Custom-shape programs spend months on geometry and capacity, then pick the connector in an afternoon from whatever the last project used. It is the cheapest part of the pack and the one we see fail most often in the field, because it is the only part that gets handled by a human during assembly and service.

Start from the current, not the catalogue

Pick the contact rating first, then the pitch that supports it. Common small-format families, by pitch:

  • 1.00 mm (JST SH and equivalents) — around 1 A per contact. Fine for wearables and sensors; too tight for anything with a motor or a high-inrush DC-DC.
  • 1.25 mm (Molex PicoBlade and equivalents) — around 1 A. Slightly more robust mechanically than 1.0 mm.
  • 1.50 mm (JST ZH and equivalents) — around 1 A, with a friction lock available.
  • 2.00 mm (JST PH and equivalents) — around 2 A. The default for anything above a few hundred milliamps.

Treat published per-contact ratings as a ceiling measured on a single mated pair at room temperature. Derate for ambient, for adjacent powered contacts, and for the fact that a pack connector may be mated and unmated during service.

Peak current is the number that matters

Size on the peak, not the average. A device drawing 200 mA average with 2 A radio bursts needs a 2 A interface. If you do not know your peak, measure it — the same current trace you built to size the cell (see sizing from a power profile) already contains it.

Wire gauge and where the heat goes

Gauge is chosen for two independent reasons: current-carrying capacity and voltage drop. In a small pack, voltage drop usually binds first — a few tens of millivolts lost in the harness comes straight out of your low-battery margin.

Thinner wire also means a more fragile joint at the tab weld. For pouch cells we default to 26 AWG for general use and 24 AWG where peak current or harness length justifies it; below 28 AWG the mechanical reliability of the termination, not the copper, is the limiting factor.

Polarity: design it out, do not document it out

A reversed pack connector destroys the host board, and a note in the assembly manual will not prevent it. Three mechanisms, in order of how much we trust them:

  1. Keyed and polarised housings. Physically impossible to seat backwards. Always the first choice.
  2. Asymmetric pin count or position — a three-pin housing with the thermistor off-centre cannot be reversed even if the shell is symmetric.
  3. Reverse-polarity protection on the board. A back-to-back FET costs a few cents and covers the case where someone sources a compatible-but-unkeyed housing during a shortage.

Use at least two of the three on anything that will be serviced in the field.

The thermistor line

If the pack carries an NTC for charge-temperature sensing — and it should, for the reasons in managing skin-contact temperature — that is a third conductor and a third contact. Decide early: a two-pin pack cannot gain a thermistor later without new tooling on the housing and a new harness part number.

Strain relief is a mechanical requirement, not a nicety

The failure mode we see returned most often is not a bad crimp. It is wire fatigue a few millimetres from the tab weld, caused by the harness flexing every time the enclosure is opened or the pack shifts inside it. Kapton anchoring at the cell, a service loop, and a housing that constrains the wire exit direction fix nearly all of it.

For stepped and curved packs the exit geometry interacts with the cell shape — worth resolving during the co-design phase described in our battery co-design workflow rather than after the tooling is cut. Tooling changes are not cheap; see what custom tooling actually costs.

What to put on the drawing

  1. Connector manufacturer part number for both halves, plus the crimp terminal part number.
  2. Wire gauge, insulation type and finished length, with tolerance.
  3. Pin-out with polarity called out explicitly, including the thermistor if present.
  4. Strain-relief detail at the cell end.
  5. Pull-test acceptance criterion for the terminated harness.

A pack drawing missing any of these will be built to somebody's assumption, and it will not be yours.