This article is sponsored by Tadiran Batteries.  

Lithium-ion batteries don’t tolerate stress well. They react with extreme sensitivity to environmental factors and brutal temperatures to mechanical pressure, shocks, vibrations, and other physical stress, which can all degrade performance, reduce capacity, and, in severe cases, trigger thermal runaway. But as these batteries break into everything from industrial and medical robots to military radios and other mission-critical systems, they’re increasingly being exposed to exactly these conditions.

These systems need new approaches both inside the battery pack through safer, more durable chemistries, and outside of it with more rugged packaging that can withstand harsh temperatures and other hardships.

As these batteries break into increasingly rough, unpredictable environments, ruggedness is becoming as important to many system companies as energy density. For example, SpaceX is building its first rugged Starlink terminal with a built-in battery pack to power its satellite connectivity. To ensure reliability and safety, battery packs used in heavy-lift drones and humanoid robots such as Hyundai’s Atlas also need to endure constant vibrations, hard impacts, and moisture exposure while delivering uninterrupted power.

The energy density in these systems is typically as high as possible to maximize runtimes in tight spaces. But this concentration of power increases the consequences of a cell failure. Rugged battery packaging helps reduce these risks by protecting densely packed cells from vibration, moisture, dust, or other environmental hazards while managing temperature highs and lows. By shielding the battery from conditions that speed up cell degradation, it can also improve service life.

There’s no one-size-fits-all battery pack, but the most robust designs adopt multiple layers of protection. Safer cell chemistries, rugged cell construction, and reinforced enclosures work together with a battery-management system (BMS) that controls the operation of the entire pack to extend its cycle life and reduce the chances of catastrophic events from overheating.

Rugged Construction from the Cell to the Pack Level

While chemistry defines the upper limits of a battery’s durability and reliability, rugged construction at the cell, module, and pack levels enables each chemistry to perform closer to its full potential in the real world.

While lithium-ion chemistries such as nickel manganese cobalt oxide (NMC) and nickel cobalt aluminum oxide (NCA) deliver very high energy density, they can have difficulties with harsh temperatures and other rugged conditions (Table 1). In addition, military- and industrial-grade nickel-based cells often operate over a much wider temperature range — from −40 to 85°C — while offering longer lifespans, lasting thousands of full charge-discharge cycles.

Lithium iron phosphate (LFP) and lithium titanate oxide (LTO) are even more durable, operating safely in extreme heat and cold while lasting thousands to tens of thousands of full charge-discharge cycles. Though these chemistries sacrifice energy density, their safety and longevity are well-suited for rugged systems. In all of these cases, rugged packaging improves protection and thermal management, allowing each battery chemistry to operate in as normalized a state as possible.

Table 1: Battery Chemistries

Use the horizontal scroll bar at the bottom of the tables to see the rest of the table. 

These cells come in a wide range of form factors that can further add durability, from cylindrical cells and brick-shaped prismatics to soft, pouch-shaped cells, all with varying levels of energy density and capacities (Table 2). Within cylindrical and prismatic cells, the cathode, anode, and separator are tightly wound like a jelly roll. While consumer-grade batteries often use crimped seals that are prone to fail, industrial-grade cells can be precision-welded into hermetically sealed enclosures.

While these cells are the fundamental building blocks of a battery, most rugged systems require multiple cells connected in series, parallel, or both to form a battery pack. The cells are housed within a ruggedized enclosure made from impact-resistant steel, aluminum, or reinforced polymer material. This case protects the cells from vibrations or prevents them from being crushed or punctured. The BMS provides additional layers of protection by controlling the charge and discharge FETs to maintain the battery’s safe operation.

Incorrect construction of the pack can make it more vulnerable to harsh temperature, mechanical shock, or other stress, which could degrade the performance of the internal cells or damage them, reducing service life.

The arrangement of the cells is one of the factors in ensuring the battery’s ruggedness. In the event one of the cells overheats and starts venting, it vents out from the positive terminal. To reduce the chance of one venting cell impacting others and causing thermal runaway, it helps to keep the positive terminals of the cells turned away from each other. It’s also desirable to put the positive terminal of the highest potential cell in the cell stack on the edge of the pack and facing out.

Isolating cells, with fuses wired to both the positive and negative terminals, ensures that each one is prevented from impacting other cells in the pack in the event it starts overheating or fails. These devices also enable the battery pack to keep operating safely even if a damaged cell gets disconnected. The fuses also help limit overheating and isolate thermal runaway to a single cell before it can spread throughout the battery pack and trigger a fire or catastrophic safety event.

As batteries perform best within a narrow temperature range, battery packs often come with cooling plates or compression pads to help regulate temperature. Thermal pastes, conductive tapes, and other thermal interface materials (TIM) can be used as well to help move heat away from the cells and onto cooling plates. Operating temperature is a critical parameter for battery packs as it impacts internal cell chemistry, electronics, and the PCB itself, which can also be equipped with a heatsink to keep cool.

Table 2: Form Factors

Temperatures that fluctuate either too hot or too cold pose danger to battery packs. As the Arrhenius equation shows, a 10°C rise in battery temperature can roughly double the rate of internal side reactions, which may grant a temporary boost to capacity during discharge. However, prolonged exposure can also degrade the battery and reduce its cycle life. In very hot conditions, the risk of cell venting and thermal runaway could lead to smoke, fires, or explosions.

In cold conditions, the opposite occurs. Frigid temperatures increase a battery’s internal resistance, slowing down the electrochemical reactions in the cells. At an operating temperature of −20°C, a battery’s discharge capacity can drop so dramatically that a cell fully charged at 20°C may stop operating after only several minutes. Charging in freezing conditions also increases the risk of lithium plating. This irreversible process can permanently reduce battery capacity and, in severe cases, create internal short circuits.

Besides cooling plates and other thermally conductive materials, battery packs use various techniques to regulate the battery pack’s temperature, reducing the risk of thermal runaway while extending the service life of the battery. By using thermistors to keep tabs on the thermal conditions in the pack, the BMS can adjust charging and discharging rates based on those. It’s also able to control heaters to warm batteries in the cold or a cooling fan to turn down heat. On top of that, it can monitor the output of the fan to make sure it’s working correctly.

Securing, Padding, and Sealing the Battery Pack

In portable military electronics and other systems subject to rough conditions, battery packs need to handle everything from being dropped on the ground to significant levels of vibration, shock, and mechanical abuse.

To prevent fatigue-related failures, engineers need to secure everything from individual cells to the BMS. Battery cells are typically connected using nickel strips welded directly to cell terminals or soldered wires attached to the battery contacts, before being soldered on the BMS board. Copper busbars offer another option to bolt cells together in heavy-duty battery packs and reinforcing them can prevent them from dislocating.

While internal structural supports help hold cells in place during operation, it helps to have redundant points of constraint on the connection joints of the cells. If one fails, the joints could still be held by backup connections.

Padding the inside of the pack with materials such as polyurethane or silicone foam helps prevent the cells or the electronics from shifting during severe vibration or physical blows to the battery. These shock-absorbing materials also often have fire-resistant properties that help isolate hot spots and slow the spread of thermal runaway between cells. They can also enhance the structural integrity of the pack by locking the cells in place and distributing physical loads.

The electronics inside the pack require similar levels of protection. Applying conformal coatings to the BMS and the PCB assembly helps prevent them from shaking loose during vibrations or physical impacts. Widely used in marine, medical, aerospace, and other mission-critical systems, these coatings are able to conform to the irregular landscape of the PCB, giving it the robustness to handle vibration, shock, water, dust, and debris.

Alternatively, engineers can apply potting compounds to encapsulate the BMS or sensitive electronics inside the pack. Potting involves filling portions of the battery pack with liquid or gel materials, such as silicone, epoxy, or polyurethane, which then hardens around the electronics to dampen vibrations as well as prevent dust and moisture. Potting compounds can help prevent stress fractures in the cells and even move heat from the most sensitive parts of the pack, helping extend the battery’s operating life.

Once the cells are secured, the battery pack must be protected from environmental hazards by sealing them in a robust enclosure.  If moisture enters the battery pack, it can cause corrosion that saps performance and potentially create electrical shorts. Dust and other debris may also interfere with electrical connections and block battery cooling fans, leading to overheating and shorter service life. Sealed battery enclosures must keep out these potential hazards while still allowing for cell venting.

Many methods exist for sealing a battery pack — from applying adhesives to the seams and joints of the battery housing, to creating a durable seal, to using gaskets made of silicone or other rubbery materials that are placed between the enclosure and cover to form a flexible seal. By reducing the number of parts in the battery housing, you can reduce the potential points of failure and regions to seal, resulting in a more robust design.

Using connectors and other components that have been rigorously tested for this purpose can help create a barrier against water, dust, and other debris. Alternatively, applying epoxy or encapsulating components in potting compounds help improve the environmental resistance of parts that aren’t sealed as tightly. While sealing the battery enclosure from the outside is important, engineers can also use internal sealing strategies to compartmentalize the pack and limit the spread of moisture if water does get inside.

However, it’s rarely safe to have a completely hermetically sealed battery pack because it can prevent the release of gases during cell venting. Without venting, pressure can build inside the enclosure, increasing the risk of rupture or catastrophic failure during thermal runaway.

The BMS: The Brains of a Rugged Battery Pack

The BMS plays a critical role in rugged battery packs, not only protecting the entire assembly from harm but also actively optimizing the operation of all cells to minimize degradation and maximize cycle life.

Often attached directly to the cells within the enclosure, the BMS acts as the caretaker for the battery pack. It runs diagnostics on the internal mechanisms of the battery, tracking a wide range of parameters such as cell voltage and pack current as well as cell and pack temperatures. These are monitored by strategically placed thermistors. If any of these parameters exceed safe operating limits or signal the early stages of thermal runaway, the BMS intervenes by limiting the charging and discharging process.

The BMS governs all of the rules and boundaries for safe, long-life battery operation. Using a model of the battery developed through rigorous testing, it continuously adjusts charge and discharge rates based on ambient and pack temperature, state of charge (SOC), and state of health (SOH). This helps prevent any over-charging or over-discharging and stops it from operating in fringe areas where battery life is most compromised. One of its other responsibilities is balancing cells to cut down on uneven cell degradation.

The data collected by the BMS can help expand the operating windows for Li-ion batteries. By keeping closer tabs on the battery, the BMS enables charging closer to full capacity and more of the battery’s capacity to be safely discharged. By tracking thermals, it can reduce charging rates to keep the cells safe in the freezing cold and limit discharge rates in heat to reduce risks of overheating or premature aging.

Thresholds for thermal protection must be set in the BMS by considering the cell’s limitations and the pack’s application-specific operating conditions. The cell’s datasheet outlines the temperature range where it’s safe to charge and discharge, which can be programmed into the BMS’s configuration file. If battery temperature falls below or rises above those limits, the BMS can shut things down until it’s safe to resume.

As the caretaker for these potentially explosive cells, the BMS often needs to be even more rugged than the pack itself. The battery-management ICs at its core often come with wider operating temperatures, typically −40 to 85°C, which is well above the limits of the cells themselves. They also often come with redundant analog-to-digital converters (ADCs), so they keep tabs on the battery under fault conditions and trigger a disconnect if danger arises.

Furthermore, the BMS must be designed at its core to operate in the presence of significant electromagnetic interference (EMI). Battery packs face unique challenges related to EMI, while dense component placement in compact designs can exacerbate the issue. EMI may also enter the pack from external sources, including RF emissions from radios or electrical noise caused by motors in drones, robotics, or sun-tracking solar panels. That can cause data inaccuracies or system failure.

Clear data signals on cell voltage, pack current, and temperature are crucial when situations arise in which the battery pack may be non-responsive, underperforming, or experiencing a thermal crisis.

To overcome these challenges, EMI-safe battery packs can use EMI filters or shielded wires as well as good grounding methods and optimized PCB layouts with sufficient space between the power and signal lines. In addition to adopting shielded wires to reduce noise, shielding materials such as sealing gaskets, absorbers, and conductive foam may be used to block EMI before it interferes with the BMS while limiting vibrations.

Inside the BMS, space is often severely limited. Rigid boards sometimes pose a challenge both due to space constraints and vibrations that can dislodge components over time, causing both performance and maintenance issues. Instead, flexible circuits can conform to irregularly shaped areas due to bending and folding benefits. The flexibility literally gives engineers more flexibility in placement to save space and weight. Flexible circuits can also replace connectors, which are a primary culprit of EMI.

As the control center of the battery, the BMS is key to keeping the battery pack safe and healthy. By constantly monitoring a multitude of criteria, it keeps the battery from straying into unsafe territory.

Testing: How Rugged is a Rugged Battery Pack?

Once they have been carefully designed, battery packs undergo a gauntlet of rigorous electrical, mechanical, and environmental tests to verify its safe, reliable operation under harsh conditions.

These tests simulate years of real-world abuse, evaluating how the battery cells (pre-assembly) and the pack (post-assembly) respond to everything from overcharging, undercharging, and short circuits to being dropped, crushed, punctured, baked, shocked, shaken, chilled, as well as exposed to moisture and humidity and splashed with or submerged under water. The battery pack and its contents are often tested against strict industrial, medical, and military standards for safety and reliability (Table 3).

Table 3: Safety Standards

A rugged battery pack is a lot more than a bundle of cells in a tough enclosure. Its performance and durability depend on engineering across the entire pack, from the choice of cells to mechanical design thermal management and qualification testing depending upon battery chemistry.