The most expensive way to develop a custom battery is to finalise the product enclosure design first and then ask a battery supplier to fit something into the remaining space. By that point, the tolerance stack is fixed, the connector position is decided, the PCB layout is committed, and the battery supplier is asked to perform a miracle in a box they were not consulted on. The result is either a compromised cell, a redesign, or a program delay. The correct approach is to engage the battery supplier at the same stage you engage your mechanical design partner.

Why projects fail: engagement timing

In a survey of 40 custom battery programs we reviewed over 2024–2025, the programs that required the most costly design changes shared a common pattern: the battery supplier was first contacted after the product industrial design was locked. At that stage, the product team had already committed to:

  • A specific enclosure volume and wall thickness
  • A PCB layout with the battery connector in a fixed position
  • An NTC thermistor location based on assumed cell geometry
  • A BMS IC selected for a specific cell voltage range

All four of these choices interact directly with cell geometry. Changing them after ID lock is expensive. Changing them before ID lock is free.

Stage 1: Space claim (week 1–2)

The battery supplier needs a 3D model of the battery envelope — the specific volume available for the cell, including the clearances required for the enclosure wall, the PCB standoffs, and the thermal interface. This is not the same as the product enclosure model. It is the battery-specific space claim: the maximum bounding box the cell can occupy in all three dimensions, with tolerances.

Deliverable from the OEM: STEP file of the battery envelope, maximum dimensions with ± tolerances, tab exit direction, minimum tab length, and target capacity at end of life (cycle 500).

Deliverable from the supplier: Feasibility note — can the target capacity fit in this envelope? If not, what capacity is achievable, and what is the gap?

Stage 2: Chemistry and voltage selection (week 2–3)

Once the envelope is confirmed feasible, the supplier proposes a chemistry and cell architecture. This involves:

  • Electrode chemistry selection (LCO, HV-LCO, NMC 111, NMC 532) based on the capacity target and cycle life requirement
  • Voltage window (standard 4.20 V or high-voltage 4.35/4.48 V) — higher voltage increases capacity density but adds complexity to the BMS and may require different charger IC selection on the OEM side
  • Cell architecture (number of electrode layers, electrode thickness) to match the capacity in the available thickness

This stage requires chemistry specification agreement in writing — it is the basis for all subsequent testing and qualification. If the OEM later requests a voltage or chemistry change, Stage 1 and Stage 2 restart.

Stage 3: Prototype fabrication and first FIT test (week 4–8)

The supplier produces 10–20 prototype cells using hand-built electrodes or laser-cut electrode plates rather than production tooling. These cells are mechanically representative of the final design but electrically may differ by ± 10% in capacity from the production target. Their purpose is FIT testing — fitting in the product enclosure to verify that:

  • The cell fits within the space claim with adequate clearance
  • The tab exits correctly and mates with the PCB connector
  • The NTC thermistor mounts correctly on the cell body
  • The cell dimensions are consistent with the mechanical design assumptions

After FIT testing, the OEM provides a signed geometry approval or a list of changes required. A first-pass approval is uncommon — plan for one to two minor geometry iterations at this stage. Each iteration adds 2–3 weeks and does not require new tooling (prototypes continue to be hand-built).

Stage 4: Pre-production tooling and first article inspection (week 8–16)

Once the geometry is approved, the supplier builds production tooling (see the tooling cost breakdown in the related article). The first cells produced with production tooling undergo a first article inspection (FAI) covering:

  • Dimensional verification against the approved drawing (all critical dimensions with CMM or caliper data)
  • Electrical parameters: OCV, capacity at C/5, internal resistance
  • Basic abuse tests: external short, overcharge, forced discharge
  • Mechanical tests: tab pull strength, pouch seal integrity

FAI typically consumes 30–50 cells. It is not a certification — it is a production readiness check. An FAI pass means the production line is capable of making cells to the agreed drawing.

Stage 5: Production qualification (week 16–24)

Production qualification runs the cell through the full IEC 62133-2 + UN 38.3 test stack (see the compliance walkthrough for what this entails). The cells for qualification must come from three separate production runs — three different batches — to verify that the process is stable. Qualification takes 8–12 weeks from sample submission to final reports.

Stage 6: Production transfer and ongoing control (week 24+)

After qualification, the supplier establishes a control plan: which parameters are measured on every lot (OCV, IR, capacity sample), which require formal lot release (dimensional, electrical), and which trigger a deviation notification to the OEM (any parameter outside the agreed specification). The control plan is a living document — it is updated when a production change (material substitution, line reconfiguration, yield improvement) is proposed.

The supplier must notify the OEM of any change to materials, electrode formulation, or production process that could affect cell performance or safety — even if the change appears to be an improvement. Many OEM-supplier disputes originate from undisclosed process changes that affected cell behaviour in the end product without the OEM's knowledge.