Ultracapacitors vs. Batteries... 3,2,1 FIGHT !!!

[xboogerx]

I was wondering if I could use Ultracapacitors as a replacement part for a battery setup? Aren't Ultracapacitors pretty much just better battery like things?

[pomprocker]

where would they get their charge from?

[izua]

Same place batteries do?

[gamefreak]

You have a lot to learn about batteries SOR's young apprentice. Batteries work by producing a charge through a chemical reaction involving, but not limited to, acid, iron, copper, lithium, semi-permiable membranes, etc.

Capacitors don't produce charges, they retain it for a period of time. They get their charge by something putting energy in and they then retain in the form of a difference between to plates.

[izua]

So, why wouldn't that allow the poster to use a ultracap instead of a battery?

[Fredrik Andersson]

Capacitors lose voltage over time. Both charging and discharging is exponential. After a certain time of discharging (which you get from the capacitors capacitance times the resistance), the capacitor loses 63% of the voltage. After the same amount of time you lose 63% more.

Say you have a circuit that needs 5v and lets say the circuit draws 100 mA. Then the total resistance for the circuit must be R = 5/0.100 = 50 ohm. You connect the circuit with a capacitor.

So now you want to have a capacitor large enough to provide power for one hour. Therefore we charge it with 15v so that after one hour we still have just above 5v. To get one hour (3600 s) we need the capacitance to be 3600/50 = 72 Farad. This is a really really large capacitance. Normally you work with micro-Farad. Given that the capacitance depends on the size of the metal plates I can only imagine how much larger than the corresponding batteries it must be.

[izua]

I agree with you, on behalf of the capacitors. However, this topic is on ultracapacitors, which due to the double-layer construction easily break the 1kF (yes, one kilo farad) limit.

To bring an argument in their favor, China is developing a bus vehicle which is powered by ultracaps, charging between stations. Google for capabus for more details

[gamefreak]

How expensive are Ultra Capacitors? How safe are they?
It seems to me that putting a large charge in something that can discharge it all in a fraction of a second, is dangerous. Why not make a salt water leyden jar and use that to power it?

Why would you need yo use capacitors? Is the project very small and wont run for an extended period?
Aren't capacitors generally used as a short term burst in energy, such as camera flashes, or a boost in an RC car?

[izua]

Salt water layden jars store high voltages, but overall, they have a very low power-weight ratio.

It seems to me that putting a large charge in something that can discharge it all in a fraction of a second, is dangerous

Well, Pb-Acid and LiPo batteries can discharge in a fraction of a second, and that's dangerous too.

[xboogerx]

like G I Joe would say "knowing is half the battle." Thanks for the knowledge.

[Admin]

If you look at the datasheets of a cap and a battery, you will quickly see that:

a battery of equal volume holds about ~100x more energy than the best of ultra caps.