car's burning hot inside, so you flip on the AC to cool it down while you sit fiddling with stuff on your passenger seat. Since you're idling you've only got 40A to play with, and your AC, battery and engine are pulling in, let's say, 30A of juice, so your alternator is running about 75% load.
You shift into gear and take off, so now you're up to 70A, and the alternator drops down to around 45%. You realize you have no music playing, so you flip on the deck, and start cranking it up. Being a class D amp, at 600W (full power), it starts to draw 55A, because you decide to play some Ludacris track with a constant rumble in the back.
Suddenly, you're pulling over 85A, and your alternator doesn't have the power to supply it, and the voltage is starting to dip below 12.7 volts. At this point, power starts to flow OUT of the battery, instead of IN from the alternator. 15A is now draining out of the battery every second, and charging has stopped. Not that big of a deal just yet, because the battery has alot of juice in it, but since the voltage has dropped down so low, your sub output also suffers.
528W is about the most your amp can put out at 12.7 volts, so you're already suffering thump at this point. If you went with a second battery, this is the point it would finally start kicking in, splitting the 15A overload across two batteries. Not really helping anything, because once you're not overloaded, they BOTH are going to draw juice to recharge.
So you pull up to a red light, and stop, so you're back to idle. Now that your alternator RPMs have dropped, you're only putting out 40A once again, and you're now 45A over what your alternator can put out, so once again, it starts coming out of the battery, and the voltage continues to drop.
Depending on the condition of your battery, it determines how long it would take for the battery to be drained below what it takes to start the car, but if you sit in a traffic jam long enough and the car stalled from getting overheated, you have to wonder... Will the battery have anything left to turn the car back on?
Light turns green, you take off, the RPMs jump back up, but now since your battery is discharged even more, it's got even less to push out to your amp, and your A/C and amp are still pulling in all the juice from the alternator. You go back to pulling 15A from your battery every second.
You decide, enough of the AC, it's not too bad outside, so you flip it off, and down the windows. That frees up 20A. You're now drawing 65A, and have 5A free to recharge the battery. Since you've been drawing over 15A per second for the past 5 minutes, it will take over 15 minutes for the power to be put back in at this rate.
...Do you see where I'm going with this?
and one more vote to get a new bigger alt.
