Selection & System Sizing
How to Specify a Custom Li-Polymer Battery
Define the cell envelope, tab or wire exit, capacity, load profile, protection, connector, swelling allowance and compliance needs.
Quick answer
Specify a custom Li-polymer battery with a maximum thickness × width × length envelope, usable capacity or energy, nominal/charge voltage, continuous and pulse current, wire or tab exit, connector and pinout, protection circuit, temperature range, swelling clearance, device cutoff and annual quantity. Thickness is especially important and must include realistic expansion and tolerance.
Start with a controlled drawing
Use maximum allowed thickness × width × length and show the measurement convention. Mark the position and direction of tabs or wires, protection circuit, connector, cable length, adhesive area and any keep-out zones. Provide a 3D device cavity drawing when enclosure ribs, screws or curved surfaces constrain the cell.
Do not treat pouch thickness as fixed. Allow for production tolerance, state of charge, temperature, aging and normal gas generation within the cell manufacturer’s limits. The device must not clamp or puncture the pouch.
Electrical specification
Record:
| Input | Required definition |
|---|---|
| Voltage | Nominal voltage, maximum charge voltage and device cutoff |
| Capacity | Minimum required Ah or Wh at a stated test rate |
| Load | Continuous current and pulse current with duration |
| Charge | Current, method and charger or power-management IC |
| Life | Expected cycles, depth of discharge and end-of-life criterion |
| Temperature | Separate charge, discharge and storage ranges |
Single-cell Li-polymer products are often described as 3.7 V nominal, but higher-voltage variants exist. Never infer the charge limit from the generic product name.
Protection and connector
Define whether the cell includes a protection circuit module and required overcharge, overdischarge, overcurrent and short-circuit behavior. Specify thermistor type and value if used. Give the exact mating connector, pinout, polarity, wire gauge, wire length and pull/strain requirements.
A connector photo is not enough because visually similar housings can have different pitch, keying and polarity.
Mechanical integration risks
- Avoid sharp edges and pressure points near the pouch.
- Keep the cell away from heat sources and moving parts.
- Do not fold or repeatedly flex the pouch tabs.
- Provide retention that controls movement without crushing the cell.
- Define drop, vibration and device-level abuse conditions.
Prototype and production approval
Prototype evaluation should check fit, charging, peak load, runtime, temperature, protection behavior and device cutoff. Production approval should control the drawing, cell source, protection circuit, connector, labeling and test limits.
Identify destination and required transport or market documents. Documents must match the final configuration; similar-cell paperwork should not be presented as proof for a custom pack.
FAQ
Can capacity be maximized to fill the complete device cavity?
The design needs clearance for manufacturing tolerance, protective components, wiring and normal cell expansion. Filling the entire rigid cavity can create mechanical stress.
Is a 3.7 V Li-polymer cell compatible with every 3.7 V device?
No. Confirm maximum charge voltage, cutoff, current, connector polarity, thermistor, protection and physical fit.