Charging of super capacitor batteries

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Charging of super capacitor batteries

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A discharged supercapacitor is like a circuit that is shorted to an energy source. Fortunately, many energy harvesting sources (such as solar cells and micro-generators) can drive a short circuit and charge a super capacitor directly from 0V. ICs that interface with various energy sources, such as piezoelectric or thermal power, must be able to drive a shorted circuit to charge the supercapacitor.

The industry has made great efforts in MPPT (Maximum Peak Power Tracking) to get the most efficient power from energy harvesting sources. This solution is feasible when the battery must be charged in a constant voltage manner. The battery charger is usually a dc/dc converter that is a constant power load to the energy source, so it makes sense to use MPPT to get energy at the most efficient point.

In contrast to batteries, supercapacitors do not need to be charged at a constant voltage, but are most efficient when charging at the maximum current that the power supply can provide. A simple and effective charging circuit for the case where the open circuit voltage of the solar cell array is less than the rated voltage of the super capacitor. The diode prevents the supercapacitor from recharging the solar cell in the absence of light. If the open circuit voltage of the energy is greater than the voltage of the super capacitor, the super capacitor needs to be overvoltage protected by a shunt regulator. The shunt regulator is an inexpensive and simple solution to overvoltage protection. Once the supercapacitor is fully charged, it does not matter whether it consumes too much energy.

The energy harvester is like a water pipe with unlimited water supply, filling a sink (like a super capacitor). If the sink is full, the water pipe is still open and the water will overflow. This is different from batteries, which have limited battery supply and therefore require a series regulator.

In the circuit, the super capacitor is 0V, and the short-circuit current is obtained from a solar cell. As the supercapacitor is charged, the current drops, depending on the voltage/current characteristics of the solar cell. But the supercapacitor always gets the maximum current possible, so it charges at the highest possible rate. The circuit in use uses the TLV3011 solar cell because it contains a voltage reference that requires only about 3μA of quiescent current, and it is an open-drain battery cell. When the regulator is turned off, the output is open. . The circuit uses a BAT54 diode because it has a low forward voltage drop at low currents, ie, a forward voltage of less than 0.1V at forward currents less than 10μA.

Micro-generators are well suited for industrial control applications, such as monitoring rotating machines because the machine vibrates while it is working. The voltage-current characteristic of a micro-generator is given, which is similar to a solar cell and can provide the maximum current for a short circuit. The micro-generator also has a diode bridge that prevents the supercapacitor from recharging the generator, which results in a simple charging circuit.

When the supercapacitor is charged, the leakage current decays over time as ions in the carbon electrode diffuse into the pores. The leakage current settles at an equilibrium value that depends on capacitance, voltage, and time. The leakage current is proportional to the capacitor core. The empirical estimation algorithm for supercapacitor equalized leakage current is 1μA/F at room temperature. The 150mF capacitor has a leakage current of 0.2μA and 0.3μA after 160 hours.

The leakage current increases exponentially with increasing temperature. As the temperature increases, the time to stabilize to equilibrium is reduced because ions diffuse faster. Therefore, these capacitors require the least amount of time to charge from 0V. This current ranges from 5μA to 50μA depending on the supercapacitor. Designers should consider testing this minimum charging current when selecting a supercapacitor for an energy harvesting circuit.