In this study I examine the relationship between busbar overcurrent capability and thermal safety in lithium-ion battery systems at three hierarchical levels: the complete battery pack, the battery module, and the individual cell. I show that the busbar, or connecting tab, is not a passive current-carrying accessory but an active thermal source that can dominate the temperature field of a lithium ion battery pack under high-rate charging and discharging. Insufficient overcurrent capability in the busbar produces a persistent localized heat source that lies outside the direct measurement range of the battery management system, and that cannot be fully compensated by an existing liquid-cooling plate. I further show that a properly sized busbar suppresses local temperature rise and reduces the risk of heat flowing back from the tab into the cell jelly roll. The results provide a design basis for coupling busbar cross-section, weld geometry, and cooling strategy in lithium-ion battery packs intended for fast charging.
1. Introduction and Motivation
Lithium-ion battery systems for electric vehicles are being pushed simultaneously toward higher energy density and higher power density. This combination increases the severity of thermal safety challenges, because the same cell that stores more energy per unit mass is also required to accept and deliver larger currents. The conventional thermal-safety tool kit includes power limitation by the battery management system, active cooling through a cold plate, cell inversion to protect the passenger compartment, and thermal-barrier materials that interrupt propagation from one cell to the next. Most of these measures target either the intrinsic safety of the cell or the performance of the active cooling loop. The overcurrent capability of the busbar, by contrast, is frequently treated as an electrical design detail rather than as a thermal-safety parameter.
This omission is significant. In a lithium-ion battery pack, busbars collect and distribute current among cells and modules. They are usually classified by material as copper busbars or aluminum busbars. Copper offers high electrical conductivity and good mechanical strength and is therefore common at module terminals and pack output poles. Aluminum offers lower density and lower cost and is therefore common at cell-to-cell connections. The two materials have different resistivity and different thermal behavior, and therefore different overcurrent coefficients. Irrespective of material, a central design rule is that the maximum allowable continuous current of the busbar must exceed the current demanded by the system under every operating condition. When the actual current exceeds the rated overcurrent value, the conductor temperature rises rapidly because of the Joule effect:
$$Q_{Joule} = I^{2} R_{busbar} \, t$$
This heat generation can evolve into pack-level overheating and, in the worst case, into a thermal-safety event that threatens both occupants and the vehicle system. My objective here is to quantify the coupling between busbar overcurrent capability and thermal-safety risk, and to demonstrate that overcurrent design belongs alongside battery management strategy and cold-plate design as a primary element of lithium-ion battery pack thermal management.
2. Theoretical Framework for Busbar Heating
The electrical resistance of a uniform conductor of resistivity $\rho$, length $L$, and cross-sectional area $A$ is:
$$R_{busbar} = \rho \frac{L}{A}$$
A busbar of rectangular cross-section with width $w$ and thickness $t$ has area $A = w t$ and perimeter $\mathcal{P} = 2(w+t)$. Combining Joule heating with convective and conductive removal, the transient temperature rise above ambient can be approximated by a lumped-capacitance form:
$$T_{b}(t) – T_{\infty} = \frac{I^{2} R_{busbar}}{h A_{s}} \left(1 – e^{-t/\tau}\right)$$
where $h$ is the effective heat-transfer coefficient, $A_{s}$ is the surface area available for heat rejection, and the thermal time constant is:
$$\tau = \frac{m c_{p}}{h A_{s}} = \frac{\rho_{m} A L c_{p}}{h \mathcal{P} L} = \frac{\rho_{m} A c_{p}}{h \mathcal{P}}$$
with $\rho_{m}$ the mass density and $c_{p}$ the specific heat capacity. For long-duration or repetitive fast-charging events, the steady-state rise is the more relevant quantity. Equating generation to removal gives:
$$\Delta T_{ss} = \frac{I^{2} \rho}{h \, \mathcal{P} \, A}$$
Equation (5) makes the design trade-off explicit. The steady-state temperature rise scales with the square of current and inversely with the cross-sectional area and perimeter. Increasing conductor area therefore reduces both resistance and current density, but it also changes the available surface area and the thermal mass. Solving for the current that produces a chosen allowable temperature rise yields the overcurrent capability:
$$I_{max} = \sqrt{\frac{\Delta T_{max} \, h \, \mathcal{P} \, A}{\rho}}$$
In practice, the allowable current density is a more convenient design variable:
$$J = \frac{I}{A}$$
$$A_{req} = \frac{I_{design}}{J_{allow}}$$
Table 1 summarizes the physical properties that govern the comparison between aluminum and copper busbars. The higher resistivity of aluminum is partially offset by its lower density, which matters for pack mass, but the thermal consequence is that an aluminum busbar of equal cross-section runs hotter than a copper one at the same current.
| Property | Units | Aluminum | Copper |
|---|---|---|---|
| Electrical resistivity at 20 °C | Ω·m | 2.65 × 10⁻⁸ | 1.72 × 10⁻⁸ |
| Thermal conductivity | W·m⁻¹·K⁻¹ | 237 | 401 |
| Density | kg·m⁻³ | 2700 | 8960 |
| Specific heat capacity | J·kg⁻¹·K⁻¹ | 900 | 385 |
| Relative current capacity for equal area | – | 0.65 | 1.00 |
| Typical use in pack | – | Cell-to-cell connection | Module and output pole |
For a rectangular aluminum busbar with $w = 50$ mm and $t = 1.6$ mm, the cross-sectional area is $A = 80$ mm² and the perimeter is $\mathcal{P} = 103.2$ mm. Using $\rho = 2.65 \times 10^{-8}$ Ω·m and an effective $h = 10$ W·m⁻²·K⁻¹, the coefficient relating temperature rise to the square of current is:
$$\frac{\rho}{h \mathcal{P} A} = \frac{2.65 \times 10^{-8}}{10 \times 0.1032 \times 80 \times 10^{-6}} \approx 3.21 \times 10^{-4} \; \mathrm{K \cdot A^{-2}}$$
At 200 A this predicts a rise of about 13 K, whereas at 500 A it predicts about 80 K. The latter is comparable to the temperature differences observed in the experimental work described below, and it already indicates that a busbar sized for a conventional charge rate becomes thermally marginal once the fast-charge current rises above roughly 400 A.
3. Experimental Platform and Test Objects
I assembled a test platform capable of pack-level and module-level charging and discharging with synchronized temperature and voltage acquisition. The platform consisted of a high-voltage charge–discharge unit, a walk-in thermal chamber, a dual-channel water chiller, a multi-channel temperature logger, and a multi-channel isolated voltage acquisition board. Table 2 lists the principal equipment parameters, and the physical arrangement is shown in the accompanying photograph of the laboratory setup.
| Subsystem | Key parameters | Purpose |
|---|---|---|
| Charge–discharge unit | 800 V, 600 A, 300 kW, accuracy ±1‰ | Pack fast charging and typical duty cycles |
| Walk-in thermal chamber | Door width 1.8 m, internal width ≈2.1 m, height 1.8 m, depth 3.5 m, −40 °C to 130 °C, ±1 °C | Temperature-controlled charging and discharging |
| Water chiller | Dual channel: 5.5 kW × 2 at 100 °C; 5.5 kW × 2 at 20 °C; 3.8 kW × 2 at 5 °C | Simulation of a 3 kW-class cooling loop |
| Temperature logger | Multi-channel, accuracy 0.5 °C, resolution 0.01 °C | Busbar, weld, and cell surface temperature measurement |
| Voltage acquisition | Six-digit multimeter for static accuracy; isolated multi-channel board with 1 mV resolution for dynamic tests | Battery management system voltage-accuracy checks |
The main test pack, designated Pack-A, is a 1P96S module-to-pack construction composed of three 1P32S sub-modules. To test the validity of the conclusions across the service life, I selected Pack-A units with different accumulated mileage and labeled them Pack-A, x# with x = 1, 2, 3. I also introduced similarly designed packs, Pack-B, Pack-C, and Pack-D, as reference groups. Table 3 presents the basic parameters of all test packs.
| Item | Unit | Pack-A | Pack-B | Pack-C | Pack-D |
|---|---|---|---|---|---|
| External dimensions | mm | 2020 × 1430 × 151 | 2020 × 1430 × 151 | 1320 × 1340 × 145 | 1320 × 1340 × 145 |
| Busbar dimensions | mm | 50 × 1.6 × 117.5 | 58.5 × 2.0 × 115 | 50.3 × 1.5 × 77.6 | 50.3 × 1.5 × 77.6 |
| Busbar cross-sectional area | mm² | 80 | 117 | 75 | 75 |
| Rated energy at 1/3C | kWh | 88.3 | 88.3 | 67.8 | 67.8 |
| Cooling method | – | Bottom liquid cooling | Bottom liquid cooling | Bottom liquid cooling | Bottom liquid cooling |
3.1 Sensor Placement for Pack-Level Tests
Before each pack test I inspected the exterior, recorded basic information, and opened the pack to install temperature sensors. I selected sixteen busbars adjacent to the main positive output pole of the module and bonded thermocouples to them, as illustrated by the internal sensor layout. The battery management system remained active so that its own temperature readings could be logged in parallel with the external sensors. This parallel arrangement is essential, because the central question of this work is not only how hot the busbar becomes but also how much of that temperature rise is visible to the battery management system.
3.2 Sensor Placement for Module and Cell Tests
For the module tests I used a finer sensor network that included both internal and external cell locations. Table 4 lists the numbering and location of each temperature-sensing line. Sensors 1–4 were placed on internal cell features, while sensors 5–11 were placed on external features and current-carrying parts.
| Number | Location |
|---|---|
| 1# | Positive-electrode laser weld |
| 2# | Negative-electrode laser weld |
| 3# | 5 mm below the positive tab root |
| 4# | 5 mm below the negative tab root |
| 5# | Center of the large cell face |
| 6# | Bottom of the large cell face |
| 7# | Top cover at the battery management system thermistor |
| 8# | Positive terminal post |
| 9# | Negative terminal post |
| 10# | Center of the negative copper busbar |
| 11# | Center of the positive copper busbar |
4. Pack-Level Constant-Current Fast-Charging Results
I first performed 2C constant-current charging on three Pack-A units with accumulated mileages corresponding to low, medium, and high service exposure. Each test was run with the liquid-cooling loop active and with the loop inactive, so that the compensating effect of the cooling system could be isolated. The pack was discharged at 1/2C until the relay opened, allowed to rest, and then charged at 2C while busbar and thermistor temperatures were recorded. Charging was stopped when the monitored thermistor exceeded 55 °C or when another protection condition was met.
Table 5 summarizes the maximum temperatures observed at the end of charging. The most important result is not the absolute busbar temperature but the difference between the busbar temperature and the highest temperature reported by the battery management system. Under water-cooled conditions this difference ranged from 24.81 °C to 28.44 °C, and under no-cooling conditions it ranged from 26.83 °C to 29.00 °C.
| Pack | Cooling condition | 2C duration (s) | Maximum busbar temperature (°C) | Maximum BMS temperature (°C) | Difference (°C) |
|---|---|---|---|---|---|
| Pack-A, 1# | No cooling | 192.0 | 62.83 | 36.0 | 26.83 |
| Pack-A, 1# | Water cooling | 182.4 | 56.86 | 32.0 | 24.86 |
| Pack-A, 2# | No cooling | 245.2 | 65.00 | 36.0 | 29.00 |
| Pack-A, 2# | Water cooling | 245.2 | 60.44 | 32.0 | 28.44 |
| Pack-A, 3# | No cooling | 224.2 | 64.52 | 37.0 | 27.52 |
| Pack-A, 3# | Water cooling | 217.1 | 56.81 | 32.0 | 24.81 |
Three conclusions follow immediately. First, the busbar-to-sensor difference exceeds the typical pack-level uniformity target of 10 °C by a wide margin. Second, the difference is almost the same with and without liquid cooling, which means that the Joule heat generated by the high current is far beyond the compensation capability of the cooling loop. Third, because the battery management system uses its own thermistor readings to index the charging map, the pack is effectively being charged according to a current profile that does not correspond to the true busbar temperature. The cell is therefore exposed to currents above the intended map values for a longer time than the control strategy assumes, which is a challenge for both safety and life.
In addition, the busbar is mechanically and thermally connected to the cell terminal post. A hot busbar can therefore feed heat back into the cell, a phenomenon I refer to as thermal backflow. This backflow raises the risk that the cell, rather than the busbar, becomes the site of the first thermal anomaly. The pack-level cooling loop does not suppress this mechanism, because the loop is thermally distant from the busbar-to-post joint.
5. Validation of Battery Management System Sensor Location
A natural objection to the preceding result is that the temperature difference might be caused by a poorly chosen thermistor location rather than by a genuine blind spot. I therefore tested the adequacy of the thermistor position directly. I placed additional sensors on the surface of three cells in Pack-A, 1# and compared their readings with the thermistor reading during a map-based fast charge.
During the early part of the charge, the thermistor location was warmer than the upper, middle, and lower surface points. As the charge progressed, the surface points rose faster and ended 4–5 °C above the thermistor location. During the high-current phase, when the current exceeded 400 A, the thermistor reading did reflect the relative high point of the cell surface temperature, and the difference between the thermistor reading and the maximum cell surface temperature remained within 5 °C. This satisfies a typical engineering requirement. Because the thermistor reading was slightly higher than the surface maximum, the map lookup based on that reading tends to be conservative rather than aggressive.
The sensor location is therefore not the core defect. The core defect is that the thermistor measures the cell, while the hottest component in the current path is the busbar and its weld. Table 6 summarizes the logic of this validation.
| Observation | Implication |
|---|---|
| Early in charge, thermistor > upper surface > middle surface > lower surface | Thermistor responds quickly to local heat sources |
| Late in charge, surface points exceed thermistor by 4–5 °C | Thermistor is close to, but not identical with, the surface maximum |
| Above 400 A, thermistor is within 5 °C of the surface maximum | Location is acceptable for cell-temperature control |
| Busbar can be 30 °C above thermistor | Location cannot detect busbar overheating |
6. Pack-Level Map-Based Fast-Charging Results
The constant-current tests establish that high current produces a large busbar-to-sensor difference. To check whether the same effect appears under a realistic charging strategy, I ran map-based fast-charge tests in which the current is adjusted as a function of state of charge and cell temperature. I selected ambient temperatures of 25 °C, 30 °C, and 38 °C to represent different usage scenarios. The liquid-cooling loop was controlled by the thermistor: below 34 °C the loop was off, between 34 °C and 39 °C the inlet temperature was 22 °C at 12 L·min⁻¹, and above 39 °C the inlet temperature was 18 °C at 15 L·min⁻¹.
Table 7 lists the peak busbar temperature, the peak battery management system temperature, and their difference. The busbar peak reached 80.79 °C at 38 °C ambient, while the corresponding sensor peak was 57 °C, giving a difference of 30.33 °C. Even at 25 °C ambient the difference was 29.37 °C. The difference did not shrink as the cooling loop became more aggressive, which again indicates that the cooling system is not addressing the dominant heat source.
| Ambient temperature (°C) | Peak busbar temperature (°C) | Peak BMS temperature (°C) | Difference (°C) |
|---|---|---|---|
| 25 | 71.30 | 50.00 | 29.37 |
| 30 | 74.41 | 50.00 | 28.74 |
| 38 | 80.79 | 57.00 | 30.33 |
These results show a system-level overheating risk under high-load operation. The busbar becomes a continuous heat source during charging, and the heat flows through the terminal post into the jelly roll. Because the battery management system measures the cell surface rather than the busbar, the over-temperature protection threshold is never reached, even though the metal conductor is well above the temperature that the design intended. A lithium ion battery pack operating in this condition accumulates thermal stress in a region that is not represented in the control model.
7. Module-Level Temperature Distribution and Thermal Backflow
To determine whether busbar heat actually propagates into the cell interior, I built a small 1P3S module using the same cell type as Pack-A. The small format allowed me to place sensors inside the cell stack and on the terminal posts, so that the internal temperature gradient could be resolved. The module was wrapped with thermal insulation on the sides and top to approximate the single-sided cooling boundary of a cell inside a pack. I tested two cooling flow rates, 1 L·min⁻¹ and 3.6 L·min⁻¹, at coolant inlet temperature 20 °C, under 30-minute and 40-minute charging strategies at 35 °C and 40 °C ambient.
| Ambient (°C) | Charge strategy | Module | Coolant flow (L·min⁻¹) | 1# weld (°C) | 2# weld (°C) | 3# tab root (°C) | 5# face center (°C) | 7# NTC (°C) | 8# positive post (°C) | 9# negative post (°C) | 10# Cu busbar (°C) | 11# Cu busbar (°C) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 35 | 30 min | 1# | 3.6 | 63.6 | 62.7 | 49.2 | 47.5 | 46.8 | 66.2 | 63.6 | – | – |
| 35 | 30 min | 2# | 1.0 | 71.6 | 74.7 | 54.6 | 52.2 | 53.5 | 74.3 | 69.7 | 70.5 | 75.8 |
| 40 | 30 min | 1# | 3.6 | 71.7 | 70.8 | 55.7 | 52.4 | 53.4 | 75.4 | 72.3 | – | – |
| 40 | 30 min | 2# | 1.0 | 79.4 | 78.6 | 58.7 | 55.5 | 59.0 | 81.6 | 76.8 | 78.0 | 83.3 |
| 35 | 40 min | 1# | 3.6 | 52.7 | 54.9 | 46.4 | 45.1 | 43.6 | 54.2 | 53.5 | – | – |
| 35 | 40 min | 2# | 1.0 | 55.6 | – | 48.5 | 46.9 | 45.5 | 60.0 | 55.2 | 56.5 | 60.0 |
| 40 | 40 min | 1# | 3.6 | 57.1 | 58.4 | 50.0 | 48.2 | 47.3 | 59.2 | 58.2 | – | – |
| 40 | 40 min | 2# | 1.0 | 60.7 | – | 52.5 | 50.8 | 49.9 | 65.5 | 60.5 | 61.9 | 65.6 |
The module data reveal a consistent vertical temperature gradient. The weld and terminal-post locations are always the hottest, the tab root is intermediate, and the large face and the thermistor location are coolest. At 35 °C ambient and 30-minute charging, the thermistor location reached 46.8 °C while the positive post reached 66.2 °C, a difference of 19.4 °C. At 40 °C ambient, the same comparison gave 53.4 °C at the thermistor location and 75.4 °C at the positive post, a difference of 22.0 °C. When the coolant flow was reduced to 1 L·min⁻¹, the difference increased further. In the most severe case, 40 °C ambient with 1 L·min⁻¹ and a 30-minute strategy, the positive post reached 81.6 °C, while the thermistor location reported 59.0 °C. The difference was 22.6 °C at the post and nearly 30 °C at the weld.
This is direct evidence of thermal backflow. The heat generated in the busbar and weld does not remain confined to the metal parts. It conducts through the terminal post into the tab, into the jelly roll, and eventually to the large face. The face center and the region below the tab root rise in phase with the post temperature, which means that the cell interior is being heated by an external source rather than only by its own electrochemical and ohmic losses.
| Ambient (°C) | Charge strategy | Flow (L·min⁻¹) | NTC temperature (°C) | Positive post temperature (°C) | Difference (°C) |
|---|---|---|---|---|---|
| 35 | 30 min | 3.6 | 46.8 | 66.2 | 19.4 |
| 35 | 30 min | 1.0 | 53.5 | 74.3 | 20.8 |
| 40 | 30 min | 3.6 | 53.4 | 75.4 | 22.0 |
| 40 | 30 min | 1.0 | 59.0 | 81.6 | 22.6 |
| 35 | 40 min | 3.6 | 43.6 | 54.2 | 10.6 |
| 40 | 40 min | 1.0 | 49.9 | 65.5 | 15.6 |
The heat path is therefore ordered as follows: weld and terminal post, then U-shaped busbar, then tab, then jelly roll, then cell surface. The weld is the hottest point because it combines the constriction resistance of the joint with the bulk resistance of the busbar. A peak weld temperature of the order of 84 °C was observed while the thermistor reported only about 55 °C, a difference of nearly 30 °C. This ordering identifies insufficient busbar overcurrent capability, together with weld quality, as the root cause of the localized overheating. Improved weld geometry and larger conductor cross-section both reduce the local current density and the local heat generation, and therefore both suppress the hotspot.
8. Effect of Busbar Cross-Section on Temperature Rise
To confirm that insufficient overcurrent capability is a design-level defect rather than a consequence of aging, I compared Pack-A with Pack-B under the same ambient temperature and the same charging strategy. Pack-A has a busbar cross-sectional area of about 80 mm², whereas Pack-B has about 117 mm², an increase of roughly 46 %. The theoretical continuous overcurrent capability of the Pack-A busbar is on the order of 200 A, while the fast-charge peak current exceeds 500 A. The corresponding current densities are:
$$J_{A} = \frac{500}{80} = 6.25 \; \mathrm{A \cdot mm^{-2}}$$
$$J_{B} = \frac{500}{117} = 4.27 \; \mathrm{A \cdot mm^{-2}}$$
Table 10 compares the measured peak temperatures. At 40 °C ambient and identical charging conditions, the Pack-A busbar reached 67.96 °C while the Pack-B busbar reached 57.95 °C, a difference of 10.01 °C. The temperature rise of the Pack-A busbar was about 28 °C, whereas the Pack-B busbar rose by a substantially smaller amount. This is consistent with the inverse dependence of temperature rise on cross-sectional area in Equation (5), and with the reduction in current density.
| Pack | Busbar dimensions (mm) | Cross-section (mm²) | Estimated continuous overcurrent (A) | Current density at 500 A (A·mm⁻²) | Peak busbar temperature at 40 °C ambient (°C) | Temperature rise (K) |
|---|---|---|---|---|---|---|
| Pack-A | 50 × 1.6 | 80 | ≈200 | 6.25 | 67.96 | ≈28 |
| Pack-B | 58.5 × 2.0 | 117 | ≈300 | 4.27 | 57.95 | ≈18 |
The comparison demonstrates that local overheating is not inevitable. It is a consequence of selecting a conductor cross-section for a conventional charge rate and then operating the pack at a fast-charge rate. The same cells, the same cooling plate, and the same control strategy produce a difference of 10 °C in peak metal temperature solely because the conductor is larger. A well-designed busbar therefore acts as a thermal buffer, spreading current over a larger area and reducing the heat flux that must be absorbed by the weld and terminal post.
9. Coupling Between Busbar Design and Cooling Strategy
The experimental results show that the liquid-cooling loop and the busbar are not independent design variables. Increasing coolant flow from 1 L·min⁻¹ to 3.6 L·min⁻¹ reduced the pack-level temperature, but it did not reduce the busbar-to-sensor difference to an acceptable value. The reason is that the cooling plate is thermally connected to the cell through a long conduction path, while the busbar heat source is connected to the cell through a short and highly conductive metallic path. The thermal resistance network can be written schematically as:
$$\Delta T_{busbar-cell} = Q_{busbar} \left( R_{weld} + R_{post} + R_{tab} \right)$$
where $Q_{busbar}$ is the heat generated in the conductor and joint, and the resistances are those of the weld, the terminal post, and the tab. Reducing $Q_{busbar}$ by increasing the cross-section, or reducing the current density by improving the weld geometry, is therefore more effective than increasing the coolant flow, because the coolant acts on the far side of a much larger thermal resistance.
A second coupling appears in the control strategy. The battery management system indexes the charging map by the temperature it measures. If the measured temperature is lower than the true temperature of the current-carrying metal, the map will permit a higher current than the thermal design assumes. This is a feedback loop in which an under-designed busbar causes a measurement error, and the measurement error causes additional current, which further increases the busbar temperature. The loop can be expressed as:
$$I_{map} = f\left(T_{BMS}\right), \qquad T_{busbar} = T_{BMS} + \Delta T_{busbar-BMS}\left(I_{map}\right)$$
Because $\Delta T$ increases approximately with the square of current, the loop has a positive feedback character until another protection limit is reached. This helps explain why the difference between busbar and sensor temperature remained near 30 °C across a wide range of ambient temperatures and cooling conditions, rather than shrinking as the cooling loop became stronger.
10. Design Guidelines for Busbar Overcurrent Capability
The results support a set of practical design rules for lithium-ion battery packs intended for fast charging. The first rule is that the continuous overcurrent rating of the busbar must be established for the maximum sustained current, not for the average current of a nominal duty cycle. The second rule is that the allowable current density should be selected with the cooling boundary in mind, because the same conductor behaves differently when it is buried inside a pack and when it is exposed to air. The third rule is that the weld and terminal post should be treated as part of the busbar thermal system, because they are frequently the hottest points.
Using Equation (6), the required cross-sectional area for a given current and allowable temperature rise can be estimated. Table 11 gives indicative current densities for several cooling and duty conditions. These values are intended as a starting point for design, not as a substitute for a full thermal model.
| Cooling condition | Duty | Allowable current density (A·mm⁻²) | Comment |
|---|---|---|---|
| Still air, enclosed | Continuous | 1.5–2.5 | Conservative limit for a buried conductor |
| Still air, exposed | Continuous | 2.5–3.5 | Improves with free convection |
| Forced air | Continuous | 3.0–4.0 | Requires verified airflow over the conductor |
| Liquid cooled plate, indirect | Continuous | 3.5–5.0 | Cooling acts through the cell, not directly on the busbar |
| Liquid cooled plate, direct busbar contact | Continuous | 5.0–6.5 | Only if the busbar has a dedicated thermal path |
| Any condition | Fast-charge pulse | Reduce by 20–30 % | Pulse heating is dominated by the weld and joint |
Two further equations are useful for sizing the joint. The contact resistance of a weld or bolted joint is:
$$R_{contact} = \frac{\rho_{c}}{A_{c}}$$
and the local heat generated at the joint is:
$$Q_{joint} = I^{2} R_{contact}$$
Because $R_{contact}$ varies inversely with contact area, a joint that is geometrically small relative to the conductor will dominate the local temperature rise even when the conductor itself is adequately sized. This is the reason the weld was consistently the hottest location in the module tests. Increasing the weld area, improving the weld penetration, and avoiding sharp current constrictions are therefore as important as increasing the conductor cross-section.
11. Implications for Lithium-Ion Battery Thermal Safety
The evidence assembled here supports a view of busbar design that differs from the conventional electrical-only perspective. In a lithium-ion battery pack, the busbar is a structural element of the thermal management system. Its overcurrent capability determines whether the pack can accept a fast-charge current without creating a persistent hotspot. Its geometry determines how much of that heat flows into the cell rather than into the cooling plate. Its material determines the temperature rise for a given current density. And its position determines whether the battery management system can see the resulting temperature at all.
The practical implications can be summarized as follows. First, a busbar with a continuous overcurrent capability below the fast-charge peak current will create a temperature difference between the metal and the cell surface that is large enough to defeat the cell-temperature-based protection logic. Second, this difference is not removed by increasing the coolant flow, because the thermal resistance between the cooling plate and the busbar is larger than the thermal resistance between the busbar and the cell. Third, the resulting heat flows into the cell and raises the internal temperature gradient, which increases the risk of accelerated aging and, in the limit, of a thermal-safety event. Fourth, a larger busbar cross-section and a better weld geometry both reduce the hotspot and the backflow, and therefore improve safety without changing the cell chemistry or the cooling architecture.
These findings are relevant to the full life cycle of a lithium-ion battery pack. The comparison of packs with different accumulated mileage showed that the busbar-to-sensor difference remained in the same range regardless of service exposure. This means that the defect is present from the beginning of life as a design margin issue, not as a wear-out phenomenon. It also means that a pack that was acceptable at a conventional charge rate can become marginal when the charge rate is increased, even if the cells themselves retain adequate capacity. The engineering background of this issue lies in the rapid evolution of market demand for fast charging: busbars are often selected early in a program on the basis of a nominal charge rate, and the later decision to increase the charge rate is not always accompanied by a re-check of the busbar overcurrent capability or of its interaction with the cooling system.
12. Summary of Quantitative Findings
Table 12 collects the principal quantitative results of this study. The numbers are drawn from the pack-level, module-level, and cell-level tests and illustrate the consistency of the busbar overheating mechanism across scales.
| Level | Test condition | Key result | Interpretation |
|---|---|---|---|
| Pack | 2C charge, water cooling | Busbar-to-BMS difference 24.8–28.4 °C | Cooling does not compensate busbar heat |
| Pack | 2C charge, no cooling | Busbar-to-BMS difference 26.8–29.0 °C | Difference is intrinsic to the current path |
| Pack | Map charge at 25–38 °C | Peak busbar 71.3–80.8 °C | Realistic fast charging creates sustained hotspots |
| Cell surface | Map charge, multiple locations | BMS reading within 5 °C of surface maximum | Sensor location itself is adequate |
| Module | 30–40 min charge, 1–3.6 L·min⁻¹ | NTC-to-positive-post difference 15–23 °C | Terminal post and weld are the hottest points |
| Module | 40 °C ambient, 1 L·min⁻¹, 30 min | Positive post 81.6 °C, weld 83.3 °C | Local temperature can exceed safe design limits |
| Pack comparison | 80 mm² versus 117 mm² busbar | Peak temperature difference 10.0 °C | Cross-section directly controls local temperature |
| Design check | 500 A peak, 80 mm² | Current density 6.25 A·mm⁻² | Above the allowable range for indirect cooling |
The consistency of the results across scales is important. At the pack level, the busbar is hotter than the sensor by an amount that is large enough to defeat the protection logic. At the module level, the weld and post are hotter than the cell surface by an amount that demonstrates heat flow into the cell. At the cell level, the internal temperature gradient follows the external metal temperature rather than the electrochemical heat generation alone. The same design variable, the busbar cross-section and its associated current density, controls all three observations.
13. Conclusions
I have systematically evaluated the relationship between busbar overcurrent capability and thermal-safety risk in lithium-ion battery systems at the pack, module, and cell levels. The main conclusions are as follows.
First, a large and persistent temperature difference exists between the busbar and the temperature reported by the battery management system. Across packs with different accumulated mileage, the 2C fast-charge difference remained above 20 °C, and in map-based fast charging it was approximately 30 °C. The battery management system therefore has a protection blind spot: by the time a conventional sensor reaches its threshold, the busbar may already be above its safe design temperature.
Second, the sensor location itself is reasonable. Multi-point cell-surface measurements confirmed that the thermistor reflects the high point of the cell surface temperature during fast charging and remains within about 5 °C of the surface maximum. The problem is not the location of the sensor but the fact that the hottest element in the current path is not the cell surface. Monitoring cell temperature alone cannot detect an abnormal busbar heat source.
Third, the heat path is well defined. Heat generated in the weld and terminal post conducts into the U-shaped busbar, the tab, the jelly roll, and finally the cell surface. The weld is the hottest location, with a peak temperature approaching 85 °C while the thermistor reports about 55 °C. The temperature gradient follows the metal parts, which confirms that the busbar is the initiating source of the anomaly.
Fourth, there is a direct relationship between busbar design and thermal safety. A larger cross-section and a better weld geometry reduce the local current density and suppress the hotspot. Increasing the coolant flow alone does not remove the busbar-to-sensor difference, because the cooling plate is thermally distant from the busbar. The cooling system and the busbar overcurrent capability must therefore be optimized together.
In summary, the busbar is a critical element of lithium-ion battery pack thermal management. A busbar with insufficient overcurrent capability becomes a continuous heat source during fast charging, creates a blind spot in battery management system monitoring, and cannot be compensated by an existing liquid-cooling system. Correctly sizing the busbar and its joints, and coordinating that sizing with the cooling strategy, is therefore an essential part of the thermal-safety design of a lithium-ion battery pack. These results provide a theoretical basis and an experimental foundation for optimizing pack thermal design, and they highlight the strong coupling between electrical connection design and thermal safety in lithium-ion battery systems.