D-Cell Battery Management System

Active capacitive cell balancing is a MOSFET-based, switch-mode electronic system that transfers electrical energy from higher-voltage cells to lower-voltage cells within the battery pack.

This is necessary because, due to manufacturing tolerances, individual cells are never perfectly identical and exhibit slight differences in leakage current (self-discharge rate). Without balancing, these differences would cause the cells to drift apart over time, leading to significant voltage imbalance within the battery.

The key advantage of active balancing over passive balancing is that it is energy-efficient and not limited to charging phases only. Active balancing operates continuously, redistributing energy whenever voltage differences occur, rather than dissipating excess energy as heat.

In many applications—such as machinery, construction equipment, trucks, marine systems, forklifts, security systems, uninterruptible power supplies (UPS), and solar energy systems—multiple batteries must be connected in series. This allows the creation of low-voltage DC systems such as 24V, 36V, 48V, 60V, 72V, 84V, or 96V configurations.

Just as individual cells differ slightly in leakage current, standalone battery units are also not perfectly identical. In series-connected systems, energy cannot be transferred directly between two-terminal batteries without additional external electronics. To avoid the need for extra hardware and complex installation, a passive balancing method is applied.

From a cost and usability perspective, this allows the system to be installed like a conventional (non-intelligent) battery setup: only the positive and negative terminals need to be connected, with no additional wiring or communication interfaces required.

The internal electronics equalize the leakage current of the batteries between 14V (≈80% State of Charge) and 15V (100% State of Charge).At 14V, the artificial leakage current is 0mA, while at 15V it increases to 112mA. Between 14V and 15V, the leakage current increaseslinearly with voltage.

As a result, any voltage above 14V is reduced by approximately 50% every 4 days, and after 28 days the battery voltage naturally settles at approximately 14.008V. This means that:

• Batteries charged above 80% will automatically equalize during float charging, or
• If left unused, they will discharge to 80% within 28 days and then enter a sleep state, after which they continue to discharge slowly and uniformly.

During active operation, balancing is continuously applied whenever the battery remains above 80% State of Charge.

The battery incorporates a dual-layer, fully hardware-based short-circuit protection system independent of the main processor.

The slow short-circuit protection operates in the millisecond range and uses a foldback-current characteristic similar to laboratory power supplies. A POWER MOSFET stage limits the maximum current to 1300A, preventing further current increase while the terminal voltage drops in a controlled manner. This protection activates in cases such as shorted motors, faulty inverters, or cable damage occurring a few meters from the battery.

The system continuously monitors current rise behavior. After detecting a fault condition, it automatically attempts to reconnect the battery with timed retries. If the short circuit is no longer present, normal terminal voltage is restored without user intervention.

In addition, a fast di/dt protection layer reacts to extremely rapid current changes within 1-10 microseconds. This protects against direct terminal short circuits (e.g. metal tools, ladders, or conductive objects bridging the terminals). By monitoring the current rise rate, the MOSFETs are switched off before a destructive current peak can form. Typical maximum current is limited to 350A ±15%.

The RMS overcurrent protection is a software-controlled energy-based safety mechanism. It evaluates the time-integrated square of the current (I²) over a 15-second window, equivalent to allowing 160A DC continuouslywithout limitation.

Short-term higher currents are permitted within defined time limits, enabling real-world operation of inductive loads such as motors, winches, starters, air-conditioning units, and fans. For example, the system allows:

• 200A for up to 32 seconds
• 300A for up to 8 seconds
• 400A for up to 4 seconds
• 780A for up to 1 second

If the limit is exceeded, the system disconnects and automatically reconnects after a cooldown period. On repeated overload events, stricter repetitive limits are applied to protect both the battery and connected equipment.

Overcurrent conditions can occur not only during discharge but also during charging. The system therefore includes dual-level hardware protection for both overcurrent and overvoltage scenarios.

In abnormal charging situations—such as using a high-current charger designed for multiple batteries while one battery is missing or disabled—the hardware protection reacts immediately. For extremely high currents (above 500A), the system disconnects within microseconds, independent of software processing.

A foldback-based hardware protection stage limits charging current to a safe maximum (620A) and prevents uncontrolled current flow in fault conditions, including incorrect system voltage connections during installation or maintenance.

Overcurrent conditions may occur not only during discharge but also during charging. For this reason, the battery incorporates a dual-level hardware overcurrent and overvoltage protection systemoperating independently from software control.

In abnormal charging scenarios — such as the use of a high-current charger designed for multiple batteries when one battery is missing, disabled, or faulty — the hardware protection reacts immediately. The system allows controlled current intake for limited durations, but for extremely high currents (above 500A), disconnection occurs within microseconds.

A foldback-based hardware protection stage limits the maximum charging current to a safe upper limit of 620A. This prevents uncontrolled current flow in fault conditions, including incorrect system voltage connections during installation, maintenance, or accidental contact with higher-voltage series packs.

If excessive voltage or current is detected while the battery is in sleep mode, the hardware protection triggers instantly, ensuring that neither the battery nor connected equipment can be damaged before normal operation resumes.

The low-voltage protection prevents excessive battery discharge by disconnecting the load when the battery reaches a critically low voltage level. This function is implemented as a software-based protection mechanism.

Voltage thresholds are evaluated based on the open-circuit (no-load) battery voltage, ensuring accurate state-of-charge assessment even under high load currents. As a result, the battery is not disconnected prematurely during heavy discharge conditions.

For example, under extreme load conditions the terminal voltage may temporarily drop below 10V while supplying very high current. In such cases, the system correctly identifies that the open-circuit voltage remains within a safe range and maintains operation until true undervoltage conditions are met.

The battery incorporates a dedicated overcharge protection mechanism to prevent damage caused by excessive charging voltage. This protection layer is implemented in software and continuously monitors battery voltage.

Overcharge thresholds are evaluated using the open-circuit voltage reference, with protection activation occurring when the voltage exceeds 15V. This approach ensures stable and reliable operation across a wide range of charging conditions.

By accurately distinguishing between load-induced voltage behavior and true overvoltage events, the system effectively safeguards battery health while maintaining optimal charging performance.

Transient voltage surges can occur when high charging currents are abruptly interrupted, particularly in systems using inductive chargers such as generators or transformer-based power supplies.

When inductive current paths are suddenly disconnected, stored magnetic energy causes a rapid voltage rise. These transient surges can stress or damage charging equipment, rectifier diodes, or battery-connected electronics if not properly managed.

To mitigate this effect, the battery integrates surge energy absorption using capacitive elements. Transient energy is temporarily stored and then safely dissipated as heat, preventing destructive voltage spikes.

During surge events, terminal voltage may rise briefly but remains within controlled limits. The system is designed so that transient energy levels and peak voltages remain within safe operating ranges, ensuring compliance with applicable safety standards and protecting both the battery and connected equipment.

The battery features a comprehensive multi-point thermal monitoring system designed to ensure safe operation under all load conditions.

Temperature is continuously measured at multiple locations, including the battery cells, terminal connections, and the PCB. Cell temperature is monitored across groups of cells, while additional sensors track thermal conditions at the power terminals and internal electronics.

The maximum permissible cell temperature is 65°C. Higher temperature limits are allowed at the terminals and PCB; however, exceeding these thresholds is considered a fault condition and results in immediate battery disconnection.

Once temperatures return to safe operating limits, the system automatically reconnects the battery, ensuring both protection and uninterrupted long-term operation without manual intervention.

The High-Frequency Current (HFC) protection prevents the battery from being stressed by abnormal high-frequency current components, which can cause excessive heating and long-term degradation.

While the RMS overcurrent protection allows up to 160 Arms — including alternating current — accurate RMS evaluation is performed up to 5kHz. Above this frequency range, such high current levels should not normally occur in properly functioning inverter or charger systems.

Fault conditions such as failed filter capacitors, shorted rectifier diodes, damaged inverter stages, or defective commutators in brushed DC motors can introduce high-amplitude, high-frequency current oscillations. These effects generate unnecessary heat, create electromagnetic interference, and may damage other electronics connected in parallel.

To mitigate these risks, the system enforces strict current limits in the 5Hz to 150kHz frequency range. Depending on frequency, only approximately 50 Arms down to 10 Arms of alternating current is permitted. If these limits are exceeded, the battery disconnects automatically.

This protection strategy also supports compliance with conducted electromagnetic emission requirements(such as EN 55022), ensuring stable operation even at high DC load currents while minimizing high-frequency current noise.