CELL BALANCING AND ITS NEED FOR ULTRACAPACITOR MODULES

An ultracapacitor (or supercapacitor) is like a short-circuit to a voltage applied across their terminals in fully discharged condition. Even if it holds a charge and carries some voltage, its low ESR still makes it appear like a short-circuit, or a very low-resistance path, inviting enormous amount of current. Charging an ultracapacitor is therefore very different from charging a battery. UC can store large amount of energy, which can be delivered to a load across its terminals in regulated way over time.

Practical considerations for charging of ultracapacitors

When connected to a voltage source, UC may appear as a short-circuit- its current is limited only by its ESR and applied voltage, reaching very high values, often dangerous for supply and connected cables, components and switchgear. Hence the charging current has to be restricted within values safe levels.

Ultracapacitors self-discharge noticeably over time, and full discharge to zero volts can take several days.

Problem in Charging UC series modules (or stacks)

A module of UCs consisting of series/parallel combinations of UCs, can be charged by applying supply across its terminals, keeping in mind the current and voltage limitations of UCs. If there are a number of cells in series, voltages across each cell can be different depending on capacitance tolerance and ESR of individual cells. Further, UCs cannot tolerate voltage beyond their rated maximum level.

In a series stack, the same current flows through every cell, but cells differ in capacitance, leakage current (self-discharge), internal resistance / ESR, temperature and aging. These differences cause some cells to reach their maximum voltage earlier than others. In a module, the charger sees only total stack voltage, so it may continue charging even though one cell is already fully charged, causing overvoltage.

For example, in a chain of 10 cells of 1000 F 2.8V rating, individual capacitance values may vary between 800 F to 1200 F (tolerance of ±20%). For simplicity, let us assume there are 5 cells of 800 F each and 5 cells of 1200 F each (extremes of tolerance). If a voltage of 28V is applied across the chain, voltages across a 1200 F cells will be 2.24 V, while that across 800F will 3.36 V.- a difference too wide to be acceptable, and damaging for 400 F cell. (Things could be worse, depending on actual capacitance values and number of cells in series).

Overvoltage during charging can:

• Accelerate aging and capacity loss.
• Cause gas generation, swelling, or in extreme cases, safety hazards.
• Push electrolyte beyond its stable window and reduce cycle life.

What is cell balancing?

To avoid such unbalanced situations, different ways are adopted to bring voltages on individual cells to as near as possible to a uniform level. This process of balancing or equalizing of voltages across all cells in a module /stack is called cell balancing. Without balancing, some cells can be overcharged even if the overall module voltage appears normal.

Cell balancing during charging is the process of keeping all series-connected cells at compatible voltages so that no single cell is overcharged while the stack/module as a whole is being charged.

During the charging cycle, once the pack has reached its full charge, it will be subject to overcharging until remaining cells in the chain reach their full charge. Temperature and pressure may build up and possibly damage that cell. During discharging, weakest cell will have maximum depth of discharge (DoD) and tend to fail before others. Voltage on weaker cells could even become reversed as they are fully discharged before other cells, resulting in early failure of the cell.

Balancing ensures that, during charging, all cells approach their upper voltage limit together, maximizing usable energy and lifetime, and also are within safe levels during discharge.

Basic charging methods for ultracapacitors

1. Constant-current charging (CC): Charging current is held constant till it is fully charged and cutoff through a terminal voltage sensing circuit. Voltage increases linearly with time during constant current charging, and stops when UC reaches its full voltage, i.e. current drops by default.
2. Constant power charging: Power from supply is held constant till final voltage is reached across terminals. Current goes down as voltage across terminals increases, such that power drawn from source remains constant.
3. Constant current / constant voltage (CC/CV): A fixed current is forced into the device from 0 V (or low voltage) up to near the rated voltage.  Once the target voltage is reached, the charger holds that voltage and the current naturally tapers as the capacitor fills. 
4. Stop / float: Many designs stop charging once current drops below a threshold. Long-term float at maximum voltage is possible, and many systems float slightly below max voltage to extend life. 

Ultracapacitors can be recharged as fast as they can be discharged; the real limit is usually limited by the charger power rating. For very large modules or packs, multi-module cooperative charging is used: several charger modules in parallel share the current, with control loops to balance currents among chargers and regulate stack voltage. 

Balancing approaches

• Passive balancing: Resistors are placed across each cell or string to bleed excess charge from higher-voltage cells. This is simple process, but wastes some energy as heat.
• Active balancing: Uses dedicated ICs or electronic circuits that move charge between cells. This is more efficient and precise, especially for large or frequently cycled modules. Active balancing is energy efficient, but costlier.
• Integrated charge-and-balance converters: This method uses modified forward or other isolated topologies that inherently direct more current to lower-voltage cells during charging, achieving simultaneous charge and equalization without complex control.

Passive balancing

Passive methods dissipate excess energy from higher-voltage cells as heat. Common techniques are:

• Shunt resistors across each cell: Fixed resistors across UCs draw a current several times larger than the typical leakage current. Continuously bleed charge from higher-voltage cells leads to slow rise in voltage across terminals during charge.

• Zener diodes or transistor/Zener combos: Conduct only when a cell exceeds a threshold voltage, shunting current once that cell is “full.”
• MOSFET-based shunts controlled by comparators: Turn on when cell voltage exceeds a setpoint, providing a controlled bleed path.

Characteristics during charging with passive balancing:

• Simple, low-cost, no extra energy transfer path.
• Balancing currents of cells are typically in the milliampere range, so correction is slow.
• Best suited for small or moderate-power ultracapacitor modules, and systems where charge/discharge rates are not extremely high and imbalance build-up is slow or modest.

Passive resistor balancing is often sufficient for many modules, especially if sized to dominate leakage currents. During a charge cycle, passive balancing lets higher-voltage cells “leak” more, so their voltage rise is slower. Lower-voltage cells continue charging at the full stack current, gradually catching up.

Active balancing

Active methods move charge between cells instead of wasting it, allowing faster and more precise balancing, especially under high-power operation. There are number of methods of balancing, and two of them are mentioned here.

• DC–DC converters per cell or per group bypass a cell with a small converter that discharges it during charging and charges it during discharge, effectively adjusting its net current.
• Op-amp or comparator-controlled circuits that source/sink current at the midpoint between cells to equalize currents.

Cell Balancing for Batteries

Cell balancing is needed for battery management as well for similar reasons, and principles are nearly same. However, there are some major differences:

• Battery voltages do not vary by large margins over the range, and fully charged condition, and fully discharged condition have difference just a few volts. For example, for 12 V rated lead acid battery has minimum permissible safe voltage of 10.8 V, while Charging window is from 10.8 V, to full charge at 13.8V. UC can be discharged right down to zero volts.
• Charging currents for UC is vey high, while that for battery is much lower, limited by chemical reactions inside the cells
• Battery charging is monitored by SoC, while UC can be monitored by just measuring voltage across terminals.