Custom Lithium Battery Pack: When Standard Batteries Don't Fit

Typical Requirements: Mechanical, Thermal, Electrical

The most common trigger for a custom pack is the installation envelope: a forestry machine housing has a defined slot, an outdoor instrument allows only a housing with a specific wall thickness, a specialist vehicle has a compartment that matches neither the length nor the height of standard formats. Here, design starts with the envelope dimensions — not the catalogue.

Thermally, many special applications impose demands that exceed the specification of a standard product: operation at −20 °C in winter service, heat generated by high pulse currents in measuring instruments, or continuous solar irradiation on vehicle bodies. These conditions determine whether passive insulation, an active heating foil, or an actively cooled housing solution is required — and which cell chemistry (LiFePO4 vs. Li-NMC) offers the more favourable thermal properties for the specific load case.

Electrically, nominal voltage, usable capacity, maximum discharge current and charging strategy are the four cornerstones of the specification. Added to these are the communication interface (CAN, SMBus, UART, proprietary), the connector ingress protection rating (at least IP54 for outdoor use, IP65/IP67 where water spray or wash-down exposure is present), and — particularly relevant in vehicle applications — the BMS safety shutdown logic in coordination with the superordinate vehicle controller.

Application Examples: Forestry, Survey, Emergency Vehicles

In forestry technology, harvesters and forwarders operate under extreme conditions: vibration, temperature swings from −25 °C in winter to above 50 °C in the engine bay during summer operation, dust, moisture and mechanical shock. A battery pack for an electrically driven forwarder must deliver a housing rigidity and shock absorption far beyond what a standard industrial battery pack offers. In addition, most forestry OEMs specify their own installation dimensions, making a custom solution practically unavoidable.

Survey and measurement instruments — geodetic instruments, mobile analysis units, drone ground equipment — require battery packs with high energy density in small volume, long storage without deep discharge, and a reliability where an unexpected field failure is not acceptable. LiFePO4 cells with low self-discharge and mature BMS protection are the reliability standard here; the form factor follows the instrument, not the other way around.

Emergency and authority vehicles — fire services, police, rescue services, military — are in some cases subject to public procurement rules that impose not only technical but also formal requirements: CE marking, type sheet, declaration of conformity, documentation for the procurement authority. At the same time, the installation spaces in specialist vehicle bodywork are individual, so every delivery is essentially a custom item. Typical voltage levels in these applications are 24 V, 48 V and 72 V — often in conjunction with a charging system fed from the vehicle power supply.

Certification Requirements and Standards Conformity

For every battery pack placed on the European market, CE marking is mandatory. Which directives apply depends on the application: the Low Voltage Directive (LVD, 2014/35/EU) applies to electrical equipment with a nominal voltage between 50 V AC and 1000 V AC or 75 V DC and 1500 V DC. Below these limits, the LVD does not apply, but the General Product Safety Regulation does. The EMC Directive (2014/30/EU) additionally applies if the battery pack contains active electronics (BMS) that can generate electromagnetic emissions.

For the transport of lithium batteries, the UN 38.3 tests apply: vibration, shock, impact, thermal stress, overcharge, short circuit, crush and external short circuit verification. These tests must be repeated for each new cell/pack combination — a test report for a similar configuration does not cover the new design. The test report is a mandatory document for air freight (IATA), sea freight (IMDG) and road transport under ADR.

For applications in machinery or vehicles, the Machinery Directive (2006/42/EC) or its successor regulation may additionally apply if the battery pack is classified as a safety-relevant component of a machine. In that case, a risk assessment to EN ISO 12100 is part of the documentation, and the BMS safety concept must demonstrably be aligned with the machine concept.

Process: Specification → Prototype → Series Release

A custom battery starts with the specification document: the document that unambiguously describes all requirements — envelope dimensions, voltage level, capacity, discharge and charge current, temperature range, ingress protection rating, communication interface, certification requirements, planned unit volume and logistics requirements. A complete specification prevents later room for interpretation and is the basis for a binding quotation.

The specification is followed by cell selection and pack design: which cell chemistry (LiFePO4 for longevity and thermal safety, Li-NMC for higher energy density) suits the load case? How are the cells connected (series/parallel configuration) to achieve the target voltage and capacity? Which BMS model meets the communication and protection requirements? The result is a defined bill of materials.

The next step is the prototype phase: manufacture of one or more functional prototypes for mechanical installation testing, electrical commissioning and environmental testing at the customer's premises. Based on the prototype, changes are defined and incorporated before the first article inspection report (FAIR) is formally closed.

The series release requires that all tests (UN 38.3, CE-compliant type test, BMS parameterisation) are completed and documented, and that series production is running under controlled conditions with incoming goods and outgoing goods inspection. Hellpower Energy supplies the complete conformity documentation — declaration of conformity, test reports, safety data sheet — as a mandatory part of every series delivery.