Super capacitors

[sivan_and]

I need to develop a super capacitor to aid in cranking automobiles i have a basic plan of:
50 Farads/2.5Volts 6 numbers in series...but how to equally divide the voltages ?
two techniques: 1) Passive by equal value resistors across each capacitor or by
2) Active methods but how ?
help in this regard will be appreciiated...Thanks

[davidsrsb]

The series resistance of these supercaps is high.

You may be better off using NiMH batteries.

[ocool_15]

I agree fully. If you can get your hands on a couple and test them you'll likely see they work don't work as well powering a small motor(say 350mA) but they work fine powering Led's for a long time. Supercaps are normally used for backup power for low drain devices like memory on timers ect.
Electrolytics would be able to handle high currents but they discharge quickly and that would become a much larger device when built.

[sivan_and]

Sorry Guys what i intend to know abt is how to connect them in series w/ equal potential difference....There are already high voltage 15Volts super capacitors in market used primarily in whicle cranking but costs very high...

[Eva]

Forget about that. Use a battery or a charger as everybody does. Motor-start batteries are among the cheapest as they are not required to whitstand discharge (they will sulfate easily if discharged, though).

Also, a car whose motor is fine and whose battery is discharged but still in good condition will start perfectly just after one minute of charging at 15A. Indeed, I have got some people puzzled from time to time by making their car start in a minute with just a medium sized battery charger.

[lndm]

Generally speaking, capacitors don't need resistive dividers, they will divide the voltage themselves. The resistors are typically there because capacitors often have large tolerances, and for safety reasons if one develops a problem.

[mzzj]

Originally posted by sivan_and
Sorry Guys what i intend to know abt is how to connect them in series w/ equal potential difference....There are already high voltage 15Volts super capacitors in market used primarily in whicle cranking but costs very high...

Are these single caps any cheaper? Low-ESR supercaps are VERY expensive at the required sizes. Yeah, Epcos have 600F 15V module, but it costs couple of thousand euros

Also, Your series combination cap is something like 8F 15v. Have you ever tought if this is going to help anything at all in start-up?
(I suppose we were talking about cars?)
It can supply 80amps for 0.5 sec and voltage has already dropped by 5volts.

At least here in Finland wintertime cold-cranking current need is something like 200 to 800Amps, 8F cap is hardly any use at these levels.

[davidsrsb]

Work out the energy storage.
50F at just under 2.5V each hold 0.5CV2
=0.5x50x6
=150J
So a stack of 6 of these is about 1KJ

Now look at AA NiMH
2.6Ah at 1.2V = 11KJ
A stack of 10 of these is 110KJ, roughly 100x more energy.

[Eva]

Originally posted by lndm
Generally speaking, capacitors don't need resistive dividers, they will divide the voltage themselves. The resistors are typically there because capacitors often have large tolerances, and for safety reasons if one develops a problem.

Series capacitors do need resistor dividers and even clamping diodes, because each capacitor has its own value of leakage current and of capacitance, whose tolerances are quite high for electrolytics.

If a suitable resistive divider with a current drain considerably higher than the maximum leakage unbalance expected is not employed, the voltage across the lowest leakage capacitors will increase until the electrolyte breaks down (thus increasing current) in order to compensate for the ones with higher leakage.

Also, during discharge the units with the lowest capacitance may become reverse biased if the voltage across the entire series-connected set is allowed to go too close to zero. Remember that capacitance is just the inverse of the rate of capacitor voltage change when 1A is being continuously drawn.

[lndm]

I agree with Eva. My statements appear to have been ambiguous.