The Bucket Effect: Why One Weak Cell Drains an Entire Pack

In any series-connected high-voltage battery string, usable capacity is always bounded by the lowest state-of-charge (SOC) or most degraded cell — a constraint known as the “weakest-cell bottleneck” or “bucket effect.” The pack cannot continue discharging once the weakest cell reaches its lower voltage limit, even if every other cell retains substantial charge. This is not a marginal issue: as cell inconsistency grows with age and thermal variation, the gap between theoretical and available capacity widens materially, reducing EV range without any change in the physical energy stored.

Passive balancing — the incumbent approach — addresses cell imbalance by dissipating excess energy from higher-SOC cells through resistors, operating at maximum currents of approximately 200 mA. This makes it both slow and wasteful: energy that could extend range is converted to heat. According to a 2016 patent from Jiangsu Hanhai Core Cloud Network Technology, passive balancing’s slow rate and resistive losses mean that unmatched cell capacity degrades pack-level charge/discharge capacity to the level of the weakest cell, and that active balancing is the mechanism that breaks this constraint.

Active balancing inverts the logic: rather than burning off the surplus in high-SOC cells, it transfers that energy to low-SOC cells using power electronics — bidirectional DC/DC converters, transformer networks, or switched matrix circuits. The weakest cell is charged rather than simply protected. The result is a higher effective floor of available capacity across the entire pack, directly translating to more deliverable energy per charge cycle. Standards bodies including IEC and research published through IEEE have long recognised cell balancing as a fundamental battery management requirement; the patent record now shows how the engineering community is solving it at production scale.

Circuit Topologies That Move Energy Instead of Burning It

The most prevalent hardware approach in the patent dataset is the bidirectional DC/DC converter topology, in which each cell in a series string is assigned a corresponding bidirectional DC/DC module and control switch. The controller continuously samples real-time cell voltages, identifies the lowest-voltage cell as the primary target, selects the higher-voltage adjacent cell as the energy donor, and switches the DC/DC module between Buck and Boost modes to charge or discharge the appropriate cell. This adjacent-cell diffusion approach propagates energy across the pack iteratively, improving voltage uniformity and ensuring the limiting cell is charged.

Transformer-based topologies offer a higher-efficiency alternative for non-adjacent cell balancing. A 2023 patent from Hangzhou Xenergy Technology proposes cascaded basic balancing units, each connecting four series cells via a first-order transformer and a switch group. Energy is transferred between any two cells with an energy imbalance through the transformer’s transmission path, achieving balancing with fewer switching devices and lower cost than switch-matrix approaches. The patent explicitly contrasts this with prior art: a full switch-matrix topology requires 12 series MOSFETs for a single charge/discharge balancing cycle, suffers from low conversion efficiency, and cannot balance multiple cell groups simultaneously.

A particularly compact implementation from Daechang Motors (2017) combines an open-circuit voltage detection unit, a matrix switching unit, and a bidirectional DC/DC converter. The matrix switch enables any-cell-to-any-cell energy transfer without multi-hop energy loss through intermediate cells — a key advantage over adjacent-only Buck-Boost topologies, where non-adjacent balancing incurs incremental losses at each intermediate conversion stage. Multi-winding transformer magnetic energy balancing, developed by Shenzhen Voltek Technology (2024), takes a further step: it converts electrical energy from multiple cell groups into magnetic energy stored in the transformer core simultaneously, then releases it preferentially into the lowest-voltage groups via MOS switch control, achieving group-to-group active equalisation without large bidirectional switch matrices.

“Active balancing achieves bidirectional charge/discharge with efficiencies routinely exceeding 80%, materially increasing actual usable capacity — while passive balancing dissipates all equalization energy through resistors at maximum currents of approximately 200 mA.”

Hierarchical Control: Managing Hundreds of Cells at Scale

High-voltage EV packs typically contain hundreds to thousands of cells organised in modules and racks, making flat single-level balancing architectures computationally and electrically impractical. The patent record shows strong convergence toward hierarchical balancing systems that address cell-level, module-level, and pack-level imbalances in coordinated layers, with sub-controllers operating in parallel to accelerate convergence.

A 2021 Korean patent from inventor Chang-in Kim structures the pack as M battery modules, each with N cells, managed by M sub-management modules that measure individual cell voltages and temperatures. All data feeds a main management module that collects M×N cell voltages and executes both voltage and temperature balance management across the full pack. A companion patent by the same inventor adds a communication module enabling remote state management and real-time balance status reporting. This decomposition reduces the combinatorial complexity of cell selection and allows sub-controllers to operate in parallel, substantially accelerating convergence to a balanced state.

The AutoGiGi Driver-Integrated Battery Balancing Device (2024) integrates simultaneous cell-level and module-level balancing by linking a cell balance control part directly to a module charging/discharging control part via a bidirectional driver. This allows both intra-module cell imbalances and inter-module imbalances to be addressed concurrently rather than sequentially, improving overall balancing efficiency and reducing the time during which some cells remain at the extremes of the SOC range. BMW’s approach (2024) achieves inter-module balancing during driving operation without a dedicated converter: when a module’s SOC falls significantly below the others, its current path is switched off via the switching system until the remaining higher-SOC modules discharge down to near the same level, at which point the previously isolated module is reconnected. This approach uses the vehicle’s traction load as the equalisation medium and is reported to be imperceptible to the driver.

Inter-Pack Balancing in Multi-Pack High-Voltage Architectures

Modern high-voltage EV architectures increasingly combine multiple battery packs to achieve greater energy capacity or architectural flexibility — for example, 400V/800V switchable systems. This creates a distinct class of balancing challenge at the pack-to-pack level, addressed by cross-pack equalisation topologies that transfer energy between physically separate packs at the individual cell level.

Aiways Automobile’s 2019 patent implements cross-pack equalisation across a primary-secondary pack configuration. The system calculates residual capacity differences between corresponding cells in the main and auxiliary packs, opens a cross-pack equalisation channel when the difference exceeds a preset threshold, and transfers energy from the auxiliary pack’s cells to the main pack’s matching cells. The benefit for EV range is explicit in the patent: online charging of the main pack extends its discharge time and total deliverable energy. Shenzhen Leran Technology (2025) extends this with a scheduling framework that first maps cell configurations across all packs, constructs cross-pack equalisation path sets ranked by “path benefit,” and manages conflicts between intra-pack and cross-pack equalisation through a queuing system — ensuring intra-pack equalisation takes priority and cross-pack channels targeting cells currently undergoing intra-pack balancing are deferred to a waiting queue.

CATL addresses the most challenging scenario — connecting an additional battery pack to an already-operating circuit — by predicting the reflux current that will occur when a slave BMS connects a new pack, and controlling connection timing to a window where the predicted reflux value is within a safe range. This eliminates the need for a dedicated balancing relay and prevents current surges that would otherwise damage cells or trigger protection shutdowns. Tesla’s pending 2026 patent frames the cross-pack connection problem around open-circuit voltage (OCV) matching: the system determines whether the OCV of a main pack and an auxiliary pack match before permitting parallel connection and initiating parallel charging, preventing inrush currents that would occur if packs at significantly different SOC levels were connected without prior conditioning. Research from institutions including NREL has similarly highlighted inrush current management as a critical safety constraint in multi-pack EV architectures.

Jiangsu XCMG Construction Machinery Research Institute’s 2025 patent introduces a three-level architecture for multi-branch parallel battery systems: battery packs as the first level, Battery Distribution Units (BDUs) as the second level, and a high-voltage junction box as the third level, with a bidirectional DC/DC module bridging the high-voltage battery system and the vehicle’s low-voltage 12V battery. In discharge equalisation mode, high-SOC branches discharge through the DC/DC module into the 12V battery; in charge equalisation mode, the low-voltage battery supplies energy back through the DC/DC to charge low-SOC branches. This dual-purpose design simultaneously achieves inter-branch SOC equalisation and low-voltage battery supplementation.

Algorithmic Path Optimisation: Minimising Losses During Balancing

Selecting the most efficient balancing sequence is a non-trivial optimisation problem, particularly when energy must traverse multiple intermediate cells to reach a distant target. Significant patent activity addresses this algorithmically, moving beyond fixed nearest-neighbour heuristics toward graph search and model-predictive approaches.

A 2017 patent from Hefei University of Technology formalises the balancing path selection problem as a directed weighted graph. Cell terminal voltages are nodes, and edge weights represent both the energy consumed and the energy transferred in one balancing cycle between any pair of cells, calculated from measured internal resistance and DC impedance of wiring and switching components. A depth-first search with backtracking exhaustively enumerates paths to find the one with highest energy-transfer efficiency. This ensures that for non-adjacent cells, the system selects multi-hop paths that minimise cumulative losses rather than following the nearest-neighbour default, which would incur unnecessary intermediate conversion steps. The approach is consistent with graph-theoretic optimisation methods recognised in computational literature published by ACM.

Chongqing University’s 2023 patent addresses the practical reality that equaliser efficiency varies nonlinearly with equalisation current. The patent constructs a state-space model for a bus-type equalisation system that integrates this nonlinearity, then applies Model Predictive Control (MPC) to minimise equalisation losses while also accounting for time efficiency — treating faster convergence as a weighted optimisation objective alongside energy conservation. This dual-objective formulation reflects the operational reality of EV use: prolonged balancing during a charging session that blocks the start of a drive is itself a capacity-availability penalty.

Zhengzhou University’s 2025 patent identifies a critical failure mode in single-topology active equalisers: when only over-charged cells or only under-charged cells exist in the pack, Any-Cell-to-Any-Cell (AC2AC) mode equalisers cannot form valid energy transfer paths, causing the balancing function to fail entirely. The proposed multi-mode topology supports Cell-to-String (C2S), String-to-Cell (S2C), and direct Cell-to-Cell modes via a configurable bidirectional switch array, ensuring that complex over-charge/under-charge combinations are handled without routing energy through intermediate strings at reduced efficiency. This kind of failure-mode analysis reflects the maturation of the field — the engineering community is now addressing edge cases that simple topologies cannot handle.

Patent Landscape: Who Is Leading and Where the Field Is Heading

The active cell balancing patent landscape, spanning approximately 55 sources filed between 1999 and 2026 across South Korea, China, Japan, Australia, and Italy, reveals a clear geographic and corporate concentration — with Chinese and Korean entities dominating recent filings and a distinct shift toward systems-level integration after 2021.

LG Chem / LG Energy Solution (Korea) is the most prolific assignee in the dataset, with patents covering voltage balancing apparatuses for battery modules (2018, 2020), inter-rack voltage balancing (2017), relay-controlled rack connection with reference voltage simulation (2016, 2017), and master-slave module equalisers (2019). Their approach consistently uses equalisation current supply circuits to correct SOC abnormalities in individual modules within larger packs, targeting pack-level usable capacity recovery. According to WIPO data, Korean entities have consistently ranked among the top filers in battery management system patents globally over the past decade.

CATL / Ningde Times (China) appears with multiple active patents covering discharge/charge control systems for parallel battery packs (2025) and motor-mediated inter-pack balancing (2025, 2026), reflecting their scale and systems-integration focus. Hyundai Motor Company (Korea) addresses the pre-charge balancing problem for parallel pack connection (2024, pending) and relay-switched series/parallel reconfiguration for module-level balancing (2025), reflecting OEM-level integration of balancing into powertrain design. BMW (Germany) differentiates with a driving-operation-integrated balancing approach that avoids dedicated converters entirely (2024), while Tesla focuses on OCV-matched pre-conditioning for safe auxiliary pack parallel connection (2026).

Trend analysis across the dataset indicates a clear evolution from single-level, single-pack cell balancing (pre-2020) toward hierarchical multi-level systems that simultaneously address cell-level, module-level, and pack-level imbalances (2021–2026), with increasing emphasis on algorithmic path optimisation and predictive control to minimise both energy loss and balancing time. Chinese university and startup filers — including Chongqing University, Shandong University, Hefei University of Technology, and Shenzhen Voltek Technology — are contributing a disproportionate share of algorithmic and transformer-topology innovations, suggesting that the next generation of efficiency gains will emerge from academic-industry collaboration in China. The IEA‘s global EV outlook consistently highlights battery system efficiency as a key lever for range improvement, reinforcing the strategic importance of this patent activity for the broader EV industry.