supercapacitor discharging

[aspirecole]

i want to build an electric bike using battery and supercapacitor.. the supercap is used during the the high peak demand which is during uphill ride..
what circuit should i use to discharge the energy from the supercap to charge the battery??

 

[mohit571]

voltage across capacitor at any time is given by V(t) = V(e-t/RC)
So first u need to know about rating of supercapacitor and discharge time constant.. following link may help u to know time https://www.circuits.dk/calculator_capacitor_discharge.htm


Discharge :

Note: If using a separate programmable load instead of an integrated bi-directional power supply, disconnect the charging power supply prior to discharging. (Don’t simply turn it off or change its set points, as many supplies will sink current when not regulating.)

Set the load to the appropriate constant current, and discharge to 0.1V, or as low as the load can be controlled.

IMMEDIATELY remove the load once the minimum voltage is reached, allowing the device's voltage to "bounce" back.

(The discharge can actually be stopped at any voltage. Depending on equipment, some units can be discharged to 0.1V, and others discharged to ½ of the initial voltage. Values of capacitance will be slightly higher when discharged to ½ initial voltage rather than 0.1V.)

Measure the following parameters:
Vw = initial working voltage Vmin = minimum voltage under load

Id = discharge current Vf = voltage 5 seconds after removal of load.

td = time to discharge from initial voltage to minimum voltage

Capacitance calculation:

Capacitance = (Id * td)/(Vw – Vf) = (Id * td)/Vd

(This change in voltage (Vw – Vf) is used because it eliminates the voltage drop due to the equivalent series resistance)

Equivalent Series Resistance (at “DC”) calculation:

ESR = (Vf – Vmin)/Id

(An LCR meter or bridge can be used to measure ESR at higher frequencies. The ESR at frequencies up to 100Hz will typically be 50-60% of the “DC” ESR. The capacitance will be much lower, due to the structure of the electrode.)



below attached fig.: The circuit in Figure uses the TLV3011 solar cell because it integrates a voltage reference, draws only approximately 3-μA quiescent current, and is an open-drain cell so that the output is open-circuit when the regulator is off. This circuit uses the BAT54 diode because it has a low forward voltage at low currents—that is, the forward voltage is less than 0.1V at a forward current of less than 10 μA.

 

[chuckey]

You can discharge the energy from the cap back into the battery by a power transistor acting as a switch, or a relay. Question :- How will the super cap get charged up?
Frank

 

[aspirecole]

the cap is charged from external source... using a dc power supply...thanx man

 

[kam1787]

my e-bike is 48V, and many are 36V you need many supercaps to give a significant current and sufficient voltage

 

[aspirecole]

if i use a supercap bank 20V and boost it to 40V. is it possible for me to charge the battery back? or boost it to 40V and connect it to the electric motor and run the cap during uphill climb?

 

[tpetar]

Voltage boosting specially when big difference exist between input and output voltage requires huge current from input power source. In this situation supercap will need higher current for charging, I dont think you plan to drive bike half of day. :smile:

Best regards,
Peter

 

[kam1787]

I doubt you will get that much current unless you parallel - using 20V then bosting to 40V will cut the current to less than half.

Comparing the cost and complexity, you might be doing better to use batteries.