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are these really capacitors? Answered


I'm only asking because either the manufacture doesn't know what they're talking about or it's a battery with it's capactance measured in farads. The say rechargable capacitor. That's the only reason why I feel causious


ok, new questions, how can you calculate the time it takes to charge a capacitor? Is there an easy way to reduce the capactence without adding another capacitor? I'm assuming that the answer of my question is probably no.

thanks for the link, I just used the calculator on the link

Ok, glad it was helpful. I had someone else tell me it was beyond them, but I forgot to whom I showed it *sigh*

Just remember that the time constant is equal to R times C.

Data sheet says these have an internal resistance of 70 Ohm, so the time constant would be around 5 seconds.

Note that this is also the time constant for the *discharge* of the capacitor. The relatively high internal resistance means that these guys aren't as well suited for applications where you need a very rapid discharge, such as a flash capacitor.

In comparison, there are some "ultracapacitors" with a capacitance in the single or multiple digit Farad range. Here's a doozie:

Maxwell Technologies BCAP3000

3000 Farad (yes- you read that right!), with an internal resistance of 0.37 milliOhm. This puppy will discharge in just a bit over a second, and can easily kill you...

By the way, don't forget that a capacitor is only charged about 63% after one time constant (V*(1-e-1/RC)

0.003 MF ! At 2.7V, I think "EASILY kill you" is a bit of an exaggeration. 0.37mOhm internal resistance it may have, but YOUR internal resistance is still like 1kOhm, not counting skin that adds another 10k+ The company datasheet for the original cap in question lumps together a bunch of "memory backup technology" in a single datasheet, including primary batteries, rechargible batteries, and ultra-capacitors. An odd way of documenting your product, perhaps. The part in question is one of the capacitor-style products.

I was considering to add the caveat that 2.7V in itself isn't that dangerous - but then I figured that at this level of energy release, there are other ways to get killed, rather than straight-out electrocution.

Imagine a short-circuit with a piece of wire... you could get a current spike of more than 7000 Amps, or close to 20kW of instantaneous power. Plus these types of caps are typically used in power applications with power multipliers or multiple caps in series... They're not quite in the same league as some of the high-voltage caps (energy storage is proportional to capacitance, but quadratic with voltage), but still something to be treated with a healthy dose of respect...

I honestly think because they are using the cap as a backup (instead of a battery) that this is the reason they word it this way....although I can't be sure.

ok, thanks guys, I was just not sure because the say "rechargeable capacitors" What kind of manufacturs says that! it's like saying... uhh... I can't think of a good analogy, but why would they state the obvious like that? It'd only cause confusion

all capacitors are rechargable..... It seems like a capacitor. Capacitance is measured in farads, but generally capacitors don't have that much capacitance, so it's measured in micro farads. I'd say it's legit!