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Temperature gradient heat map across a BESS battery rack
Rahul Jalthar

Cell Temperature Gradients in BESS: Safe ΔT Limits and What Causes Uneven Heating

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⚡ Quick Answer: What Is a Safe Temperature Gradient in a BESS Pack?
A temperature gradient is the difference in temperature between the hottest and coolest cells in a pack at the same moment, often written as ΔT. Many BESS specifications target a maximum gradient of around 5°C across a rack, with premium liquid-cooled systems aiming closer to 2-3°C. A larger temperature gradient does not just mean one hot spot. It means cells are aging at different rates within the same pack, which widens the performance gap that cell matching worked to close in the first place.

1. Why Temperature Uniformity Is a Different Problem Than Cooling Capacity

Choosing between air and liquid cooling answers one question: how much heat can the system remove overall. It does not answer a second, separate question, however: does that heat leave every cell at the same rate? A BESS can have more than enough total cooling capacity. Even so, it can still run a large temperature gradient, if heat leaves some cells faster than others.

This distinction matters because gradient problems do not always show up as an overheating alarm. A pack can sit comfortably within its overall safe temperature range. Meanwhile, one corner of the rack quietly runs several degrees hotter than another, cycle after cycle. Nothing trips. Nothing alarms. The pack simply ages unevenly, and nobody notices until the SOH numbers start to diverge.

2. What Counts as a Safe Temperature Gradient

Exact gradient limits vary by manufacturer, cell chemistry, and system design. As a result, treat any single number as a target to verify, not a universal rule. That said, a few reference points are commonly cited in BESS specifications.

  • Around 5°C maximum cell-to-cell gradient is a commonly specified ceiling for air-cooled and moderately cooled BESS racks.
  • 2-3°C is a tighter target that premium liquid-cooled systems often aim for, particularly at utility scale, where thousands of cells raise the stakes of even small mismatches.
  • Gradient limits typically apply within a single rack or module first. They then get checked again at the full-system level, since gradients between racks can run larger than gradients within one rack.

Ask your supplier for their specific gradient target, not just their overall operating temperature range. A wide operating range, such as -20°C to 55°C, says nothing about how tightly matched cell temperatures stay relative to each other inside that range.

3. Three Root Causes of Uneven Cell Heating

Temperature gradient causes: coolant flow, cell position, busbar resistance

Temperature gradients rarely come from one single cause. Instead, three factors typically combine to create them.

Coolant Path Position

In a liquid-cooled rack, coolant usually enters at one point and exits at another, picking up heat along the way. Cells nearest the coolant inlet sit in cooler fluid. Cells nearest the outlet, by contrast, sit in fluid that has already absorbed heat from cells earlier in the path. As a result, outlet-side cells often run measurably warmer than inlet-side cells. This happens purely because of their position in the flow path, not because of anything different about the cells themselves.

Cell Position Within the Pack

Cells near the edge of a rack or enclosure sit closer to the outside walls, where some heat escapes to the surrounding air. Cells buried in the center of a dense pack, on the other hand, have neighbors on every side, so that heat has fewer places to go. Center cells, therefore, often run hotter than edge cells, even under identical cooling and identical current.

Current Path and Busbar Resistance

Current does not always split perfectly evenly across parallel cell groups. Small differences in busbar length, connection quality, or contact resistance mean some current paths carry slightly more current than others. Since heating from resistance follows I²R, even a small current imbalance produces a disproportionate heating difference. This connects directly to internal resistance variation covered in our cell matching guide: cells or groups with higher resistance generate more heat at the same current. As a result, a resistance mismatch and a temperature gradient often reinforce each other.

4. How a Temperature Gradient Accelerates Divergent Aging

Battery aging reactions speed up with heat. Researchers publishing in PMC (National Center for Biotechnology Information) found that inhomogeneous cell temperature inside a pack is a real, measurable driver of uneven degradation, not just a theoretical concern. Applied to a pack with a real gradient, this means the hottest cells are not just uncomfortable. They are quietly aging faster than their cooler neighbors, cycle after cycle.

This is where uneven heating and cell matching intersect. A pack that started out well matched, as covered in our cell matching guide, can still drift apart over time. A persistent hot zone can push those cells toward faster capacity fade. Meanwhile, cooler cells barely age at all. The BMS then has to work harder to compensate for a gap that thermal design, not manufacturing variance, actually created.

Cold cells create a different problem. Below their optimal range, cells deliver less power. They also accept slower charge rates. In practice, this means the coolest cells in a pack can become the limiting factor for dispatch power. This happens even though they are aging the slowest of anyone in the rack.

5. How the BMS Responds to What It Can Actually See

Temperature gradient sensor placement comparison in a BMS module

A BMS cannot manage a gradient it cannot measure. Sensor placement, therefore, matters as much as sensor accuracy. A design with one temperature sensor per module, placed at a single convenient point, will miss gradients happening between that sensor’s location and the rest of the module.

More thorough designs, instead, place multiple sensors per module. These sit at known high-risk points — near coolant outlets, at pack centers, and at busbar connections. This ties directly into the safety diagnostic algorithms covered in our BMS algorithms guide, since a BMS can only flag a developing hot spot if a sensor actually sits close enough to detect it before the gradient becomes a real problem.

6. Questions to Ask Your Supplier

  • What is your specified maximum cell-to-cell temperature gradient, not just the overall operating temperature range?
  • How many temperature sensors does each module have, and where are they physically placed?
  • For liquid-cooled systems, what is the coolant flow path? What gradient exists between inlet-side and outlet-side cells?
  • Do you have field or test data showing SOH divergence between hot-zone and cool-zone cells over time?
  • How does the BMS respond if a persistent gradient develops? Does it just log the data, or does it adjust balancing or dispatch limits?

Conclusion: A Temperature Gradient Is a Slow Problem That Looks Like No Problem at All

Overheating alarms are easy to notice. Temperature gradients, however, are not. A pack can run entirely within its safe range. It can still age unevenly, cell by cell. Nobody measured the gradient closely enough to see it. Ask suppliers for their specific gradient limit, not just their operating range. Then ask how many sensors actually watch for it.

For the manufacturing-stage half of this problem — how mismatched cells enter a pack in the first place — see our cell matching guide. Matching and thermal design solve two different sources of the same underlying issue: cells in one pack quietly drifting apart from each other over time.

☀️ Need a Thermal Design Review for Your BESS Project?
Sunlith Energy reviews cooling architecture, sensor placement, and gradient specifications for BESS projects from 50 kWh upward. Contact us before you finalize a thermal design.

Frequently Asked Questions About Cell Temperature Gradients

What is a temperature gradient in a battery pack?

A temperature gradient is the difference between the hottest and coolest cell temperatures in a pack at the same moment, usually written as ΔT. It is a separate measurement from the pack’s overall operating temperature range. That is because a pack can sit within a safe range overall while still having a large gap between its warmest and coolest cells.

What causes temperature gradients inside a BESS pack?

Three factors typically combine to cause gradients. Coolant path position matters, since cells near a coolant outlet run warmer than cells near the inlet. Cell position within the pack matters too, since center cells trap more heat than edge cells. Finally, uneven current distribution from busbar resistance differences creates uneven I²R heating across parallel cell groups.

How does uneven heating affect cell aging?

Hotter cells within a gradient age faster than cooler cells in the same pack, since battery degradation reactions speed up with heat. Over time, this can widen the performance gap between cells, even in a pack that started out well matched. As a result, the BMS ends up compensating for a gap that thermal design created, rather than manufacturing variance.

What is a safe temperature gradient for a BESS pack?

Exact limits vary by manufacturer and system design. However, a maximum gradient of around 5°C is commonly specified for air-cooled and moderately cooled systems, while premium liquid-cooled systems often target 2-3°C. Always confirm the specific figure with your supplier rather than assuming a standard number applies.

How many temperature sensors does a BESS module need?

There is no single universal number. Still, a module with only one sensor at a single convenient location cannot detect a gradient occurring elsewhere in that module. More thorough designs, therefore, place multiple sensors at known high-risk points, such as near coolant outlets, pack centers, and busbar connections.

Further Reading

Cell matching sorting line grouping battery cells by voltage and resistance
Rahul Jalthar

Cell Matching Before Pack Assembly: Why It Matters Before the BMS Ever Balances a Cell

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⚡ Quick Answer: What Is Cell Matching?
Cell matching is the process of sorting battery cells by voltage, capacity, and internal resistance before they go into a pack, so cells with similar characteristics end up grouped together. It happens on the factory floor, before assembly. This is not the same thing as BMS balancing, which corrects drift after the pack is already built and in use. Skipping cell matching does not make a pack unsafe by itself, since the BMS still protects it. However, it does mean the BMS has to work much harder from day one. As a result, the pack’s real-world capacity and cycle life will likely fall short of what the cell datasheet promises.

1. Why Cell Matching Happens Before the BMS Gets Involved

Cell matching is a manufacturing step that happens before a single cell ever reaches a pack. Even cells from the same production batch are not identical. Small differences in electrode coating thickness, electrolyte fill, and formation cycling leave every cell slightly different. Capacity, voltage, and internal resistance all vary a little, even when the datasheet lists one number for all of them. In a single cell, this variation does not matter. Once dozens or hundreds of cells connect into a pack, though, it matters a great deal.

The BMS will eventually correct some of this drift through balancing, as covered in our complete battery management system guide. Cell matching, however, happens earlier. It is a manufacturing step, not a BMS function, and it exists to reduce how much correction the BMS has to do later.

2. Three Criteria Used to Sort Cells: Voltage, Capacity, and Resistance

Cell matching criteria: voltage, capacity, and internal resistance measurement

Cell matching typically screens for three characteristics. Each one affects the pack differently. As a result, a thorough process checks all three rather than relying on just one.

  • Voltage (or SOC) matching — technicians group cells by their resting voltage after a defined charge or discharge point. This is the simplest check to run. It also catches the most obvious mismatches quickly.
  • Capacity matching — technicians charge and discharge test each cell to measure actual usable Ah, then group cells with similar capacity together. This matters most for series strings, since the lowest-capacity cell sets the ceiling for the whole string.
  • Internal resistance matching — technicians measure resistance using one of two methods, DCIR or ACIR, then group similar-resistance cells into the same parallel group. This matters most for parallel groups, since a lower-resistance cell otherwise takes more than its fair share of current.

High-volume manufacturers often combine all three, and internal resistance testing itself splits into two distinct methods worth understanding.

DCIR vs ACIR: Two Ways to Measure Internal Resistance

DCIR (DC internal resistance) testing applies a current pulse to the cell and measures the resulting voltage drop. Technicians then calculate resistance directly from Ohm’s law. This method closely reflects how the cell behaves under a real load, since it uses an actual current step rather than a small signal. The tradeoff is speed: each pulse needs time to apply and settle, which slows down high-volume sorting.

ACIR (AC internal resistance) testing instead applies a small alternating current signal, commonly at 1 kHz, and reads the resulting impedance directly. This method runs much faster than DCIR, which is why many production sorting lines use it as a first-pass screen. However, ACIR mostly captures the cell’s high-frequency ohmic resistance. It does not fully capture the slower electrochemical charge-transfer resistance that DCIR testing reveals.

In practice, many manufacturers use ACIR for fast first-pass screening across an entire incoming batch, then apply DCIR pulse testing to verify cells before they go into the same series string or parallel group. A supplier who only mentions one of these two methods is likely doing the faster, less thorough version alone.

3. Series Strings vs Parallel Groups: Different Priorities

Cell matching effect on series strings and parallel groups in a battery pack

Series and parallel connections fail differently when cells are mismatched. For this reason, they need different matching priorities.

In a series string, cells share the same current, but their voltages differ based on individual state. The weakest cell — the one with the lowest capacity — reaches its low-voltage cutoff first during discharge. Likewise, it hits its high-voltage cutoff first during charge. As a result, that one weak cell limits the usable capacity of the entire string. This happens even though the other cells still have energy left. This is why capacity matching matters most for series strings.

In a parallel group, cells share the same voltage, but current splits between them based on internal resistance. A cell with lower resistance pulls more current than its neighbors. In turn, it works harder and ages faster. Over time, that uneven current sharing can widen the resistance gap further, creating a feedback loop. Left unchecked, this loop drives localized accelerated aging in the same cells, cycle after cycle. That localized wear is what leads to premature pack failure, well before the rest of the pack reaches end of life. For a buyer, that translates directly into a shorter calendar life and a worse return than the datasheet cycle life implied. This is why resistance matching matters most for parallel groups.

☀️ Resistance matching matters most for parallel groups.
💡 The Thermal Feedback Loop: Internal resistance mismatch and localized heating reinforce one another. For a deeper look at how temperature imbalances accelerate this degradation, read our guide on Cell Temperature Gradients in BESS

4. What Happens If You Skip Cell Matching

Skipping cell matching does not make a pack dangerous on its own. A properly designed BMS still enforces voltage and temperature limits, regardless of how well matched the cells are. What changes, instead, is how hard the BMS has to work, and how much capacity the pack actually delivers.

If cells arrive at noticeably different SOC and go into a pack without matching, the BMS must run a large initial balancing pass. This happens the first time the pack charges. Passive balancing currents are typically small — often just tens to a few hundred milliamps — compared to the pack’s full Ah rating. Correcting a large initial mismatch this way can take many hours. In some cases, it takes several charge cycles before the pack reaches a properly balanced state.

Beyond the slow start, an unmatched pack often never fully closes the gap. If capacity variation between cells is large enough, ongoing balancing keeps the weakest cell from falling further behind. Still, balancing cannot manufacture capacity that a weak cell simply does not have. The pack’s usable capacity, therefore, ends up set by its weakest link, cycle after cycle.

5. Top-Balance vs Bottom-Balance: Which Comes First

When manufacturers match cells by connecting them in parallel before final assembly, the SOC point at which this happens changes the outcome.

Bottom-balance matching connects cells in parallel at a low SOC, often close to how they arrive from the manufacturer. This approach is simple and fast. However, it only aligns the cells at the bottom of the charge curve. The pack will likely still need a top-of-charge balancing pass once assembled and charged for the first time.

Top-balance matching, instead, charges the parallel-connected cells to a high SOC before final assembly, typically near the top of the charge curve. This produces a better-aligned pack from the first charge. That is because the region where mismatch matters most for safety and full capacity gets addressed early. The tradeoff is time: bringing a large batch of cells to a matched high-SOC state takes more equipment and more hours before assembly can begin.

6. Cell Matching at Scale: How Manufacturers Grade Cells for Utility BESS

At utility scale, matching thousands of cells by hand is not practical. Instead, high-volume manufacturers run automated sorting lines. These measure voltage, capacity, and resistance for every incoming cell. Grading software then groups cells into matched sets before they ever reach the assembly line.

For a BESS buyer, this raises a practical question worth asking directly: does the supplier grade and match cells before assembly, or does the pack rely entirely on the BMS to fix mismatch after the fact? Independent testing resources such as Battery University document just how differently DCIR and ACIR readings can diverge on the same cell, which is exactly why asking a supplier which method they use, and at which stage, is worth doing directly.

A supplier who can show incoming cell test data is doing meaningfully more quality control than one who simply points to their BMS’s balancing feature. Look, in particular, for a specific matching tolerance — for example, a defined percentage spread in capacity, or a defined milliohm band in resistance.

7. Questions to Ask Your Cell or Pack Supplier

  • Do you test and match cells by voltage, capacity, and internal resistance before assembly, or only one of these?
  • For internal resistance, do you use DCIR, ACIR, or both — and at which stage does each method apply?
  • What matching tolerance do you use? For example, what percentage spread in capacity, or what milliohm band in resistance?
  • Do you keep incoming cell test data on file? Can you provide it for the specific batch used in our order?

For series strings, how do you decide which cells go together — capacity, resistance, or both? Our BMS algorithms guide covers how the BMS itself later measures DCIR for SOH estimation, which is a useful comparison point when you ask this question.

  • Is matching done at a low SOC, a high SOC, or both, before final assembly?

Conclusion: Matching Sets the Ceiling the BMS Can’t Raise

A BMS is very good at correcting small, ongoing drift between cells. It is not designed, however, to compensate for a pack that started out badly mismatched. Cell matching before pack assembly sets the baseline the BMS then has to maintain for the life of the system. A well-matched pack lets the BMS do its normal job: fine-tuning small differences over time. A poorly matched pack, by contrast, forces the BMS into a losing battle against a gap it cannot close, cycle after cycle.

When evaluating a cell or pack supplier, ask specifically how they match cells before assembly, including whether they use DCIR, ACIR, or both. Do not just ask how the BMS balances them afterward. For supplier evaluation more broadly, see our BESS supplier BMS evaluation guide. The cell matching answer says a lot about how much real capacity and cycle life you can expect to see in practice.

☀️ Need Help Evaluating a Cell Matching Process?
Sunlith Energy reviews incoming cell test data, matching tolerances, and pack assembly quality control for BESS projects from 50 kWh upward. Contact us before you finalize a cell or pack supplier.

Frequently Asked Questions About Cell Matching

Is cell matching the same as BMS balancing?

No. Cell matching happens before assembly. It is a manufacturing step that sorts cells by voltage, capacity, and internal resistance, so similar cells end up grouped together. BMS balancing, on the other hand, happens after assembly, correcting the small drift that develops during normal use. Matching reduces how much balancing the BMS has to do; it does not replace it.

What is the difference between DCIR and ACIR matching?

DCIR testing applies a current pulse and calculates resistance from the voltage drop using Ohm’s law, closely reflecting real load behavior. ACIR testing applies a small AC signal, commonly at 1 kHz, and reads impedance directly, which runs much faster but mostly captures high-frequency ohmic resistance rather than the full picture. Many manufacturers use ACIR for fast first-pass screening, then confirm with DCIR before final grouping.

What is the difference between capacity-based and resistance-based sorting?

Capacity-based sorting groups cells with similar usable Ah, and matters most for series strings, since the lowest-capacity cell sets the ceiling for the whole string. Resistance-based sorting, by contrast, groups cells with similar internal resistance, and matters most for parallel groups, since a lower-resistance cell will otherwise pull more than its fair share of current.

Does skipping this step make a battery pack unsafe?

Not directly. A properly designed BMS still enforces voltage and temperature limits, no matter how well the cells were matched. That said, skipping this step does mean the BMS must run a larger initial balancing pass. In turn, the pack’s real-world capacity may fall short of the datasheet value, since the weakest cell limits the whole pack.

Should I ask my BESS supplier for this test data?

Yes. Ask whether the supplier tests and matches cells by voltage, capacity, and internal resistance before assembly, and which resistance method they use. A supplier who can provide incoming cell test data for your specific batch is demonstrating a real quality control process, not just relying on the BMS to compensate after the fact.

Is top-balance or bottom-balance better?

Top-balance, which aligns cells at a high SOC before assembly, generally produces a better-aligned pack from the first charge. That is because it addresses the top-of-charge region where mismatch matters most. Bottom-balance is faster, but the pack will likely still need a top-of-charge balancing pass once assembled.