The engineering decision
Accept a cell for the application only after its loaded-voltage and temperature behavior meets the equipment limits at the required current, state of charge and environment.
Have these inputs ready
- Cell test conditions and discharge curves
- Equipment voltage window and cutoff
- Current versus time, including repetition
- Cell temperature, SOC and aging condition
Same cell. Two operating points.
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Separate charge, energy and power
Amp-hours integrate current over time. Watt-hours integrate terminal voltage multiplied by current. A nominal-voltage-times-capacity estimate is useful for screening, but the energy delivered to equipment follows its actual loaded-voltage curve and stopping conditions.
The lowest usable voltage is a system boundary. An inverter can reach its undervoltage threshold while the cells still retain charge. A cell-level protection event, a hot connector or a thermal limit can end the mission earlier still. Record the first limit reached rather than reporting only the final Ah result.
| Quantity | Measurement | Design decision |
|---|---|---|
| Capacity (Ah) | Integral of current to the defined cell cutoff | Charge available at that test rate |
| Usable energy (Wh) | Integral of terminal power to the equipment limit | Mission runtime |
| Power capability | Loaded voltage during each current event | Acceleration and actuator support |
| Thermal behavior | Temperature versus time in the enclosure | Continuous and repeated load limits |
What changes between 0.2C and 5C?
Take a hypothetical 30 Ah cell and assume 4 mΩ effective resistance at the stated operating point. Increasing current from 6 A to 150 A multiplies the resistive voltage drop by 25 and instantaneous resistive heating by 625.
The 90 W result estimates the resistive contribution under this fixed-resistance model. Cell heat also includes reversible electrochemical heat, and pulse resistance depends on the observation interval. Predicting temperature requires a validated heat-source model, heat capacity, heat transfer and time.
| Current | C-rate | I × R drop | I²R loss |
|---|---|---|---|
| 6 A | 0.2C | 0.024 V | 0.144 W |
| 60 A | 2C | 0.240 V | 14.4 W |
| 150 A | 5C | 0.600 V | 90.0 W |
Find the operating corner that determines the pack
Screen manufacturer curves at the relevant discharge current and temperature. Then test the assembled pack, including cells, joints, switches, fuse, connectors and cables. Cold cells, low SOC and aged cells should be represented in the validation matrix where they fall inside the service requirement.
At constant equipment power, falling battery voltage raises input current. That can deepen sag and increase heating. A test performed at constant current can therefore underrepresent the end-of-discharge demand of a regulated load.
Parallel cells reduce ideal branch current, but unequal interconnect resistance and thermal conditions can prevent equal sharing. Select the series count from the full voltage window and determine parallel count from energy, current and packaging together.
Recognize the failure signature
| Observation | Possible mechanism | Next evidence |
|---|---|---|
| Good low-rate Ah; early loaded cutoff | Sag reaches equipment input limit | Synchronized terminal V and I |
| One series group drops faster | Capacity, resistance or SOC mismatch | Group voltage and controlled cell testing |
| Terminal voltage low; cell stack stable | Current-path loss | Segment voltage-drop measurements |
| First pulse passes; later pulses fail | Heat accumulation or protection recovery | Temperature and event log across repetitions |
Measurement & validation procedure
| Test / review | Record | Acceptance condition |
|---|---|---|
| Reproduce the mission | Terminal V, current, delivered Wh and stop reason | Required mission finishes within equipment limits |
| Test service corners | Low SOC, specified temperature extremes and aging state | Required events pass at each specified corner |
| Inspect the current path | Temperature at cells, FETs and joints | Every component stays within its approved operating envelope |
Continue this engineering task
Related engineering guidance
Prepare the evidence for review
- Cell data sheet and test report
- Recorded load trace
- Equipment cutoff and operating temperature
- Pack wiring and shutdown log
Technical sources & claim support
References support the stated engineering principles. Worked examples use their explicitly stated assumptions.
- Molicel INR-21700-P45B product data sheet, revision 1.2 ↗
Cell specifications; discharge-rate and temperature characteristics
Cell-specific capacity, impedance and rate/temperature data; worked examples state their own input values.
- Keysight: How to Test Coin Cell Batteries ↗
Coin Cell Battery Testing and Modeling: four-wire sensing, pulse resistance, pulse width/rest time and defined cutoffs
Measurement principles illustrated with coin cells. Select the current profile, fixture, sampling, temperature and cutoff conditions for the actual cell or pack.
- Texas Instruments BQ76952 data sheet, SLUSE13B ↗
Sections 7 and 10–12: electrical characteristics, measurement, protection and FET control
Example of a monitor/protector architecture; capabilities depend on the selected IC and complete circuit.
- Richardson & Howey: Sensorless Battery Internal Temperature Estimation (IEEE TSTE, 2015) ↗
Sections III-A and V–VIII; DOI 10.1109/TSTE.2015.2420375; author manuscript
Validated on one 2.3 Ah cylindrical LFP cell; model parameters and accuracy do not transfer to other cells.
Key claims and evidence
| Claim | Basis | Reference |
|---|---|---|
| Cell data separates capacity and condition-dependent discharge performance | documentation | Molicel INR-21700-P45B product data sheet, revision 1.2 |
| Pulse resistance requires defined test conditions | documentation | Keysight: How to Test Coin Cell Batteries |
| At 150 A and 4 mΩ, resistive loss is 90 W | calculation | calculation |
| Application acceptance needs the first system limit, not Ah alone | inference | Molicel INR-21700-P45B product data sheet, revision 1.2; Texas Instruments BQ76952 data sheet, SLUSE13B |