How to Develop a Custom Battery Pack: A Practical Guide for Engineers and OEMs

Step 1: Define Requirements Completely

The first step is the most important — and the most frequently underestimated. Before cell chemistry or BMS are discussed, the following parameters must be fully and bindingly defined: nominal voltage and voltage range (e.g. 48 V nominal, 40–54.6 V), nominal capacity and usable energy content (Ah at which C rate and which DoD), maximum discharge and charge current (continuous current and pulse peak with duration), operating temperature range (charging and discharging stated separately), form factor and installation situation (external dimensions, weight limit, mounting points, connector position), and ingress protection class and environment (IP rating, vibration and shock loading per IEC 60068).

Incomplete requirements force compromises later — often at a point where design changes are expensive. A written specification document covering all the above points is a prerequisite for a reliable quotation and prevents misunderstandings between customer and developer.

Step 2: Cell Chemistry Selection and BMS Specification

LiFePO4 (LFP) is the more robust choice for most industrial applications: thermally more stable, higher cycle durability (typically 2,000–4,000 full cycles to 80 % SoH), less hazardous to dispose of, and less prone to thermal runaway. The drawback is lower gravimetric energy density (approximately 90–160 Wh/kg at cell level) and the flat discharge curve that complicates SoC calculation. Li-NMC offers more energy per kilogram (approximately 150–260 Wh/kg) and is better suited for applications where weight is critical and the temperature environment is controlled.

The BMS must match the chemistry, the cell configuration, and the communication protocol of the machine controller. Core parameters for the BMS specification: number of series cells and parallel strings, balancing strategy (passive or active), communication interface (CAN, CANopen per CiA-454, SMBus, RS485), protection functions (over/undervoltage, over/undertemperature, overcurrent, short circuit), behaviour at limit events (hard disconnect vs. orderly throttling via the vehicle controller), and diagnostic functions (SoC, SoH, individual cell voltages, fault log).

Cell selection and BMS selection should happen in parallel, because both depend on each other. A cell with a specific capacity and internal resistance defines how the BMS must dimension balancing and protection thresholds.

Step 3: Prototype Build, Testing, and Iteration

The prototype serves to verify the assumptions from steps 1 and 2 under real conditions — not to produce a saleable product. Typical test points in the prototype phase: capacity and range verification under the actual application load profile, temperature distribution in the pack under full and sustained load (thermal imaging), mechanical fit in the installation situation (connector, mounting, cable routing), BMS communication with the real vehicle controller (not just with the BMS manufacturer's PC tool), and behaviour at limit conditions (overcurrent shutdown, deep discharge protection, restart after fault).

Iterations between prototype and requirements are normal and must be planned for. Common findings from this phase: the pack generates more heat in operation than the simulation predicted, the connector position conflicts with the vehicle harness routing, or the BMS boot behaviour does not match the controller start-up cycle. Finding these issues early is the goal — not avoiding them.

Step 4: Certification — CE, UN 38.3, Safety Testing

Lithium battery packs placed on the European market are subject to several regulatory requirements. CE marking requires conformity with the Low Voltage Directive (LVD 2014/35/EU), the EMC Directive (2014/30/EU), and — when integrated into machinery — the Machinery Directive (2006/42/EC, superseded by the Machinery Regulation 2023/1230/EU from 2027 onwards). For industrial batteries, the Battery Regulation (EU) 2023/1542 is also relevant.

UN 38.3 testing is mandatory for all lithium batteries that are transported — regardless of whether they need to be declared as dangerous goods. The test sequence covers altitude simulation, thermal test, vibration, mechanical shock, external short circuit, impact, overcharge, and forced discharge. An accredited test laboratory issues the UN 38.3 test report, which is a mandatory document for insurers, customs authorities, and vehicle manufacturers.

Certification is not a step at the end — it must be considered in the design from the start. Choosing cell selection and pack design under CE and UN 38.3 requirements avoids costly rework. A test laboratory or certification consultant should be involved no later than the prototype phase.

Step 5: Series Readiness and Supply Chain

A successful prototype demonstrates that the design works. Series readiness additionally means: reproducible manufacturing with defined test steps (end-of-line test per pack), stable cell supply over the planned product lifetime, a documented quality assurance system, and the ability to qualify a replacement if a cell is discontinued.

For OEMs particularly relevant: supply chain transparency. Which cells are used — and from which manufacturer? Are they available from multiple distributors? What is the minimum order quantity for replacement packs? Does the battery manufacturer have a defined process for the event that a cell is discontinued? These questions separate a technically capable supplier from a production-ready partner.

Hellpower Energy supports OEM customers in the DACH region from requirements capture to series delivery — with an in-house engineering team at the Hausleiten, Austria site. Enquiries with a specification document or technical description are evaluated directly by the engineering team.