Electronic – Supercapacitors in series

capacitor chargingover-voltage-protectionseriessupercapacitor

Can I charge series of 4 Maxwell BCAP3000 supercapacitors with constant current method using an adjustable Power Supply, and exceed the voltage rated for single cell?

What I mean is to charge the series of 4 ultracaps, that should be able to charge up to 10.8V, and have voltage higher than 2.7V on the power supply. I'm sorry if this question is somewhat silly, but this is my first take on supercaps connected in series. I've done single cell tests but never exceeded rated voltage on it. Therefore I have this small concern to solve.

They are rated for 2.7V. I have the manufaturers' balancing transistor boards attached to the cells, so single cell should be protected, but I am unsure if they are working since no LED is shining so far.

EDIT: Answers and comments provided solved my problem so this topic can be closed. Thanks for all of the concern and replies.

Best Answer

Can I charge series of 4 Maxwell BCAP3000 supercapacitors with constant current method using an adjustable Power Supply, and exceed the voltage rated for single cell?

Any capacitor put in series will increase the voltage rating of the capacitor. Keep in mind that supercapacitors are different from normal capacitors because of their very low ESR (Equivalent Series Resistance). The ESR could create problems if the current is being sourced fast enough to heat the capacitors (if the ESR rating varies by say 50% then the highest ESR capacitor will source most of the heat, in this case you will need to balance the capacitors.

You also need balancing if the capacitor values are different:

Since the individual ultracapacitor cell voltage is relatively limited compared to the majority of application requirements, it is necessary to series connect the ultracapacitors to achieve the voltage required. Because each ultracapacitor will have a slight tolerance in capacitance and resistance it is necessary to balance, or prevent, individual ultracapacitors from exceeding its rated voltage. Consider a string of 3 ultracapacitors with the following performance:
C1 = 100F and 0.011 ohms
C2 = 110 F and 0.012 ohms
C3 = 95 F and 0.010 ohms

If each ultracapacitor is initially at 0 volts and the string of ultracapacitors is charged to 7.5 volts at a constant current then C3 will reach 2.5 volts before C2 or C1. dt = IC•dV Thus if the string is not at 7.5 volts C3 will continue to charge above its rated voltage of 2.5 volts. In order to address this issue, balancing is required to maintain the ultracapacitors within its rated voltage. Balancing can be achieved through two different methods, active balancing or passive balancing.

Source: www.maxwell.com/images/documents/PG_boostcap_product_guide.pdf

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