Adding a capacitor, except not for audio.

[ciper]

Ive always wondered:

Car audio has becom so popular that you can get 2 farad 20 volt capacitors for under 100$. Smaller units, like .5 farad can be had for 25$ or so if you look around.

Having a steady supply of power regardless of engine speed for the ignition and ecu should provide better performance no? Cranking the starter over also should be stronger and produce less strain on the battery right?

I think Im going to add one, even if I rarely listen to anything but AM radio. What do you guys think?

[vrg3]

I don't think you'll see much performance increase. The ECU's stuff all has a high power supply rejection ratio because it runs off a well-regulated 5-volt supply. Plastic-housing MAF sensors don't share this reference with the ECU but they do internally also have a 5-volt supply with a high power supply rejection ratio.

I suppose if the battery and alternator are somewhat lacking, it could help with ignition, particularly at idle. I can't say for sure because I don't know much about ignition dwell times on our cars.

It won't hurt anything, so if you're so inclined I say go ahead and give it a shot. Remember to discharge the cap if you ever try to reset a computer by disconnecting the battery, of course.

[ciper]

Well, think about it this way. If you can get it for 25 dollars and it could only help (if not just for music and starting) I think its a good thing. I know that low power to the starter is what usually damages it on other vehicles (like if the ground is corroded).

I know to discharge, I was thinking to just lick it everytime

[Yukonart]

I'm just trying to figure out where you'd wire it in if you want the starter to benefit from it. I know that power will flow both ways from the cap, but if the battery is in the way of the starter, won't the battery get a charge back from the cap once the voltage drops?

I don't know all this 100%, but I've been paying attention to the things my very experienced audio installer tells me.

[evolutionmovement]

Are you experiencing a starter issue? They seem very rare in Subarus. My starter's original @ 237k.

I like to use metal handled tools to discharge big caps.

Steve

[ciper]

No, I just dont like killing the battery every time.

Starter issues might be rare, but not starting issues. Look at the relay problems people have had.

[evolutionmovement]

I must be extremely lucky with my electrical issues, then. I've never had any trouble with anything. I even replaced what I suspect was the original battery (Subaru - how many people get them at the dealer?) about 2 yrs ago. I did have to replace a transmission, though (mostly my fault anyway).

Steve

[vrg3]

Art - I'm not sure what you mean about power flowing "both ways." But the way I would imagine wiring the cap would be in parallel with the battery. That way the cap is essentially bypassing the battery power supply.

ciper - Think about the amount of power it takes to start the engine, though.

The factory service manual gives three specs for the starter motor's draw:
Unloaded: 11 volts, 90 amps, 3350 rpm.
Loaded: 8 volts, 370 amps, 10 ft-lb, 900 rpm.
Stall: 5 volts, 800 amps, 20 ft-lb.

(These are for the automatic, which I presume you have, ciper. )

I would guess that starting the engine requires similar amounts of energy as holding the starter motor loaded for 1 second. The energy required, then, is 8 V * 370 A * 1 s = 2960 J

So, now, how much capacitance would you need if you wanted the cap to supply all the starting power? Assuming the battery is healthy, it would have charged the capacitor up to about 12.8 volts while the car sat.

2960 J = [C * (12.8 V)^2]/2
C = 36.1328125 F

So a half-Farad cap could supply roughly 1.4% of the energy required to start the engine. I don't think that would even be noticeable.

But like you say, it could only help and it's fairly cheap. Since your in California though I'd say spend the $25 on a solar charger instead.

Manuals are also easier on the battery to start, so maybe you should convert to a 5-speed? (kidding!)

[Guest]

Ive always wondered:

Car audio has becom so popular that you can get 2 farad 20 volt capacitors for under 100$. Smaller units, like .5 farad can be had for 25$ or so if you look around.

Having a steady supply of power regardless of engine speed for the ignition and ecu should provide better performance no? Cranking the starter over also should be stronger and produce less strain on the battery right?

I think Im going to add one, even if I rarely listen to anything but AM radio. What do you guys think?

Capacitors are not small batteries. They are useful for car stereo applications because they can discharge considerable amounts of energy in a very short time, like when the bass hits and the amp suddenly draws amperage faster than the alternator can compensate for it. Discharging a capacitor happens in an extremely short period of time. They don't have the endurance for cranking an engine or anything like that.

However, they are sometimes used in parallel with motors for EMI supression purposes, or on larger motors to help them through their initial startup.

Cliff's Notes: Putting a capacitor in an automotive circuit that is not experiencing small intermittant losses of current is not useful.

[ciper]

I understand the function of a capacitor normally, and "on larger motors to help them through their initial startup" was one of my goals.

The time between tooth engaugement and whent the crank starts to turn is a larger draw than when the crank starts to turn and the engine fires up.