Does anyone know about how fast capacitors lose their charge?

 

If I go back about 35 years to my basic electricity classes simple capacitors loose their charge in what are called time constants (TC). A capacitor is considered completely discharged after 5 TC.

The actual charge lost in the first TC is 62.5% of the charge. Each consecutive TC drops 62.5% of the remaining charge.

So how much is one TC? The TC is a measure of time (hence time constant). Each TC is equal in time to the other TCs (hence constant). One TC is actucal a number calculated by resistance times capacitance. If either value is infinte the capacitor never discharges. So again how long is one TC? It depends on the size of the capacitor and the resistance.

Does any of this apply to super capacitors? I would assume it does, but that wasn't covered in my basic electricity classes.

 

From what I've read, they'll hold half their charge for about 24 hours or so, at least for the sort that were discussed for vehicle use. On the face of it, that sounds bad, but what you have to factor in is that batteries can only be trickle-charged, so you don't get much from regeneration. Ultracapacitors can usually accept a charge as fast as you can generate it. Imagine 100% regenerative braking. You could start off every morning flat and not care if it only took you the first three stops to be topped off.

 

IIRC, Time Constant refers not to the capacitor by itself, but to the capacitor discharging through a given resistance (or maybe it was impedence, i.e. taking into account inductance). In any case, TC is a property of a circuit, not a capacitor (if I'm remembering correctly, that is).

I would guess that the issue with a capacitor self-discharging--i.e. with an open external circuit ("open" meaning incomplete, i.e. switched off) is one of there being a leakage path internal to the capacitor, i.e. due to the fact there aren't perfect insulators. But I don't think TC refers to that.

 

The time constant is all about caps in circuits. A perfect cap would hold a charge forever out of a circuit.

 

Capacitors would be great for mild-hybrids, but the next step is plug-in hybrids, and batteries definitely have the higher ground here because they can store 20 times the energy (and hence range) as even the best ultracapacitors.

That is unless ECASS and EESTOR actually come up with the goods they claim to have.... Should EESTOR in particular deliver, that's pretty much the end of the gasoline industry.

 

I'm surprised that GM isn't going the capacitor route for BAS, since that system seems to be designed primarily as a start/stop system that also provides brief acceleration assistance. It seems that for this use, a capacitor-based hybrid system would really shine.

 

yes, most of you are mostly right....
the time constant is the time that a capacitor discharges to about 62% of its initial voltage, and it equals the capacitance in Farads times the effective load resistance across the capacitor in Ohms. One Ohm / 1 Farad = 1 second to lose about 2/3 of its charge.

has anyone said how many Farads of capacitance we're talking about here for these SuperCapacitors? and in real life, the supercaps would probably be disconnected from the system by a relay, so the discharge path resistance would be whatever the internal leakage resistance paths are, inside the supercaps. probably millions of ohms, if they're any good, but i have no real guess here. One million Ohms / One Farad = 1 million seconds. maybe ten days PER FARAD, and these puppies we're talking about here will probably be on the order of tens or hundreds, if not thousands of Farads. unless they leak like sieves internally, self-discharge won't be a problem.

now as for how fast to discharge or recharge, please don't jump to the assumption that piling all the amps from the M2 into these babies for ten minutes will recharge them. I=C*dv/dt. one amp will charge one farad at one volt per second.

you need to know how many amps the generator can be allowed to pump out during regen, and without any idea of how many Farads (or hundreds or thousands) we're talking about here, you can't know how long it would take to recharge one.

so, not knowing how many Farads we'll have, nor how fast they self-discharge, (although we probably can get numbers for generator output), it's not clear to me whether that part of the discussion makes any sense at this time at all.

wait 'til we get some real-world numbers.

but in the world of engineering, you're all right: the supercaps WILL take the load off the batteries for acceleration bursts and other high-current loads. a really good capacitor looks like a VERY low internal-resistance battery to the world around it, and a supercap with tons of Farads can start to look like a BIG, LOW-internal-resistance battery, so my bet is that some balance between battery and supercap capacity will be chosen, most likely based on pure economics, and the balance will shift as the supercap technology develops. (of course, don't forget that the battery folks, interested in selling batteries, won't be sitting on THEIR butts, either!, but that's what makes a horserace, and that's what benefits cars in general, and hybrids in particular! more "power" to us!)

there's more and more fun to come!

 

ps... from one of the links...

The product weighs 400 pounds and delivers 52 kilowatt-hours.
The batteries fully charge in minutes as opposed to hours.

um, guys,... if you want to charge that baby from zero to full, you STILL need to pump 52 kilowatt-hours of energy into it. that's 3120 KILOWATT-MINUTES. my HOUSE, with everything electrical in it turned on at one time draws MAYBE 25 kilowatts. So it would take two minutes to charge that baby with the equivalent of 14 times what my house uses with: both A/C units, the electric dryer, electric range, every light in the house, every fan in the house, and every tv, radio and entertainment system running, all the PCs, .....

of course, that's just two minutes worth, but how many kilowatt-hours do you burn every MONTH? no pun intended, but currently, i run about 30KWH a month in the winter and maybe 2-3 times that in the summer. at about $100 a month per 30 KWH.

start pumping 50 KWH into your car every how often? and you'd better start doing some math, making some extra money on the side, or start designing MUCH better propulsion systems for cars.

10? 20? horsepower to drive your car down the road and up some hills? multiply by over 700 to get Watts. 7000-14,000 watts to move the car? for every hour of drive time? hmmm... 52 KWH...$200? 7-14 hours of driving?

not in MY budget.

check my math, ok?
thanks!

 

You mean that you run about 30kwh a day, which runs about $100 per month (900 kwh per month, 9 cents per kwh). Correct?