Supercapacitors in series

Question

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.

Answer 1

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

Answer 2

There is no indication when using the SAB devices (likely the Maxwell balancing board you are using). It's based on balancing the leakage currents of the series connected supercaps and will not protect you from poor balancing at high charge current. Since your cap values can vary by more than 20% (datasheet) you will have unbalanced terminal voltages, so must provide this protection.

Yes you can easily end up stressing a capacitor due to overvoltage.
If you are going to charge the capacitor string at say 10A (your PS in CC mode), then you must allow for the possibility that any one of the caps may enter overvoltage (whatever you select as the terminal voltage ..in this case 2.7V maximum). At that point in time you need to stop charging that capacitor so must shunt the 10A around the cap, which will stop it charging.
At 2.7V and 10A you obviously need to dissipate 27W in the shunt while the other caps continue to charge.

Given you have 4 caps in series you will need a shunt balancer across each cap to protect it.
How you create your protection mechanism depends on what maximum charging current you want.

For example if you set the maximum current at say 1A, then you only have to dissipate 2.7W in each shunt which would be much easier to design. Worst case you will have 3 of the 4 shunts active, so a total of about 8W. At 10A you may need to dissipate 80W worst case.

There is some added complexity due to the ESR, but in the case of the caps you are using this is very low so can be almost ignored (max delta V would be 290mV @ 1A).