Custom Battery Pack Design & Development for Non-Standard OEM Projects

A standard battery — or one with minor modifications such as a different connector, wire length, or label — may work if the voltage, capacity, physical dimensions, and discharge current of an existing product already match the device. A custom battery pack design is typically needed when the device requires a non-standard voltage or series/parallel configuration, when the available space does not match any existing cell geometry, when the load profile demands specific protection settings, or when the charger imposes constraints that a standard protection circuit cannot accommodate. Providing your device specifications early allows the engineering team to recommend the most practical approach — whether that is a modified standard product or a fully custom development.

Each cell chemistry has a normal full-charge voltage. When the device or charger imposes a different maximum charging voltage, the engineering review evaluates two approaches.

First, selecting a cell chemistry and series count whose standard full-charge voltage falls within the charger's operating range.

Second, where the cell manufacturer's specifications permit and charging control is adequate, operating below the normal full-charge level. This uses a partial state-of-charge range — usable capacity and runtime are reduced, and not all chemistries are suitable for sustained restricted-voltage operation.

In either case, the review covers charging current limits, termination behavior, and capacity impact. For multi-cell series configurations, cell-voltage monitoring and balancing should be evaluated to prevent any cell from exceeding its safe limit under worst-case conditions.

The charger regulates normal charging. The BMS overcharge threshold provides an independent safety layer — these are separate functions. Final compatibility must be confirmed through design review and device-level testing with the production charger, cells, and protection circuit.

BMS or protection circuit selection must account for the device's actual current profile — not just the nominal load. The continuous operating current, peak current amplitude, peak duration, repetition frequency, and recovery time all affect whether the protection circuit will allow normal device operation without tripping. The overcurrent threshold should accommodate real peak demand with appropriate margin. Beyond the protection IC settings, the MOSFET ratings, wiring resistance, and connector current ratings also limit the system's actual current-handling capacity. A protection circuit rated for a given current does not guarantee that the complete battery pack will deliver that current without excessive voltage sag or heat generation.

Nominal capacity is measured under controlled conditions that may not reflect actual device operation. Real-world runtime depends on the actual load profile, the device's discharge cutoff voltage, power conversion efficiency, operating temperature, and battery aging over time. The device's cutoff voltage, the cell's safe operating range, and conversion losses between the battery and the load all affect how much of the nominal capacity is usable in practice. Higher discharge currents further reduce usable capacity due to internal resistance losses. Runtime estimation should be validated on the actual device under representative conditions — THOR Power does not guarantee specific runtime or backup-time figures without supporting test data.

The finished battery pack typically requires a larger installation envelope than the bare cell. The PCM or BMS board, NTC sensor, wire exit path, connector, insulation, and protective wrapping all contribute to the final dimensions. Manufacturing tolerances, assembly clearance, and applicable cell swelling allowances — particularly relevant for LiPo pouch cells over extended cycling — should be factored into the device enclosure design. The wire outlet direction, connector position, and fixing method also affect final assembly. THOR Power confirms the expected finished-pack envelope before sampling so the device housing can reserve appropriate clearance. For projects using LiPo pouch cells, see Custom LiPo Battery Packs. For cylindrical-cell configurations, see 18650/21700 Battery Packs.

Sample validation should reproduce real device conditions — not just check voltage and capacity on a bench. Key validation points include: finished dimensions and mechanical fit in the actual device enclosure, connector and polarity verification, charging behavior with the actual charger, continuous and peak load performance, voltage sag under load, runtime under representative conditions, temperature rise during operation and charging, and protection circuit behavior including overcurrent trip and recovery. A sample that powers on a device is not automatically validated for all operating conditions or approved for mass production — the transition requires explicit specification lock and production readiness confirmation. For the recommended validation approach, see How Custom Battery Projects Work.

Certification requirements depend on the final battery pack configuration, target device, and destination market.

UN38.3 is a transportation safety test for lithium batteries, applying to the specific pack configuration shipped, not only the bare cell.

IEC 62133-2 is a safety standard for portable rechargeable lithium batteries — applicability depends on the target market and regulations.

UL certification may be required for certain product categories in North America. Not all battery packs require UL — the applicable standard depends on the end product and its regulatory category.

CE marking indicates conformity with applicable EU legislation. Regulation (EU) 2023/1542 introduces requirements for sustainability, labeling, due diligence, and end-of-life management, phased in over several years — supply chain due diligence has been postponed to August 2027 by Regulation (EU) 2025/1561. A CE-marked battery requires a Declaration of Conformity; specific obligations depend on the battery category.

Cell-level certifications do not automatically cover the finished pack. Applicable requirements should be identified during project planning before production begins.