how fast should a cap leak charge; is this right??

[BerniniCaCO3]

I just purchased a 1.8F capacitor to use to supply the instantaneous huge draw for an electric fan that I've added to my car, replacing the ailing clutch fan. I was wondering how to charge it, and if it's supposed to leak charge as quickly as it seems to even when wholly disconnected from any circuit?




I just got a brandy new capacitor in the mail yesterday, was going to install it tomorrow, decided to charge it today. The model is a Pyle Audio "PLMRCAP18 1.8 F 20V Round Marine Capacitor" bought from amazon.

It IS a bad sign that in several places on the package and on the capacitor itself, it calls itself a "maring" capacitor instead of a "marine" capacitor....

Anyway.
I charged it as directed: I hooked it up to my battery, in parallel, (positive to positive, neg. to neg.) with the resistor in series, which as I understand, limits a current spike especially when the capacitor is at 0V and 0ohms resistance and the battery is at 12.5x volts.

It very rapidly climbed to 10.2x volts and stopped there, presumably the resistor was consuming 2.3v.
At this point I took the resistor out and hooked the battery up directly, and the capacitor instantly came to equilibrium with battery voltage.

Now, I unhooked it, thinking it fully charged. I decided to check the voltage with my own voltmeter. It was , really just a few seconds after unhooking from the battery, now at 10.xx volts. I held the voltmeter to it, and watched the voltage drop quickly, to 9.xx volts, 8 volts... Just in case the voltmeter itself was consuming all the power this 1.8Farad capacitor could supply, I let it be and came back in 5 minutes. At this point it was down to .73 volts and holding steadyish.

So at the very least, open-circuit, NO load WHATSOEVER, terminals not connected in any way, my capacitor dropped of its own accord from 10 volts to less than 1 in between voltmeter measurements just minutes apart.
This makes the idea of precharging a capacitor absolutely laughable; it will discharge itself to .7 volts before I've even half finished soldering its connections!

Is this normal? For capacitors to leak this quickly internally? Or should that capacitor have held some charge for at least a few minutes... if not days, frankly, given a claimed 1.8Farads.

I'm fearing that there's an internal short that is both consuming energy, and perhaps seriously castrating the actual capacitance to be much less than 1.8F

But I thought I'd check with you guys first before I cry for my money back, just in case this is normal behavior, seeing as I'm really not experienced working with capacitors.
And if I do need to return this, what brands/models have you found highest in quality and most durable?


....another option that will achieve the same goal (of not loading the engine with a current spike to the alternator!), instead of accomplishing the spike with a capacitor, is to delay and smooth the spike with a homemade inductor.
Of course, then I'd have to wind up my own; whereas capacitors come factory-made.

[indianajo]

I've got a 30 year old 10000 uf mallory computer grade capacitor, that when I charge it to 9 V, loses about 6 millivolt a second. This takes a long time to discharge.
Car systems usually use the lead-acid battery to take spikes of current draw. The exception is car audio systems where the inductance of the wiring from the battery is so significant at the frequencies involved, so huge capacitors have been developed.
You can measure the capacitor leakage by determining what resistor it takes to hold your capacitor to about a steady voltage less than your power supply, and useing v=i/r to determine the current. The v is the voltage across the dropping resistor.
I put an electric fan on my 59 Ford to replace the belt drive fan, because the 1984 motor is too close to the radiator. I just wired the fan up with 16 ga wire, and a fuse, to a switch under the dash. (It is only needed in the summer). I installed a 71 accessory relay around my 59 ignition switch, as I have several other electrical accessories not invented in 1959, like halogen headlights, electric windshield wipers, and electric defroster. I also replaced the 25 amp generator with a 1971 65 amp alternator. The electric fan does not make the alternator belt chirp. (supposed to have dual belts, I only have drive pulleys for one).

[Mooly]

On the question of the capacitor... the value has no bearing on how long it "holds charge" on a perfect cap.

What is relevant is the internal leakage and actually smaller caps tend to be better but that's no help to your problem. I suspect that any ultra large cap will have a very high leakage current tbh but not as much as you are experiencing. That sounds faulty/fake whatever.

Do you really need it anyway ? A 12 volt car battery is pretty robust as regards high surge loading. The terminal voltage shouldn't drop too much and then only for a few milliseconds.

[simon7000]

As a super cap it is bad. It should hold a charge for weeks. As a car audio aftermarket boost capacitor it is consistent with that use.

I once bought a pair of three way auto aftermarket loudspeakers. The woofer worked. The midrange had wires to it's terminals but there was no voice coil. The tweeter had no wires attached. Of course there was no crossover. They sold lots of them to the auto sound guys! No HF hearing left, no test equipment and plain cheap.

[DF96]

Maybe a 'Chinese' copy?

[dgta]

The whole idea is wrong. The battery in your car is already a charge storage device of many F. If wired properly with the appropriate gauge wire and solid connections, the battery can supply short spikes of hundreds of amps to the fan. You don't need a cap or anything else. Just make sure whatever controls the fan (switch or thermoswitch) can handle the current and spikes.