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BrianG · 12.22.2008, 03:13 AM

Testing caps is pretty easy actually, and you can even verify the capacitance value.

1: Make sure it is totally discharged to 0v. Those large caps will still develop a charge if left alone for a little while, so once it is discharged, hook the leads together (short them) for about 10 minutes.

2: Hook a voltmeter across the cap.

3: Get a known resistor value. Something like 10 ohms @ 10w would work. Measure this with a meter to get an exact value.

4: Charge the cap through the resistor. Time how long it takes for the voltage to get to 63.2% of the supply.

5: Then, take this time (in seconds) and divide by the resistor value. This is the capacitance.

So, let's say it took 3 seconds for a cap to get to 9.1v on a 14.4v supply (63.2%) charging through a 10 ohm resistor. 3 seconds divided by 10 ohms is 0.3, which is 300,000uF.

The "rule of thumb" is to have 1F per 1000w of power. But, a cap will only help so much if the electrical system is weak. Headlights are around 100w each, the heater/blower motor pulls current, not to mention the computer, ignition, headunit, battery charging, etc. The electrical system has to supply all these devices PLUS your 600w system. And class AB amps are around 65% efficient, so the amp is actually requiring around 923w of power. At 14.4v, that's 64A.

Now, the dimming you're seeing is because lights get quite a bit brighter with just a little voltage. So, your alternator is supplying ~14.4v, but then when a bass note hits hard, that voltage drops to, say, 13v. 1.4v difference is pretty noticeable on lights. You probably notice the dimming less when the car is off because the static voltage is at the battery voltage (~13.8v) and batteries can handle pulse loads a little better.

Some people just add better/more batteries, but a battery is at ~13.8v. The alternator is still providing that extra ~0.6v, and if it is already loaded with stuff, the voltage will still dip. You can get an aftermarket high output alternator, but they can be costly. And most people don't realize that a stock alternator actually puts out more current at idle than a high output one. So, at a stoplight, you'll probably still dim.

In short, I wouldn't worry too much about the dimming. You aren't going to eliminate it since you'll always have voltage drop at high currents. But, if the dimming is very bad, it could be a sign of an overworked alternator...

	(#45)
BrianG RC-Monster Admin Offline Posts: 14,609 Join Date: Nov 2005 Location: Des Moines, IA	12.22.2008, 03:13 AM Testing caps is pretty easy actually, and you can even verify the capacitance value. 1: Make sure it is totally discharged to 0v. Those large caps will still develop a charge if left alone for a little while, so once it is discharged, hook the leads together (short them) for about 10 minutes. 2: Hook a voltmeter across the cap. 3: Get a known resistor value. Something like 10 ohms @ 10w would work. Measure this with a meter to get an exact value. 4: Charge the cap through the resistor. Time how long it takes for the voltage to get to 63.2% of the supply. 5: Then, take this time (in seconds) and divide by the resistor value. This is the capacitance. So, let's say it took 3 seconds for a cap to get to 9.1v on a 14.4v supply (63.2%) charging through a 10 ohm resistor. 3 seconds divided by 10 ohms is 0.3, which is 300,000uF. The "rule of thumb" is to have 1F per 1000w of power. But, a cap will only help so much if the electrical system is weak. Headlights are around 100w each, the heater/blower motor pulls current, not to mention the computer, ignition, headunit, battery charging, etc. The electrical system has to supply all these devices PLUS your 600w system. And class AB amps are around 65% efficient, so the amp is actually requiring around 923w of power. At 14.4v, that's 64A. Now, the dimming you're seeing is because lights get quite a bit brighter with just a little voltage. So, your alternator is supplying ~14.4v, but then when a bass note hits hard, that voltage drops to, say, 13v. 1.4v difference is pretty noticeable on lights. You probably notice the dimming less when the car is off because the static voltage is at the battery voltage (~13.8v) and batteries can handle pulse loads a little better. Some people just add better/more batteries, but a battery is at ~13.8v. The alternator is still providing that extra ~0.6v, and if it is already loaded with stuff, the voltage will still dip. You can get an aftermarket high output alternator, but they can be costly. And most people don't realize that a stock alternator actually puts out more current at idle than a high output one. So, at a stoplight, you'll probably still dim. In short, I wouldn't worry too much about the dimming. You aren't going to eliminate it since you'll always have voltage drop at high currents. But, if the dimming is very bad, it could be a sign of an overworked alternator...