Battery Test Methods: Cell, Module and Pack Cycling

Testing at three levels

The same physics applies from cell to pack, but the scale and safety envelope shift dramatically.

• Cell testing works at low voltage (typically 0–5 V per cell) and demands tight current accuracy and high readback resolution, because coulomb counting at the cell level is how capacity and coulombic efficiency are derived.
• Module testing aggregates cells into the tens-of-volts range and introduces balancing and thermal-gradient effects that a single cell never shows.
• Pack testing reaches hundreds of volts (EV packs commonly 400–800 V, with 1000 V+ architectures emerging) and high current, so the test source must combine high voltage, high power and a controlled sink path for discharge.

A wide-range supply such as the 0–2250 V class can address all three levels from one instrument, which is why labs consolidate on high-ceiling bidirectional sources rather than buying a separate cycler per level.

Charge-discharge profiles that matter

Characterisation is built from a handful of standard profiles, each isolating a different property:

1. CC-CV charge. Constant current to a voltage ceiling, then constant voltage while current tapers, the universal lithium-ion charge profile.
2. CC / CP discharge. Constant-current discharge gives capacity in amp-hours; constant-power discharge better mirrors how a motor or converter actually loads a pack.
3. Cycle life. Thousands of repeated charge-discharge cycles to track capacity fade and resistance growth over the battery's life.
4. HPPC / DCIR pulses. Hybrid pulse power characterisation and DC internal-resistance pulses extract impedance versus state of charge, the data that feeds battery emulation models (SoC/OCV/Ri).
5. Drive-cycle replay. Streaming a recorded current-versus-time profile (for example a WLTP trace) to validate real-world behaviour.

These profiles are exactly where instrument sequencing and the four-quadrant (charge and discharge, both polarities) capability of a bidirectional DC power supply earn their place: the same channel sources the charge and sinks the discharge without re-cabling.

Why regenerative cycling cuts energy and heat

A traditional dissipative load discharges a battery by burning the energy in resistive elements, dumping it as heat. In a cycle-life test that runs for weeks, that means two compounding costs: the electricity to discharge, and the HVAC load to remove the resulting heat from the lab.

A regenerative source instead returns discharge energy to the AC grid. With recovery efficiency reaching the low-90% range (the N35500 specifies up to 93%), the great majority of the energy pulled out of the battery on each discharge is fed back rather than wasted. Over thousands of cycles the savings are substantial, and the thermal load on the room collapses because the heat that a resistive bank would have produced is never generated in the first place.

The secondary benefit is thermal stability of the test itself: less waste heat near the cells means tighter, more repeatable temperature control, which directly improves the quality of capacity and impedance data.

Repeatability and safety versus real cells

Two distinct activities are easy to conflate: testing a real battery, and emulating one. When you are validating a battery management system, charger or inverter, putting a real pack on the bench is slow, hazardous and irreproducible, every cell ages between runs. Battery emulation replaces the pack with a programmable source that reproduces the SoC/OCV/internal-resistance behaviour from a model library, giving a perfectly repeatable, instantly re-settable, non-flammable target. The reference platform offers seven built-in battery libraries plus custom curves for exactly this.

The practical division: use real-cell cycling to characterise the battery; use emulation to test everything that connects to a battery. Both rely on the same accurate, fast, bidirectional source, which is why a single instrument class spans both jobs.

Frequently asked questions