Super Caps
I want to get this cap: http://search.digikey.com/us/en/products/EEC-HW0D506/P11066-ND/300482 , it is 50F and 2.3V. But, here's the question: The voltage is too low to be useful to me, so could I put three of these in series to get a 6.9V 16.7F capacitor? If this would work, could I charge them all together in series at 6.9V or would I have to charge them each individually?
Comments
http://www.allaboutcircuits.com/vol_1/chpt_13/4.html
Paul
EDIT:
I'm too slow on reply.
Will let you know.
Putting capacitors in series will decrease the capacitance, the same as putting resistors in parallel (same formula).
Yes, the charging voltage will be "split" across the capacitors but the split will be uneven depending on the condition of the individual cap; that's why bleeder resistors are used (think voltage divider).
Working with that large of capacitance - be careful!! There's a bunch o'energy there just watin' to vaporize something.
Your calculations and conclusions are correct....in theory. Problem is the capacitance of the individual capacitors have to be closely matched for the voltages on each one to be the same. Not sure about this cap but most electrolytics are rated at +-20%. Voltage wise this would be the same as having 3 resistors (nominally 1000 ohms) in series. One could be as low as 800 ohms or as high as 1200 ohms. Putting 6.9 volts on them would not result in 2.3 volts on each one.
I thought someone at Parallax has a super cap / solar powered robot?
If it drove into a dark area, it would backup and look for somewhere with more light.
Maxwell ( now general atomics ) uses them to 48v or more ! in series !
and at kF ratings ...........
so I know you can series them for charging and not have huge issues ..
we know Super cap are VERY leaky .... so there is our resistor . no real need for a external one .
and joules is 1/2C V^2 so the V is needed ..............
Peter
http://forums.parallax.com/showthread.php?124075-Solar-Panel-Boe-Bot-need-advise&highlight=supercap
Go down to post #7 and following
This can be as simple as a resistor ladder across each cell, or active balancing circuits involving an op amp / comparator. I've pulled apart my maxwell boostcap (6 x 350F in series) and it has a small active balancing circuit between each pair of cells.