In this investigation, I examined how aluminum busbar overcurrent capability governs the thermal safety of Li-ion battery systems from three hierarchical levels: the full Li-ion battery pack, the Li-ion battery module, and the individual Li-ion battery cell. My central premise is that a Li-ion battery busbar is not merely a passive current collector. It is a thermally active component whose joule heating can become a persistent local heat source. When the busbar current-carrying capacity is lower than the current demanded by fast charging, the resulting temperature rise can exceed the compensation ability of a liquid-cooling plate and can remain outside the direct measurement field of the battery management system. This creates a structured thermal blind spot in the Li-ion battery pack.
I organized the work around a simple but demanding question: if a Li-ion battery busbar is undersized for the actual fast-charge profile, can the cooling system and the battery management system still protect the Li-ion battery pack? My measurements show that the answer is no. The Li-ion battery busbar can reach temperatures far above the values reported by the battery management system, and the liquid-cooling system cannot fully remove the concentrated heat generated at welds, terminals, and busbar bends. The result is a direct coupling between electrical design, thermal management, and Li-ion battery safety.
1. Electrical and Thermal Framework
I began by defining the overcurrent condition of a Li-ion battery busbar in terms of current density. For a busbar of cross-sectional area \(A\), the current density is
$$J = \frac{I}{A}.$$
The joule heat generated per unit volume is
$$q_g = \frac{I^2 R}{V} = J^2 \rho,$$
where \(I\) is current, \(R\) is electrical resistance, \(V\) is conductor volume, and \(\rho\) is electrical resistivity. For a busbar of length \(L\), the resistance is
$$R = \rho \frac{L}{A}.$$
Therefore, the total joule heating rate is
$$Q_g = I^2 R = I^2 \rho \frac{L}{A}.$$
This equation already reveals the risk in a Li-ion battery busbar. If the operating current rises because of fast charging, the heat generation increases with the square of current, while the heat removal path remains limited by the available surface area and thermal resistance. A small reduction in cross-sectional area can therefore produce a large increase in local temperature.
I modeled the steady-state temperature rise of the busbar using a lumped thermal resistance network:
$$T_b – T_a = Q_g R_{\theta},$$
where \(T_b\) is busbar temperature, \(T_a\) is ambient or coolant reference temperature, and \(R_{\theta}\) is the effective thermal resistance. The thermal resistance contains conduction, convection, and radiation components:
$$R_{\theta} = R_{\theta,\mathrm{cond}} + R_{\theta,\mathrm{conv}} + R_{\theta,\mathrm{rad}}.$$
For convective cooling to a liquid-cooled plate, I used the approximation
$$Q_c = h A_s (T_b – T_c),$$
where \(h\) is the effective heat-transfer coefficient, \(A_s\) is the effective surface area, and \(T_c\) is the coolant temperature. Combining generation and cooling gives
$$I^2 \rho \frac{L}{A} = h A_s (T_b – T_c).$$
The maximum current that keeps the busbar below a temperature limit \(T_{\mathrm{lim}}\) is therefore approximated by
$$I_{\mathrm{max}} \approx \sqrt{\frac{h A_s (T_{\mathrm{lim}} – T_c) A}{\rho L}}.$$
This expression is central to my interpretation. It shows that overcurrent capability is not a single material property. It is a system property that depends on cross-sectional area, surface area, thermal resistance, coolant temperature, and the allowed temperature rise. A Li-ion battery pack can therefore appear to have sufficient cooling while still containing a Li-ion battery busbar that is electrically undersized.
I also used the C-rate definition for charge and discharge:
$$C_{\mathrm{rate}} = \frac{I}{C_n},$$
where \(C_n\) is the nominal capacity in ampere-hours. In my tests, a 2C charge on a large Li-ion battery pack produced currents that exceeded 500 A during part of the charge, while the theoretical continuous current capability of the smaller busbar was only around 200 A. This mismatch is the core overcurrent condition.
2. Experimental Platform and Li-ion Battery Pack Samples
I built the test platform around a high-voltage charge and discharge system, a walk-in temperature chamber, a dual-channel water chiller, a multi-channel temperature acquisition system, and a multi-channel voltage acquisition system. The equipment allowed me to measure the Li-ion battery pack, Li-ion battery module, and Li-ion battery cell under controlled temperature and cooling conditions.
| Equipment | Key specification | Role in my Li-ion battery tests |
|---|---|---|
| Charge and discharge system | 800 V, 600 A, 300 kW, accuracy ±1‰ | Fast charging and representative operating conditions for the Li-ion battery pack |
| Walk-in temperature chamber | −40 °C to 130 °C, control accuracy ±1 °C | Controlled ambient temperature for Li-ion battery pack and module charging |
| 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 | Liquid-cooling simulation for the Li-ion battery pack and module |
| Temperature acquisition | Multi-channel, accuracy 0.5 °C, resolution 0.01 °C | Direct measurement of busbar, weld, terminal, and cell surface temperatures |
| Voltage acquisition | Six-and-a-half-digit multimeter and isolated multi-channel system, 1 mV accuracy | Static and dynamic voltage monitoring of the Li-ion battery pack |
I selected a Li-ion battery pack designated Pack-A as the main test object. Pack-A is a 1P96S MTP Li-ion battery pack composed of three 1P32S small modules. I used three Pack-A samples with different mileage values to test whether the conclusion holds across different states of health. I also introduced similar Li-ion battery packs Pack-B, Pack-C, and Pack-D as control groups. The basic parameters are summarized below.
| Item | Unit | Pack-A | Pack-B | Pack-C | Pack-D |
|---|---|---|---|---|---|
| Outline 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 |
The busbar cross-sectional area of Pack-A is about 80 mm², whereas Pack-B has about 117 mm². This difference may appear small in geometric terms, but it becomes decisive under fast charging. Using the simplified maximum-current relation, the theoretical continuous current capability of the Pack-A busbar is on the order of 200 A, while the fast-charge peak current exceeds 500 A. This is a severe overcurrent condition for a Li-ion battery busbar.
I prepared the Li-ion battery packs by inspecting them, opening them, and attaching temperature sensors to 16 busbar positions near the main positive output pole. I also used the existing battery management system temperature sensors. For module and cell tests, I placed internal and external temperature sensors at welds, tab roots, large faces, top cover, terminals, and copper bars. The following table lists the temperature-sensing positions used for the module-level Li-ion battery tests.
| Number | Position in the Li-ion battery module |
|---|---|
| 1# | Positive electrode laser weld |
| 2# | Negative electrode laser weld |
| 3# | Positive tab root, 5 mm below the root |
| 4# | Negative tab root, 5 mm below the root |
| 5# | Large face center |
| 6# | Large face bottom |
| 7# | NTC top cover |
| 8# | Positive terminal |
| 9# | Negative terminal |
| 10# | Negative copper bar center |
| 11# | Positive copper bar center |
3. Test Matrix and Charging Conditions
I designed five test groups. The first group used constant-current 2C charging of Pack-A samples with and without water cooling. The second group validated the battery management system temperature acquisition position at the cell surface. The third group used a realistic fast-charge map at 25 °C, 30 °C, and 38 °C. The fourth group used a small 1P3S Li-ion battery module to measure internal cell temperature gradients. The fifth group compared Pack-A and Pack-B busbars under the same fast-charge condition to isolate the effect of cross-sectional area.
| Test group | Object | Condition | Purpose |
|---|---|---|---|
| Constant-current fast charge | Pack-A samples | 2C charge, with and without water cooling | Quantify busbar over-temperature and BMS blind spot |
| BMS acquisition validation | Pack-A cells | Map charging, surface sensors at multiple heights | Verify whether NTC position represents cell surface maximum |
| Map fast charge | Pack-A | 25 °C, 30 °C, 38 °C ambient | Assess real fast-charge thermal risk |
| Small module test | 1P3S Li-ion battery module | 35 °C and 40 °C ambient, 30 min and 40 min charging, 1 L/min and 3.6 L/min coolant | Measure heat transfer from welds and terminals into the Li-ion battery cell |
| Busbar area comparison | Pack-A and Pack-B | 40 °C ambient, same fast-charge profile | Link cross-sectional area to temperature rise |
For the constant-current test, I discharged each Li-ion battery pack at 1/2C until the relay opened. After a rest period, I charged at 2C and monitored busbar temperature and battery management system temperature. I stopped charging when the NTC temperature exceeded 55 °C or another protection condition was reached. The liquid-cooling loop opened at 34 °C and closed at 32 °C, with a coolant temperature of 22 °C and a flow rate of 12 L/min. For map charging, I used a current profile indexed by state of charge and cell temperature. The cooling thresholds were 34 °C and 39 °C, with coolant at 22 °C or 18 °C and flow rates of 12 L/min or 15 L/min depending on maximum temperature.
I defined the busbar-to-BMS temperature difference as
$$\Delta T_{\mathrm{BMS}} = T_{\mathrm{busbar}} – T_{\mathrm{BMS}}.$$
This difference is a direct measure of the Li-ion battery thermal monitoring blind spot. I also defined a normalized blind-spot index:
$$M = \frac{T_{\mathrm{busbar,max}} – T_{\mathrm{BMS,max}}}{T_{\mathrm{limit}} – T_{\mathrm{BMS,max}}}.$$
When \(M\) is larger than 1, the busbar can reach its safety limit while the battery management system still reports a temperature below the protection threshold. This condition is dangerous for a Li-ion battery pack because the control system believes that the Li-ion battery is operating within limits.
4. Constant-Current Fast Charging of the Li-ion Battery Pack
My first measurements compared the busbar temperature and the battery management system temperature during 2C charging. I tested three Pack-A samples with different mileage values under both water-cooled and non-water-cooled conditions. The results show a consistent and large separation between the actual busbar temperature and the reported temperature.
| Li-ion battery pack sample | Cooling condition | 2C duration (s) | Busbar maximum temperature (°C) | BMS maximum temperature (°C) | Difference (°C) |
|---|---|---|---|---|---|
| Pack-A, 1# | No water 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 water 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 water cooling | 224.2 | 64.52 | 37.0 | 27.52 |
| Pack-A, 3# | Water cooling | 217.1 | 56.81 | 32.0 | 24.81 |
The temperature difference remained above 20 °C in every case. In the water-cooled cases, the busbar reached 56.81 °C to 60.44 °C while the battery management system reported only 32.0 °C. This means that the NTC sensors were not detecting the true maximum temperature of the Li-ion battery pack during fast charging. More importantly, the liquid-cooling system did not eliminate the problem. The difference under water cooling was only slightly smaller than under no cooling. I interpret this as evidence that the busbar heat generation is concentrated and structurally coupled to the Li-ion battery terminals and welds, so the cooling plate cannot compensate for the local overcurrent condition.
Using the joule heating equation, I estimated the relative heat generation change with current. If the current doubles, the heat generation increases by a factor of four:
$$\frac{Q_{g,2}}{Q_{g,1}} = \left(\frac{I_2}{I_1}\right)^2.$$
This explains why a busbar designed for a moderate charge rate becomes a severe local heat source under fast charging. In a Li-ion battery pack, the busbar is connected to the cell terminals, so the high temperature can conduct back into the Li-ion battery cell. I refer to this as thermal backflow. The backflow adds to the cell’s own heat generation and can accelerate aging or trigger thermal runaway under extreme conditions.
5. Validation of the BMS Temperature Acquisition Position
I next tested whether the battery management system temperature sensor location was fundamentally wrong. I placed temperature sensors at the upper, middle, and lower points of the cell surface and compared them with the NTC position. During the initial charging stage, the NTC temperature was higher than the upper, middle, and lower surface temperatures. As charging continued, the surface temperatures increased faster and eventually exceeded the NTC temperature by about 4 °C to 5 °C after charging ended. During the high-current phase above 400 A, the NTC temperature still represented a relative high point and remained within about 5 °C of the cell surface maximum.
| Charging stage | Observed temperature order | Interpretation for Li-ion battery safety |
|---|---|---|
| Initial low-current stage | NTC > upper surface > middle surface > lower surface | NTC is slightly conservative and reflects the upper region |
| High-current stage above 400 A | NTC near the relative maximum, within about 5 °C | NTC position is reasonable for cell surface monitoring |
| Post-charge rest | Upper, middle, and lower surfaces exceed NTC by 4 °C to 5 °C | Heat continues to diffuse after current stops |
This result is important. It shows that the problem is not simply a poor NTC location. The NTC can track the cell surface temperature reasonably well. The problem is that the NTC does not measure the busbar, weld, or terminal temperature. The Li-ion battery management system therefore monitors the Li-ion battery cell but not the Li-ion battery busbar heat source. In a fast-charge event, the busbar can overheat while the cell surface remains below the protection threshold. This is a structural blind spot, not a calibration error.
I expressed the measurement gap as
$$\varepsilon_T = T_{\mathrm{hotspot}} – T_{\mathrm{NTC}},$$
where \(T_{\mathrm{hotspot}}\) is the maximum temperature in the Li-ion battery pack and \(T_{\mathrm{NTC}}\) is the temperature reported by the battery management system. My measurements show that \(\varepsilon_T\) can exceed 25 °C during 2C charging and approach 30 °C during map charging. This error is much larger than the normal sensor tolerance and cannot be corrected by a simple offset.
6. Map Fast Charging and Real Operating Risk
Constant-current tests are useful because they isolate the overcurrent effect, but real Li-ion battery packs usually use a charge map that reduces current as temperature and state of charge change. I therefore tested Pack-A under a realistic fast-charge map at 25 °C, 30 °C, and 38 °C ambient temperature. The busbar temperature was much higher than the battery management system temperature in all three cases.
| Ambient temperature (°C) | Busbar peak temperature (°C) | BMS peak temperature (°C) | Difference (°C) |
|---|---|---|---|
| 25 | 71.3 | 50 | 29.37 |
| 30 | 74.41 | 50 | 28.74 |
| 38 | 80.79 | 57 | 30.33 |
At 38 °C ambient, the busbar peak temperature reached 80.79 °C while the battery management system reported about 57 °C. The difference was about 30 °C. This is a serious thermal safety risk for a Li-ion battery pack. The busbar becomes a persistent heat source during fast charging, and the heat can conduct into the Li-ion battery cell through the terminals and welds. The liquid-cooling system reduces the bulk temperature but does not remove the local hotspot. I observed that the difference did not shrink significantly when cooling was active. This confirms that the overcurrent capability of the Li-ion battery busbar and the cooling system must be designed together.
I estimated the heat transferred from the busbar into the Li-ion battery cell using a one-dimensional thermal resistance model:
$$Q_{\mathrm{back}} = \frac{T_b – T_{\mathrm{cell}}}{R_{\theta,b-\mathrm{cell}}}.$$
When \(T_b\) is high and \(R_{\theta,b-\mathrm{cell}}\) is low, the backflow heat can be substantial. At the weld and terminal regions, the metal path is short and the thermal resistance is low. This is why the weld and terminal temperatures are often the highest points in the Li-ion battery module.
7. Small Module Tests and Internal Cell Temperature Gradients
To measure the thermal backflow effect directly, I used a small 1P3S Li-ion battery module with internal and external temperature sensors. The small module allowed me to place sensors inside the cell and around the welds, terminals, and large faces. I tested two coolant flow rates, 1 L/min and 3.6 L/min, at 35 °C and 40 °C ambient temperature, with 30 min and 40 min charging strategies.
The full maximum temperature results are summarized below. Positions 1# to 11# follow the module temperature-sensor numbering defined earlier. A slash indicates that the sensor was not available for that specific test configuration.
| Condition | Module | Flow (L/min) | Coolant (°C) | 1# | 2# | 3# | 4# | 5# | 6# | 7# | 8# | 9# | 10# | 11# | Ambient (°C) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 35 °C, 30 min | 1# | 3.6 | 20 | 63.6 | 62.7 | 49.2 | / | 47.5 | 45.8 | 46.8 | 66.2 | 63.6 | / | / | 35.0 |
| 35 °C, 30 min | 2# | 1 | 20 | 71.6 | 74.7 | 54.6 | 56.0 | 52.2 | 49.5 | 53.5 | 74.3 | 69.7 | 70.5 | 75.8 | 35.0 |
| 40 °C, 30 min | 1# | 3.6 | 20 | 71.7 | 70.8 | 55.7 | / | 52.4 | 50.2 | 53.4 | 75.4 | 72.3 | / | / | 40.8 |
| 40 °C, 30 min | 2# | 1 | 20 | 79.4 | 78.6 | 58.7 | 60.2 | 55.5 | 52.7 | 59.0 | 81.6 | 76.8 | 78.0 | 83.3 | 40.0 |
| 35 °C, 40 min | 1# | 3.6 | 20 | 52.7 | 54.9 | 46.4 | / | 45.1 | 43.4 | 43.6 | 54.2 | 53.5 | / | / | 35.0 |
| 35 °C, 40 min | 2# | 1 | 20 | 55.6 | / | 48.5 | 49.8 | 46.9 | / | 45.5 | 60.0 | 55.2 | 56.5 | 60.0 | 35.0 |
| 40 °C, 40 min | 1# | 3.6 | 20 | 57.1 | 58.4 | 50.0 | / | 48.2 | 46.4 | 47.3 | 59.2 | 58.2 | / | / | 40.0 |
| 40 °C, 40 min | 2# | 1 | 20 | 60.7 | / | 52.5 | 54.3 | 50.8 | / | 49.9 | 65.5 | 60.5 | 61.9 | 65.6 | 40.0 |
The NTC position, 7#, was consistently much cooler than the positive and negative terminals, 8# and 9#. At 35 °C ambient and 30 min charging, 7# reached 46.8 °C while 8# reached 66.2 °C, a difference of 19.4 °C. At 40 °C ambient and 30 min charging, 8# reached 81.6 °C while 7# reached 59.0 °C, a difference of 22.6 °C. With lower coolant flow, the differences generally increased. The positive terminal was often the hottest point, and the large-face center and tab-root temperatures rose together with the terminal temperature. This confirms that heat from the busbar and weld region conducts into the Li-ion battery cell and creates an internal temperature gradient.
I calculated the thermal gradient between the terminal and the NTC as
$$\nabla T = \frac{T_{8\#} – T_{7\#}}{L_{8-7}},$$
where \(L_{8-7}\) is the effective distance between the terminal and the NTC measurement point. The high gradient indicates a concentrated heat source near the terminal and weld. This gradient is not captured by the battery management system because the NTC is placed on the top cover or cell surface, away from the busbar weld.
I also observed that increasing coolant flow from 1 L/min to 3.6 L/min reduced the overall temperature, but it did not eliminate the terminal-to-NTC difference. This is consistent with the earlier pack-level results. The cooling system can remove heat from the large cell surface, but it cannot easily remove heat from the narrow busbar-to-terminal path. The Li-ion battery busbar therefore acts as a parallel heat source that bypasses the main cooling path.
8. Effect of Busbar Cross-Sectional Area
The most direct evidence for the overcurrent design flaw came from comparing Pack-A and Pack-B. The two Li-ion battery packs have similar architecture but different busbar dimensions. Pack-A has a busbar cross-sectional area of about 80 mm², while Pack-B has about 117 mm². Under the same 40 °C ambient and the same fast-charge condition, the Pack-A busbar reached 67.96 °C, while the Pack-B busbar reached 57.95 °C. The difference was 10.01 °C, and the temperature rise of Pack-A was about 28 °C.
| Li-ion battery pack | Busbar area (mm²) | Approximate continuous current capability (A) | Fast-charge peak current (A) | Busbar peak temperature at 40 °C ambient (°C) | Difference versus Pack-B (°C) | Temperature rise above ambient (°C) |
|---|---|---|---|---|---|---|
| Pack-A | 80 | About 200 | Greater than 500 | 67.96 | +10.01 | About 28 |
| Pack-B | 117 | About 290 | Greater than 500 | 57.95 | Reference | About 18 |
The theoretical current ratio from cross-sectional area is
$$\frac{I_{\mathrm{cap},B}}{I_{\mathrm{cap},A}} = \frac{A_B}{A_A} = \frac{117}{80} = 1.4625.$$
If the allowable current scales approximately with area, Pack-B should carry about 46% more continuous current than Pack-A for the same temperature rise. The measured temperature difference is consistent with this expectation. The larger busbar reduces current density and spreads heat over a larger volume. It also provides more surface area for heat transfer to the terminals and cooling path. This is why a properly sized Li-ion battery busbar can suppress local temperature rise without changing the battery chemistry or the cooling plate design.
I also considered the effect of weld quality. A poor weld increases contact resistance, which adds to the busbar resistance:
$$R_{\mathrm{total}} = R_{\mathrm{busbar}} + R_{\mathrm{weld}} + R_{\mathrm{contact}}.$$
The total heat generation becomes
$$Q_g = I^2 (R_{\mathrm{busbar}} + R_{\mathrm{weld}} + R_{\mathrm{contact}}).$$
If the weld resistance is high, the weld becomes the hottest point. My module tests showed that the laser weld positions 1# and 2# often reached very high temperatures, sometimes above 80 °C. This means that busbar overcurrent and weld quality are coupled risks in a Li-ion battery pack. A good electrical design must consider both the conductor cross-section and the joining process.
9. Thermal Safety Model for Li-ion Battery Busbars
Based on my measurements, I propose a simple thermal safety model for Li-ion battery busbars. The model combines current density, thermal resistance, and monitoring coverage. The first condition is electrical:
$$J_{\mathrm{rms}} \le J_{\mathrm{allow}},$$
where \(J_{\mathrm{allow}}\) is the allowable current density for the chosen material and temperature limit. The second condition is thermal:
$$T_b = T_a + I^2 R_{\theta} \le T_{\mathrm{lim}}.$$
The third condition is monitoring:
$$T_{\mathrm{BMS}} \ge T_b – \Delta T_{\mathrm{allow}},$$
where \(\Delta T_{\mathrm{allow}}\) is the maximum acceptable difference between the real hotspot and the reported temperature. In my tests, \(\Delta T_{\mathrm{allow}}\) was exceeded by a large margin. The fourth condition is cooling:
$$Q_c \ge Q_g – Q_{\mathrm{storage}},$$
where \(Q_{\mathrm{storage}}\) is the heat stored in the busbar and nearby Li-ion battery components. If the cooling capacity is less than the generation rate, the busbar temperature rises until the heat loss balances the generation. The steady-state temperature is then determined by the total thermal resistance.
I combined these conditions into a safety margin:
$$S = \frac{T_{\mathrm{lim}} – T_{\mathrm{busbar}}}{T_{\mathrm{lim}} – T_{\mathrm{BMS}}}.$$
When \(S\) is less than 1, the busbar is closer to its limit than the reported BMS temperature suggests. In my Pack-A tests, \(S\) was well below 1 during fast charging. This means that the Li-ion battery pack was operating with a hidden thermal margin deficit. The battery management system believed that there was thermal headroom, but the busbar was already close to or above the safe limit.
I also defined a cooling compensation factor:
$$F_c = \frac{T_{\mathrm{no-cool}} – T_{\mathrm{cool}}}{T_{\mathrm{no-cool}} – T_a}.$$
For Pack-A constant-current tests, the reduction from no cooling to water cooling was small relative to the total temperature rise. The busbar temperature difference between cooled and uncooled cases was only a few degrees, while the busbar-to-BMS difference remained above 20 °C. Therefore, \(F_c\) was low. This confirms that the cooling system cannot compensate for an undersized Li-ion battery busbar. The overcurrent problem must be solved at the conductor design level.
10. Design Guidelines Derived from the Measurements
I derived several practical guidelines from the experiments. These guidelines apply to Li-ion battery packs, Li-ion battery modules, and Li-ion battery cells with aluminum busbars.
| Design area | Guideline | Reason from my Li-ion battery tests |
|---|---|---|
| Busbar cross-section | Size the busbar for the fast-charge peak current, not only the continuous current | Pack-A busbar was about 80 mm² and reached 67.96 °C at 40 °C ambient, while Pack-B at 117 mm² reached 57.95 °C |
| Current density | Keep the peak current density below the material limit with a safety factor | Joule heating scales with \(I^2\), so peak current dominates local temperature |
| Weld design | Minimize weld resistance and verify weld quality | Weld positions reached high temperatures and contributed to cell backflow |
| Thermal monitoring | Monitor busbar, weld, and terminal temperatures in addition to cell NTC | Busbar-to-BMS difference exceeded 25 °C and sometimes 30 °C |
| Cooling integration | Do not rely on the cooling plate to fix an undersized busbar | Water cooling reduced bulk temperature but did not remove the local hotspot |
| Fast-charge map | Recalculate the map whenever busbar geometry or current profile changes | A busbar sized for normal charging can become unsafe under fast charging |
I also developed a required cross-sectional area estimate for a target temperature rise. Starting from the steady-state heat balance,
$$I^2 \rho \frac{L}{A} = h A_s \Delta T,$$
I solved for the area:
$$A_{\mathrm{req}} = \frac{I^2 \rho L}{h A_s \Delta T}.$$
If the surface area \(A_s\) itself depends on the busbar geometry, the calculation becomes iterative. For a rectangular busbar of width \(w\) and thickness \(t\),
$$A = wt,$$
and the perimeter is
$$P = 2(w+t).$$
The surface area over length \(L\) is approximately
$$A_s \approx PL = 2(w+t)L.$$
Substituting into the required-area expression gives
$$wt \Delta T \approx \frac{I^2 \rho}{2h(w+t)}.$$
This relationship shows why increasing both width and thickness is more effective than increasing only one dimension. A thin but wide busbar may have a large cross-sectional area, but its thermal path and stiffness may still be poor. A thick but narrow busbar may have high local current density near the edges. The best design distributes current uniformly and provides a short, wide thermal path to the terminals and cooling structure.
11. Comparison of Pack-Level and Module-Level Findings
I compared my pack-level, module-level, and cell-level results to ensure that the conclusion is consistent across scales. The same pattern appeared at every level: the busbar or terminal region became the hottest point, the battery management system measured a lower temperature, and the cooling system did not eliminate the local gradient.
| Level | Observed hotspot | Reported or nearby temperature | Maximum difference | Safety implication for Li-ion battery systems |
|---|---|---|---|---|
| Li-ion battery pack | Aluminum busbar | BMS NTC | About 30 °C | Protection may not trigger before busbar over-temperature |
| Li-ion battery module | Positive terminal and laser weld | NTC top cover | About 22 °C | Heat conducts into the Li-ion battery cell |
| Li-ion battery cell | Weld, tab, and terminal region | Cell surface NTC | Up to about 30 °C | Internal gradient increases aging and thermal risk |
The consistency across scales is important. It means that the problem is not an artifact of one sensor or one operating condition. The problem is structural. The Li-ion battery busbar is a current-carrying component with its own thermal resistance and heat generation. If it is undersized, it becomes a local heat source that the rest of the thermal system may not be able to manage.
I also compared the constant-current and map-charging results. In constant-current tests, the busbar-to-BMS difference was 24 °C to 29 °C. In map-charging tests, the difference was 28 °C to 30 °C. The map strategy reduced the current at high state of charge and adjusted cooling, but it did not close the gap. This tells me that the map cannot fully protect the Li-ion battery busbar if the busbar itself is undersized. The map controls the Li-ion battery cell temperature based on the NTC, but the busbar temperature follows its own thermal path. The control logic and the thermal reality are not aligned.
12. Thermal Runaway Risk Pathway
I do not claim that every busbar over-temperature event leads directly to thermal runaway. However, my measurements identify a credible risk pathway for a Li-ion battery pack:
1. Fast charging demands a current above the busbar continuous rating.
2. Joule heating in the busbar increases with \(I^2 R\).
3. The busbar temperature rises faster than the cell surface temperature measured by the NTC.
4. The battery management system does not see the hotspot and continues the charge.
5. Heat conducts through the weld and terminal into the Li-ion battery cell.
6. The local cell temperature rises above the intended operating range.
7. If the condition continues, separator degradation, electrolyte decomposition, and gas generation can occur.
8. In extreme cases, the Li-ion battery cell may enter thermal runaway.
This pathway can be expressed as a chain of thermal resistances and heat sources. The busbar heat source is
$$Q_b = I^2 R_b.$$
The heat entering the Li-ion battery cell is
$$Q_{\mathrm{cell}} = \frac{T_b – T_{\mathrm{cell}}}{R_{\theta,b-\mathrm{cell}}}.$$
The cell temperature rise is then
$$\Delta T_{\mathrm{cell}} = Q_{\mathrm{cell}} R_{\theta,\mathrm{cell}}.$$
If the busbar temperature is high and the thermal resistance to the cell is low, the cell temperature rise can be significant even when the cell’s own electrochemical heat generation is moderate. This is why I emphasize the busbar as a thermal safety component, not only an electrical component. In a Li-ion battery pack, the busbar can be an external heat source attached directly to the cell terminals.
13. Synergistic Optimization of Overcurrent and Cooling
The most important conclusion from my work is that overcurrent capability and cooling must be optimized together. A larger busbar reduces current density and heat generation. A better cooling path reduces the thermal resistance from the busbar to the coolant. A better monitoring strategy ensures that the battery management system sees the true hotspot. None of these measures is sufficient alone.
| Measure | Effect on busbar temperature | Effect on BMS blind spot | Effect on Li-ion battery thermal safety |
|---|---|---|---|
| Increase busbar cross-sectional area | Strong reduction in \(I^2 R\) heat generation | Indirect, because the hotspot becomes cooler | High, because it addresses the root cause |
| Improve weld quality | Reduces contact resistance and local heating | Indirect | High, especially at terminals and tabs |
| Add busbar temperature sensors | No direct cooling effect | Strong reduction in blind spot | High, because protection can trigger earlier |
| Increase coolant flow | Moderate reduction in bulk temperature | Small, because local gradient remains | Moderate |
| Adjust fast-charge map | Reduces peak current and heat generation | Moderate, if map uses busbar temperature | Moderate to high |
I recommend a co-design process in which the busbar cross-section, weld design, terminal geometry, cooling path, and thermal sensor placement are evaluated together. The design should be validated at the pack level, module level, and cell level. A pack-level test alone may miss the internal gradient. A cell-level test alone may miss the system-level current distribution. My three-level approach showed that the risk appears at all levels and is most clearly quantified when the levels are compared.
14. Practical Implications for Li-ion Battery Pack Design
The practical implications of my study are straightforward. First, the Li-ion battery busbar should be selected using the fast-charge peak current and the maximum ambient temperature, not only the nominal continuous current. Second, the battery management system should monitor the busbar or terminal temperature directly, or use a thermal model that estimates the busbar temperature from current and coolant conditions. Third, the cooling system should be designed to remove heat from the busbar and terminal path, not only from the cell bottom. Fourth, the fast-charge map should include a busbar temperature constraint.
I can express the required protection condition as
$$I_{\mathrm{charge}} \le I_{\mathrm{busbar,lim}}(T_{\mathrm{cool}}, T_{\mathrm{amb}}, \mathrm{SOC}),$$
where \(I_{\mathrm{busbar,lim}}\) is the busbar current limit as a function of coolant temperature, ambient temperature, and state of charge. In current practice, the charge map is often based on cell temperature and state of charge. Adding a busbar limit would close the protection gap. The busbar limit can be calculated from the thermal model:
$$I_{\mathrm{busbar,lim}} = \sqrt{\frac{T_{\mathrm{lim}} – T_{\mathrm{ref}}}{R_{\theta} R_b}}.$$
Here \(R_b\) is the busbar electrical resistance and \(R_{\theta}\) is the busbar-to-reference thermal resistance. This equation can be calibrated from tests such as those I performed. The calibration should include different coolant flow rates, ambient temperatures, and states of health.
15. Summary of Quantitative Findings
I summarize the main quantitative findings in the following table. These values are drawn from my Li-ion battery pack, module, and cell tests.
| Finding | Value | Test level |
|---|---|---|
| Busbar-to-BMS difference under 2C charging with water cooling | 24.81 °C to 28.44 °C | Li-ion battery pack |
| Busbar-to-BMS difference under 2C charging without water cooling | 26.83 °C to 29.00 °C | Li-ion battery pack |
| Busbar peak temperature under map charging at 38 °C ambient | 80.79 °C | Li-ion battery pack |
| Busbar-to-BMS difference under map charging | 28.74 °C to 30.33 °C | Li-ion battery pack |
| Terminal-to-NTC difference in small module | About 19 °C to 23 °C | Li-ion battery module |
| Maximum terminal temperature in small module | 81.6 °C | Li-ion battery module |
| Pack-A busbar peak versus Pack-B at 40 °C ambient | 67.96 °C versus 57.95 °C | Li-ion battery pack comparison |
| Pack-A busbar cross-sectional area | 80 mm² | Li-ion battery pack |
| Pack-B busbar cross-sectional area | 117 mm² | Li-ion battery pack |
The numbers show a clear pattern. The busbar is consistently hotter than the battery management system temperature, and the difference is large enough to defeat normal protection logic. The cooling system reduces the absolute temperature but does not remove the gradient. The busbar cross-sectional area has a direct and measurable effect on peak temperature. These results support the conclusion that busbar overcurrent capability is a primary thermal safety variable in Li-ion battery packs.
16. Limitations and Future Validation
I recognize several limitations in this work. First, the tests focus on aluminum busbars in a particular family of Li-ion battery packs. Copper busbars may behave differently because copper has higher electrical conductivity and different thermal properties. Second, the weld geometry and manufacturing variation can affect local resistance. Third, the fast-charge map and cooling strategy are specific to the tested Li-ion battery packs. Fourth, the long-term aging effect on busbar resistance and thermal contact resistance requires additional study.
I also note that the engineering context is changing. Early Li-ion battery pack designs often selected busbars based on normal charge rates. As the market moves toward higher fast-charge rates, the original busbar may no longer have sufficient overcurrent capability. The cooling system may also have been designed for a different heat load. Therefore, any increase in charge current should trigger a re-evaluation of the busbar, weld, terminal, and cooling design. This is especially important for Li-ion battery packs that are already in service or that are being upgraded through software changes.
Future work should include coupled electrothermal modeling of the Li-ion battery busbar, transient temperature prediction during fast charging, and online estimation of busbar temperature from current and coolant data. It should also include accelerated aging tests to determine how busbar resistance, weld resistance, and thermal interface resistance evolve over the life of the Li-ion battery pack.
17. Conclusions
In this study, I systematically evaluated the relationship between aluminum busbar overcurrent capability and thermal safety in Li-ion battery systems at the pack, module, and cell levels. My conclusions are as follows.
First, the Li-ion battery busbar can be significantly hotter than the temperature reported by the battery management system. Under 2C charging, the busbar-to-BMS difference remained above 20 °C, and under map charging it approached 30 °C. This creates a monitoring blind spot in which the Li-ion battery pack can experience busbar over-temperature while the protection system reports a safe cell temperature.
Second, the battery management system temperature sensor position can be reasonable for cell surface monitoring, but it does not measure the busbar, weld, or terminal hotspot. The problem is not simply sensor placement. The problem is that the monitored location and the heat-generating location are different. A Li-ion battery pack needs either direct busbar temperature measurement or a validated thermal model that estimates the busbar temperature.
Third, heat from the busbar conducts through the weld and terminal into the Li-ion battery cell. The small-module tests showed terminal-to-NTC differences of about 19 °C to 23 °C, with terminal temperatures reaching 81.6 °C. This thermal backflow can raise the internal cell temperature and increase the risk of accelerated aging or thermal runaway under severe conditions.
Fourth, the busbar cross-sectional area has a direct effect on temperature rise. The Pack-A busbar with about 80 mm² reached 67.96 °C at 40 °C ambient, while the Pack-B busbar with about 117 mm² reached 57.95 °C under the same condition. The larger busbar reduced current density and spread heat over a larger volume, demonstrating that proper overcurrent design can suppress local temperature rise.
Fifth, the cooling system cannot fully compensate for an undersized busbar. Water cooling reduced the bulk temperature but did not eliminate the busbar-to-BMS difference. The local hotspot remained because the heat generation was concentrated at the busbar, weld, and terminal. Therefore, overcurrent capability and cooling must be optimized together. A Li-ion battery pack design that relies on cooling alone to manage an undersized busbar is inherently risky.
Overall, my results show that the Li-ion battery busbar is a critical thermal management component. Its overcurrent capability is not a secondary electrical detail. It is a primary safety variable that affects the Li-ion battery pack, the Li-ion battery module, and the Li-ion battery cell. A well-designed busbar, combined with good weld quality, direct thermal monitoring, and coordinated cooling, can significantly reduce local temperature rise and improve the thermal safety of Li-ion battery systems.
I conclude that future Li-ion battery pack designs should treat the busbar as a first-class thermal safety element. The design process should include fast-charge current requirements, peak current density limits, weld resistance control, terminal thermal paths, busbar temperature sensing, and cooling system coordination. When these factors are optimized together, the Li-ion battery pack can achieve higher fast-charge capability without sacrificing thermal safety or long-term reliability.