Why per-cell temperature control matters in battery testing

Ask any battery engineer which test variable they worry about most and temperature will be near the top. It changes how fast a cell ages, how much capacity it delivers and how its resistance evolves. Yet in most labs, temperature is controlled for the air around the cells, not for the cells themselves. This post explains why that distinction matters and what changes when you control temperature at each cell.

Temperature drives ageing

The side reactions that consume lithium and electrolyte, such as SEI growth, speed up with temperature, broadly following Arrhenius behaviour. Low temperatures bring their own failure modes, notably lithium plating during charge. That is why cycle-life and qualification campaigns specify a test temperature, and why many run the same protocol at several temperatures to separate thermal effects from cycling effects.

It also means a few degrees of uncontrolled variation can show up as differences between replicate cells that have nothing to do with the cells.

The chamber model, and where it falls short

The conventional setup is a cycler on the bench and the cells inside an environmental chamber. It works, and for some tests it is the only option. But it has three structural limits.

1. The chamber controls air, not cells

Cells heat themselves while they cycle, through resistive (I²R) losses and reversible entropic heat. At higher C-rates a cell can sit several degrees above the air around it, and the gap changes through every charge and discharge. A chamber regulates its air sensor; the cell surface is a different, moving number.

2. Position matters inside the chamber

Air temperature varies with airflow, distance from the heater or evaporator, and how densely the chamber is loaded. Two cells running the same protocol in different corners can see different temperatures, which shows up as unexplained spread between replicates.

3. One chamber, one setpoint

Every cell inside the chamber shares a temperature. A study at 25, 35 and 45 °C needs three chambers, or three campaigns run one after another, which multiplies either the equipment or the calendar time.

Controlling temperature at the cell

The alternative is to put each cell in its own temperature-controlled fixture and close the loop on a sensor at the cell. A solid-state thermal fixture suits this well: it can heat or cool, it has no moving parts, and it sits directly against the cell holder.

On the Cyclone Eye CBT-15, each cell channel is paired with a second channel that drives the cell’s thermal fixture, regulated on a PT100 sensor at the cell. An 8-channel unit therefore runs four temperature-controlled cells, each at its own setpoint.

• The cell is the controlled variable. Self-heating is measured and compensated rather than ignored.
• Several temperatures in parallel. A multi-temperature study runs on one bench at the same time, instead of across chambers or campaigns.
• Less bench infrastructure. No chamber to house, power and maintain next to every cycler.
• Temperature data you can trust. The temperature recorded alongside each cycle is the temperature of that cell.

Where you still need a chamber

Per-cell control has limits, and they are worth stating plainly. Per-cell thermal fixtures work within a band around room temperature; on the CBT-15 that is ±25 °C from ambient. Deep sub-zero testing, high-temperature ageing beyond that band, and abuse testing such as thermal runaway still belong in a chamber built for them. Many labs will use both: per-cell control for most cycle-life and multi-temperature work, and a chamber for the extremes.

What it means for your data

Cycle-life testing is slow, so every source of noise costs weeks. Holding the cell, rather than the air, at its setpoint removes one of the largest sources of variation between replicates, and running temperatures in parallel shortens the calendar time for multi-temperature studies. Cleaner, denser data also makes early life prediction more reliable, which is where much of the time saving in battery development now comes from.