10 Questions You Should to Know about lithium ion capacitor

Capacitors store charge without chemical reaction - the charge is stored on the surface of the dielectric inside (the capacitance is actually a function of area - those tiny ceramic caps are stacks of thousands of very thin layers). The discharge curve is linear (in ideal capacitor, real ones aren't quite). Which means, if you discharge half the current, the voltage is half what it was at the start (which means that energy stored goes as the square of voltage it's charged to). If you do the math out, even beefy "BFCs" common in electronics store pitiful amounts of energy (supercapacitors have much higher capacitance, but only at low voltages, and still small compared to batteries). Capacitors last for many many charge/discharge cycles, particularly tants and ceramics (electrolytics dry out over time)

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Batteries store energy in chemicals inside the battery, and electrochemical reactions generate the energy while the battery is discharging. Charging reverses the process by applying a voltage to drive the electrochemical reaction the other direction. This is less efficient than charging or discharging a capacitor, and the internal structure is slowly degraded with charging and discharging, resulting in performance (capacity, etc) falling over time. The discharge curve is usually flatish, so at 50% charge, a battery has almost the same voltage it had at 90% - and then it falls off sharply near the end (when there's no more chemical to react). This makes them better power sources.

This is quite an amazing forum. Instead of a "nope, nothing we know of" I get a "we can help you if you just tell us more"

I was not aware of how esoteric my question was. I thought someone would just be able to point me to some tutorials.

Also, I too am waiting for a spec sheet. However, apparently we have the options to hand in specs and get a custom capacitor.

Lets turn the game around. I am interested in your suggestions on how you would go about this.

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We have following constraints:

• The system is completely encapsulated. It is physically not possible to connect a cable to it in order to charge a power source.
• The system is temperature sensitive. It is critical that it not heat up beyond skin temperature.
• Smaller is better.
• The system will pull 1.8V DC at 110mA max.

There are two usage scenarios
a) 25 minutes, constant 100mA, - Charging speed is an important factor
b) 48 hours, max 110mA, depending on 'events'. Average current is approximately 40mA

For usage scenario a) I was thinking that capacitors might work, especially since they can be charged very fast. For scenario b) I would go with a LiPo, but I am really interested in any alternate suggestions, or whether it might be possible to cover both usage scenarios with one system...

@crossroads. Thanks for the suggestions. I was thinking something more or less along those lines for the general setup, just didn't know if I was on the right track or not. I will check out the specific regulator you suggested.

Cheers

p.

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