Battery Cell Balancing Circuit Design

A series string of cells can only deliver as much as its weakest member, and the members drift apart with every cycle. Balancing is the function that keeps them together, and it is one of several jobs a battery management board performs.

Why Cells Drift Apart

Cells from the same production batch differ slightly in capacity, in internal resistance and in self discharge. Charged in series, they all receive the same current, so the one with the smaller capacity reaches its upper voltage limit first.

The charger stops when any cell reaches the limit, which leaves the others partly charged. Over many cycles the difference grows, and the usable capacity of the pack falls even though every cell is healthy.

Temperature differences across the pack make the drift worse. A cell near the middle of a stack runs hotter than one at the end, ages faster and loses capacity faster, which is why the thermal design of the pack and the balancing function are connected.

Passive Cell Balancing

passive balancing places a resistor across each cell and switches it in when that cell is above the others. The excess energy is dissipated as heat, and the pack continues to charge while the higher cells are held back.

It is simple, cheap and reliable, and its only real disadvantage is the energy wasted as heat. The balancing current is small, typically tens of milliamps, so the process is slow and is normally done during the constant voltage phase of the charge.

The resistor has to dissipate its share of the power continuously during balancing, and the board has to conduct that heat away. A pack with a large number of cells balancing at once can dissipate several watts inside a sealed enclosure.

Active Cell Balancing

active balancing moves charge from a higher cell to a lower one instead of wasting it. The transfer can be through capacitors switched between adjacent cells, through inductors, or through a small isolated converter per cell.

The efficiency is much higher and the balancing current can be larger, so a pack balances in a fraction of the time and the energy is preserved. The circuit is more complex, and the components are more expensive and take more board area.

A simple form uses one capacitor switched between pairs of adjacent cells. It works well when the imbalance is small and the cells are close in voltage, and it becomes slow when the difference is large, because the charge transferred per cycle is small.

Measuring state of charge

state of charge is estimated rather than measured. The two common methods are counting the charge in and out of the pack and relating the open circuit voltage to the state of charge after a period of rest.

Coulomb counting drifts because the current measurement has an offset and the capacity changes with age. The open circuit method is accurate but needs the pack to rest for an hour, which is rarely available in a working product.

A practical implementation combines them: the voltage method corrects the drift in the counting method whenever the pack has rested, and the counting method fills in the rest of the time. The combination is more accurate than either alone.

Monitoring Hardware

Every cell needs a voltage measurement, and the measurement has to be made at the cell terminals rather than at the ends of the pack. A multiplexer and a single converter is the cheap approach, and one converter per cell is the accurate one.

The measurement channel has to tolerate the common mode voltage, which rises with the position of the cell in the stack. A differential input with a high common mode range, or an isolated channel, is required for the upper cells.

Current measurement uses a shunt or a Hall sensor, and its accuracy sets the accuracy of the charge counting. A shunt with a Kelvin connection and a low temperature coefficient is the usual choice, and the signal conditioning follows the practices described in our guide to mixed signal board design.

Safety and Fault Handling

The protection functions are independent of the balancing functions. Overvoltage, undervoltage, overcurrent and overtemperature are detected and acted on by hardware, and the balancing is stopped whenever a fault is present.

A cell that is out of balance beyond a threshold indicates a failing cell rather than a normal drift. Reporting that condition and refusing to continue charging is safer than balancing around it, because a self discharging cell is a precursor to a thermal event.

The communication between the management board and the system should fail safe. A pack that loses communication should revert to a safe state rather than continue at full current, and the watchdog that enforces it belongs in hardware.

When Balancing Happens

Balancing during charge is the most common arrangement, because the charger is present and the cells spend a long time at the top of their range. Balancing during discharge wastes energy that the pack needs.

Balancing during rest or during storage is used in large packs where a long balancing session is acceptable. The management board stays awake and balances the cells while the equipment is idle, which spreads the heat over a long period.

The rule for when a cell is balanced is set from a voltage threshold and a minimum difference. Balancing a cell that is only a few millivolts above its neighbours wastes energy and adds heat for no useful result.

Cell Chemistry and Pack Size

The chemistry sets the voltage window and therefore the balancing threshold. A lithium iron phosphate cell sits at a very flat three point three volts for most of its range, so the voltage difference between cells says little about the charge difference until the ends of the range are reached, and balancing is only meaningful near the top.

A nickel based pack has a steeper voltage curve and a wider window, so a voltage difference is a better indication of the imbalance. The same balancing circuit then behaves quite differently depending on the cells it is connected to, and the thresholds cannot be copied between chemistries.

The size of the pack decides how much balancing current is worth providing. A small pack with a few amp hours balances quickly with tens of milliamps, while a large pack needs a proportionally larger current or a longer balancing session, and the heat produced during balancing scales with the current rather than with the pack size.

Verification and Faults

Verify the balancing by deliberately unbalancing a pack and recording the time to restore it. The measurement also shows whether the balancing current is what the design intended, because a resistor that has drifted or a switch with a high resistance reduces it.

Check the temperature of the balancing resistors with the pack fully balancing and the enclosure closed. A design that is comfortable on the bench and marginal in the enclosure is the usual outcome of testing only in open air.

A pack that loses capacity over a few months despite balancing usually has a cell with a high self discharge. Comparing the open circuit voltage of the cells after a week of rest identifies it. The release checks that keep such a board consistent are collected in our PCB design release checklist, and the assembly points are listed in judging PCB quality.

FAQ

Is passive balancing good enough? For most packs, yes. It is simple and reliable, and the wasted energy is small compared with the capacity of the pack.

When should balancing happen? During the constant voltage phase of the charge, when the cells are at the top of their range and the charger is present.

How accurate must the cell voltage measurement be? A few millivolts. The balancing decision is made from a difference of a few tens of millivolts, so the error has to be well below that.