capacitor bank help

[bbaca]

Hi,

I'm currently on a team that is building a beam powered (light) robot for a competition. We are moving away from the spotlights, so as the distance increases there will be less power coming off the panels. To help offset this we want to put a capacitor bank onboard to store excess energy at the beginning to spend later.

I have searched on the internet and have not found definitively how to build a bank. This is what I believe is correct. We need to put several capactiors in series to bring the voltage up, and each capactior will be put in parallel with a resistor (each resistor will be identical to divide voltage evenly). It would look something like this:

       /-^V^V^-\
+Vcc --         -- (repeated until enough, then grounded)
       \--||---/

Is this correct? Is there anything else we will need? There is a voltage regulator in front of the bank, so we know the voltage will not rise too high (plus we are giving it extra anyway).

Thanks,
bbaca

 

[Comcokid]

Supercapacitors are used for energy storage. Maxwell sells them as Ultracapacitors. Cooper has them as PowerStor. They go by a number of different names. Sizes range from 1F at 2.5 volts to capacitor arrays big enough to power an electric bus. Check the application notes.

To get the voltage higher, you have to put them in series. It is important to have equal capacitance so one in the series string does not get charged past it's voltage rating. Sometimes resistors are put in parallel, but for a single robot, you should be able to work out a means of measuring the capacitance and put matched capacitors together.

Note - Eng-Tips does not allow reply for homework. I am assuming this project really is team/competition effort.

 

[bbaca]

Thanks for the reply!

I am planning on using 23 50F NessCaps rated at 2.7V (Digikey part# 589-1004-ND). I needed it to be above 56V and couldn't store more than 4kJ (competition rule).

You make it sound as if I do not need resistors, and that simply wiring these guys up is series is enough? And am I right to believe the benefit of the resistors is to be sure the voltage on each capacitor is near the same (within the tolerance of the resistors)? Would the resistors slow the charging of the capacitors, or discharge them any appreciable amount? The light remains on for only 60 seconds, so we cannot wait a few seconds for a slow capacitor bank.

And no, its not homework. You can look at the competion website if you wish:

http://www.elevator2010.org/

Thanks,
bbaca

 

[Comcokid]

I know that for some simple 5 and 12V applications, resistors are not used - the capacitors are matched. But, 56 volts will take about 24 capacitors. I don't recall the approximate value of the resistors used, but since the competition takes place within minutes, significant discharge through the resistors should not be a problem.

The one time I was experimenting with these capacitors I was trying to store energy for days, so my approach was different. I personally would try to make due without resistors, until I'm sure they're needed.

There is probably other ways to limit charge on an individual cap, such as a zener, voltage reference (shunt type) or something similar that won't conduct until the voltage reaches the trip point. You could also use extra capacitors in series so the average charge per capacitor is only around 2 volts rather than nearer the maximum of 2.4 or 2.5 volts.

 

[VEBill]

Solar cells inherently limit their own current (wouldn't it be nice if the current was not limited by the incoming photon count?).

For example, one may connect a 12v solar panel across a lead acid car battery (even a dead one) and the solar cells will happily dump current into the battery without complaint. In this example the solar panels get hot, but that's from being left in the hot sun all day, and certainly not from the relatively trivial current that they can generate (even into a dead short).


Did you see (Discovery Channel or similar) the laser-powered gadget that could be blasted hundreds of feet into the air? The passive device was spun-up for stability, and then a high-powered laser was blasted into the unique reflector on the back and the air at the focal point exploded and launched it up into the sky.