Reading a Prismatic Cell Datasheet: Key Specs Explained

Short answer: A prismatic cell datasheet specifies nominal capacity, C-rate limits, cycle life, internal resistance, operating temperature and mechanical constraints. The figures only become actionable when you also know the test conditions under which they were measured — particularly depth of discharge, temperature and charge current. Without those conditions, a cycle number is meaningless.

Every prismatic LFP and NMC cell ships with a datasheet that runs to several pages. Most of the numbers on it are straightforward. A handful require careful reading because the same headline figure — say, 3,000 cycles — can represent either a robust cell or a marginal one, depending on what the manufacturer tested to produce it. This article works through the critical parameters in order of how frequently they cause specification errors.

Nominal capacity versus usable capacity

Nominal capacity is the total charge a cell can deliver under standardised reference conditions, expressed in ampere-hours (Ah). It is a laboratory benchmark: typically a 0.2C constant-current discharge from full charge to the manufacturer's lower cut-off voltage, at 25 °C.

Usable capacity is what your application actually extracts. It is always lower than nominal because:

• Voltage window: most pack designers operate within a narrower voltage band than the full cell window to protect cycle life. A cell rated to discharge to 2.5 V (LFP) may be controlled to 2.8 V in the field.
• Temperature: capacity falls at low temperatures, typically 10–20 % at 0 °C and 30–40 % at −20 °C for LFP cells, depending on chemistry and C-rate.
• C-rate: higher discharge rates reduce delivered capacity due to internal resistance losses.

When sizing a pack, design to usable capacity at worst-case operating temperature and the highest C-rate the application requires, not to the nominal figure at 25 °C.

C-rate: continuous versus peak

C-rate expresses current relative to capacity. A 1C discharge of a 100 Ah cell is 100 A; a 2C discharge is 200 A.

Datasheets typically state two current limits:

• Continuous discharge current: the rate the cell can sustain indefinitely without exceeding its temperature limits. This is the figure to use for steady-state loads — drivetrain cruise power, continuous inverter output.
• Peak discharge current: a higher rate permitted for a limited duration (commonly 10–30 seconds). The permissible peak and its maximum duration are both stated on the datasheet; both matter.

Charge C-rate limits are separate figures. Fast-charge acceptance is constrained at low temperatures and at high state of charge (SOC), which is why datasheets often include a charge rate de-rating table across temperature.

Parameter	What to read on the datasheet	What to verify
Nominal capacity	Ah at 0.2C, 25 °C	Usable Ah at your operating C-rate and temperature
Continuous discharge	Maximum sustained current (A or C)	Duration limit, if stated
Peak discharge	Maximum pulse current (A or C)	Pulse duration (s) and recovery time
Charge rate	Standard and fast-charge current	De-rating at low temperature and high SOC
Cycle life	Number of cycles to stated capacity retention	DoD, temperature, charge/discharge C-rate
Internal resistance	DC internal resistance (mΩ)	Measurement method (DCIR or AC impedance), SOC and temperature
Operating temperature	Charge and discharge ranges (°C)	Storage temperature range
Swelling force	Maximum allowable pressure (N or kN)	Expansion over cycle life

Cycle life and the test conditions that define it

This is where the most significant specification errors occur. A cycle life figure — 2,000 cycles, 3,000 cycles, 6,000 cycles — carries no actionable information unless the datasheet or a referenced test protocol states:

1. Depth of discharge (DoD): cycles measured at 80 % DoD represent considerably less stress per cycle than 100 % DoD. A cell claiming 3,000 cycles at 80 % DoD may reach only 1,800 cycles at 100 % DoD.
2. Temperature during cycling: 25 °C is the standard test temperature. Operation at 35–40 °C accelerates ageing in most lithium chemistries; operation below 10 °C may increase cycle count but reduce capacity per cycle.
3. Charge and discharge C-rate during the test: cycle life measured at 0.5C charge / 0.5C discharge will generally be higher than at 1C / 1C. The test C-rate should match, or be more demanding than, your application duty cycle.
4. End-of-life criterion: the cycle count to 80 % capacity retention (the industry standard) and to 70 % retention are different figures. Confirm which threshold the manufacturer uses.

If the datasheet omits any of these four conditions, treat the cycle figure as unverified. Request the underlying test protocol or a cycle life curve (capacity versus cycle number) before using the figure in a system design.

IEC 62619:2022, the main safety standard for stationary lithium battery applications, requires that cycle testing conditions be documented as part of the safety case (IEC, iec.ch). This is not an accident: the standard recognises that an undocumented cycle figure cannot be validated.

Internal resistance

Internal resistance (IR) determines heat generation under load and is the primary driver of voltage sag at high C-rates. Datasheets state IR in milliohms (mΩ); lower is better, but the measurement method matters:

• DC internal resistance (DCIR): measured as ΔV/ΔI across a current pulse, typically at 50 % SOC and 25 °C. Most representative of real discharge conditions.
• AC impedance (EIS): measured at 1 kHz; tends to give a lower figure because it excludes diffusion resistance. Less representative of DC load conditions.

IR increases with age and at low temperature. A cell with a fresh IR of 0.25 mΩ may reach 0.40–0.50 mΩ at end of life, which translates directly to higher heat generation and lower effective voltage. This matters for thermal management design.

Operating temperature window

The datasheet states separate temperature ranges for discharge, charge and storage. The charge range is always narrower than the discharge range: most LFP cells cannot accept charge below 0 °C without lithium plating on the anode, regardless of how slowly the current is applied. This is a hard limit, not a de-rating.

For applications where the cell will operate or be stored below 0 °C — refrigerated transport, outdoor stationary storage in northern climates — verify whether the datasheet includes a low-temperature charge inhibit specification and design the BMS accordingly.

Swelling force and mechanical mounting

Prismatic cells expand during charge and contract during discharge. Over a cell's lifetime, there is also irreversible growth as electrode materials change structure. Datasheets for prismatic cells state a maximum allowable compression force (in Newtons or kilonewtons) and sometimes the expected thickness change over life (in millimetres or as a percentage).

This specification directly constrains the mechanical design of the module or pack. Insufficient clamping allows the cell stack to move, reducing electrical contact quality and accelerating degradation at the electrode tabs. Excessive clamping compresses the electrodes, increases internal resistance and can cause mechanical failure. The battery modules from Avantis Energy are designed with the swelling characteristics of specific cell types in mind — it is one of the reasons that mixing cell types within a standardised module format requires engineering review.

Swelling force data is also relevant for second-life applications: a cell at 80 % capacity retention will have a different mechanical profile than a new cell. The International Energy Agency has noted the growth of second-life stationary applications as a structural trend (IEA, iea.org); accounting for aged-cell mechanical behaviour is part of responsible second-life pack design.

Putting the datasheet to work

The practical sequence when evaluating a prismatic cell datasheet is:

1. Confirm nominal capacity is measured under conditions close to your operating conditions, or apply the relevant de-rating.
2. Check that continuous discharge C-rate covers your maximum sustained load with margin.
3. Verify cycle life test conditions (DoD, temperature, C-rate, end-of-life criterion) against your application duty cycle.
4. Note DCIR and confirm the thermal model can handle the resulting heat at peak current.
5. Check the charge temperature window against your lowest expected ambient temperature.
6. Record the swelling force limits and expansion tolerance and carry them into the mechanical design.

For LFP versus NCM chemistry considerations that also appear on the datasheet — particularly around voltage curve flatness, energy density and cycle life — see the comparison on NCM batteries vs LFP batteries.

Frequently asked questions

What is the difference between nominal capacity and usable capacity on a prismatic cell datasheet?
Nominal capacity is measured at 0.2C and 25 °C to the full voltage window. Usable capacity is what your application extracts, accounting for a narrower voltage window, operating temperature and discharge rate. In practice, usable capacity is typically 80–95 % of nominal under normal operating conditions.

Why does a cycle life figure on a prismatic cell datasheet mean nothing without test conditions?
Cycle life depends on depth of discharge, temperature, charge and discharge C-rate, and the end-of-life capacity threshold. A cell tested at 80 % DoD and 25 °C will show a substantially higher cycle count than the same cell tested at 100 % DoD or elevated temperature. Without those conditions, the figure cannot be validated or compared.

What is the difference between continuous and peak C-rate on a cell datasheet?
Continuous discharge C-rate is the current the cell can sustain without exceeding its thermal limits. Peak C-rate is a higher current permitted for a short, specified duration — typically 10 to 30 seconds. Both figures, and the peak duration, must fall within your application's requirements for the design to be safe.

Why can prismatic lithium cells not be charged below 0 °C?
Below 0 °C, lithium ions plate onto the graphite anode as metallic lithium rather than intercalating into it. This is irreversible, reduces capacity and creates a safety risk. The charge temperature window on the datasheet reflects this limit; a BMS must enforce it regardless of charge rate.

What does swelling force mean on a prismatic cell datasheet, and why does it affect pack design?
Prismatic cells expand during charging and over their service life. The datasheet states the maximum allowable compressive force in Newtons or kilonewtons. Too little clamping allows cell movement and contact degradation; too much compresses the electrodes and raises internal resistance. Mechanical enclosures must be designed to maintain the specified pressure range across the full range of temperature and cycle expansion.

Specify with confidence

For detailed datasheets on the cells and modules Avantis Energy supplies from stock, view the full product range or ask a technical question — we are happy to review your application requirements and confirm which specifications to prioritise.