Super capacitors

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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
 
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.
 
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.
 
sivan_and said:
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.
 
lndm said:
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.
 
Voltage distribution in a series stack of ultracapacitors is initially a function of capacitance. After the stack has been held at voltage for a period of time, voltage distribution then becomes a function of internal parallel resistance (leakage current).
For example, consider a stack of 6 capacitors initially charged to 15 volts. If the cells have identical capacitance, the voltage should divide evenly, so that each capacitor charges up to 2.5 volts. If the cells have any variation in capacitance, individual cell voltages will vary based on capacitance. The cells with greater capacitance will be charged to lower voltages, and the cells with smaller capacitance will be charged to higher voltages. This is because each cell conducts the same current, and voltage is a function of current and capacitance. The average voltage will still be 2.5 volts (15 volts / 6 cells).
Any cell at voltage discharges through an internal “parallel resistance”. The current through this parallel resistance is referred to as leakage current. The leakage current has the effect of self-discharging the cell.
After some time held at voltage, the voltages on the individual cells will vary based on the differences in leakage current, rather than on the differences in capacitance. The cells with higher leakage should have lower cell voltages, and vice versa. This is because the higher leakage current discharges the cell, lowering its voltage. This voltage must then be redistributed onto other cells in the series (provided the series string is held on a constant voltage source).
One technique to compensate for variations in leakage current is to place a bypass resistor in parallel with each cell, sized to dominate the total cell leakage current. This effectively reduces the variation of equivalent parallel resistance between the cells. For instance, if the cells have an average leakage current of 10uA +/- 3uA, a 1% resistor which will bypass 100uA may be an appropriate choice. The average leakage current will now be 110uA, +/- 4uA. Introduction of this resistor has decreased the variation in leakage current from 30% to 3.6%.
If all the parallel resistances are the same, the cells with higher voltages should discharge through the parallel resistance at a higher rate than the cells with lower voltages. This will help to distribute the total stack voltage evenly across the entire series of capacitors.
Then we must consider the effect of increased leakage current due to the addition of bypass resistors for balancing. A typical trade is based on time to balance vs. leakage current; the faster the balancing circuit responds, the greater the leakage. A 10:1 ratio of bypass leakage current to cell leakage current may take a few days to balance a severely unbalanced system, where a 100:1 ratio may balance in a few hours. Once the system is balanced, response time is less of an issue unless a system is being severely cycled.
For high duty cycle applications, active balancing is a must.
And to negate the effect of reverse charging weak capacitors (Similiar condition occurs as in Nicd-NiMh Battery packs)adding a schottky diode across each cap is a solution !!
 
Hmm, high charging current then.
I don't like the assumption that the capacitances are all similar, supercaps are badly toleranced like all electrolytics.

This means that some of the units are going to see more than 2.5V. If they just leak fine, but if the overvoltage damages the cell then you have a problem. In which case you need a 2.5V shunt regulator across each cell, capable of handling the full charging current.

You may also need reverse polarity protection to prevent lower capacitance cells being reverse charged - depends on the data sheet.
 
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