HV Battery pack design considerations

Importance of Well-Thought-Out battery pack design

The world is transitioning from Internal Combustion Engine (ICE) vehicles to Electric Vehicles (EVs). Automotive manufacturers are seeking to gain an edge over their competitors by investing heavily in R&D for EVs. One key component of every EV is its energy storage system. EV manufacturers must compete with the fast refueling and (in most cases) long range of ICE vehicles. A lot of thought and development is required to create a battery pack that is efficient, safe, and compact. If key components of a battery pack are not carefully considered and compared to available alternatives, performance can suffer—leading to a less competitive vehicle.

This article aims to provide insight into the process of selecting some key components in a High Voltage (HV) battery pack. 

High-level battery pack overview

A basic high-voltage battery pack includes the following components:

  • Battery cells to store electrical energy
  • Busbars to connect the cells and other components
  • Contactors to disconnect the battery
  • Current sensing devices to monitor battery current
  • Battery Management System (BMS) to monitor and control the battery

The selection of these components involves important design choices. These choices depend not only on the components themselves but also on the battery pack’s intended application. For instance, an EV battery pack has different requirements than one used for home energy storage.

Image: Battery module overview
Image source: https://skill-lync.com/student-projects/project-1-mechanical-design-of-battery-pack-268

Battery cells

Battery cells are the main energy storage element. They come in various construction types, shapes, sizes, and capacities. Each cell type has its own pros and cons, so it’s crucial to compare them to select the most suitable option for your application.

The capacity of a cell is expressed in Ampere-hours (Ah). For example, a 200Ah cell can theoretically provide 200A for one hour. With cell voltage included, energy is measured in Watt-hours (Wh).

Cell construction types

Different materials can be used to create battery cells. A few examples of these are: Lead-Acid, Lithium Iron Phosphate (LFP), and Nickel Manganese Cobalt Oxide (NMC). Some key differences between them are:

  • Nominal voltage
  • Energy density
  • Maximum discharge current
  • Chemistry-dependent characteristics like C-ratings

Suppliable current of a battery cell is expressed in a C-rating. The C-rating depends on its capacity:

  • For a 100Ah cell, 1C equals 100A.
  • If this cell has a maximum discharge value of 2C, the maximum discharge current is 200A.
  • A 50Ah cell at 1C discharges at 50A.

Lead-Acid, LFP, and NMC battery cells use different materials and chemical reactions that give them their energy storage capabilities. This article focuses on their electrical properties, not their chemical details.

Lead Acid

Lead-Acid battery cells are commonly found in vehicles — both in ICE vehicles and EVs. They are used as energy storage for the low-voltage power rail (12V or 24V, for example). Most electric devices in the vehicle are powered from this low-voltage rail. When starting the vehicle, power is drawn from this battery until another energy provider takes over.

This battery type can deliver a short pulse of high current. In ICE vehicles, this is used to start the engine via the starter motor. Once the ICE is running, the alternator supplies electrical power to the vehicle.

In an EV, a Lead-Acid battery can power the control electronics for the high-voltage (HV) battery system. After startup, a DC/DC converter typically supplies power to the vehicle’s electronics directly from the HV battery pack.

A Lead-Acid battery cell has a nominal voltage of approximately 2V. To create a 12V battery, six of these cells are connected in series.

While Lead-Acid batteries can be useful for powering the low-voltage rail in EVs, they are not ideal for use in the HV battery pack. Compared to other cell types, Lead-Acid batteries have:

  • Lower energy density per volume
  • Lower energy density per weight
  • Shorter lifespan
  • Limited continuous current supply

Although they can deliver high current pulses, their performance in continuous charge/discharge scenarios is limited compared to other modern battery chemistries such as LFP and NMC.

Lithium Iron Phosphate

Lithium Iron Phosphate — commonly shortened to LFP — is a type of Lithium-ion (Li-ion) battery cell. Compared to Lead-Acid batteries, it offers higher energy storage density and lower weight per Watt-hour (Wh). In practice, this means that an LFP battery holding the same amount of energy as a Lead-Acid battery will be smaller and lighter.

LFP cells also support higher charge and discharge currents and have a longer lifespan than Lead-Acid cells.

When cells are placed in series, they need to be balanced using a Battery Management System (BMS). This adds complexity to the system but is necessary to ensure all cells remain within a safe and consistent operating voltage range. Without balancing, cell voltages may drift apart over time, reducing the overall performance and capacity of the battery pack.

Some examples of EVs that currently use LFP cells in their battery packs include:

  • Volvo EX30
  • Tesla Model 3 (LFP variant)

Nickel Manganese Cobalt Oxide 

Nickel Manganese Cobalt Oxide (or NMC for short) is another type of Lithium-ion (Li-ion) battery cell. Compared to the LFP cells, they have a higher energy density per volume and per weight. 

Some examples of EVs which use this are again the Volvo EX30, but this time in its 64 kWh battery pack (compared to the 49 kWh LFP version). 

Overview Lead-Acid, LFP & NMC

Image: Energy density comparison between different battery cell types
Source: https://www.epectec.com/batteries/cell-comparison.html
Source table values: 
https://www.epectec.com/batteries/cell-comparison.html

Battery Management System 

When using cells such as LFP in series, a BMS is required to make sure each cell is at the same State of Charge (SoC). This means that each cell is charged to the same level. Without the use of a BMS, the SoC of each cell will start drifting. Cells that are charged the least will become fully discharged, which for LFP is internally damaging. This will reduce the battery pack’s operating voltage and start limiting the current the pack can supply.

Source: https://www.monolithicpower.com/en/learning/resources/active-balancing-how-it-works-and-its-advantages?srsltid=AfmBOorp1ZL-3QUsTmOOkJayB1_QjGtNHGHuDNp4TZBLv1hxC3Wl2szL

Another function of the BMS is to monitor the battery itself and verify that it is in a safe state.
The battery’s operational state is monitored by measuring:

  • cell voltages
  • battery pack discharge current
  • cell/battery pack temperatures

The total discharge current over time and the cell voltages can be used to estimate the depletion of the battery pack. This is usually expressed as State of Charge (SoC).

Balancing types

An important feature of a BMS is balancing the cells in the battery pack. This ensures the cells have the same charge capacity. One cell is not 100% equal to another, meaning some cells during charging will be full faster than others, and during discharging, some will empty faster than others.

The voltage of a cell depends on the charge in the cell. This can be seen in the figure below. By monitoring the voltage of each cell, fully charged cells can be identified. If there are cells that haven’t finished charging, the cells with higher voltages can be slowly discharged using a resistor. This is called passive balancing. An effect of this method is that heat is generated over the resistor. If the resistor heats up too much, the BMS can slow the balancing to cool it down — resulting in longer balancing and longer wait time for a fully charged battery.

Source: https://powmr.com/blogs/news/lifepo4-voltage-chart-and-soc

Another method of balancing is called active balancing. This redirects the charge from higher-voltage cells to lower-voltage ones. There are several ways to do this, but it requires more circuitry to do it safely and avoid short-circuiting cells — compared to passive balancing.

Passive and active balancing both have their pros and cons:

Active balancing handles large-capacity cells more efficiently without producing heat in resistors but needs a more complex and thoroughly tested balancing system.

For smaller-capacity cells, passive balancing is interesting because the lower balancing current doesn’t matter much.

For larger-capacity cells, passive balancing may still be used, but higher balancing current is needed — which generates more heat and requires careful management.

Charger communication 

The BMS should know the operational limits of both the battery pack and the individual cells (this should be configured in the BMS). When a battery pack needs to be charged, the BMS determines the correct voltage and current levels for charging. If the battery pack temperature approaches the lower or upper thresholds, the BMS should reduce the charge current to protect the cells.

When comparing available BMS devices on the market, one key parameter to consider is their compatibility with existing chargers. If a BMS supports a broad range of chargers out of the box and can be easily configured to support additional ones, this is a major advantage. It significantly reduces the development effort required to integrate the battery pack with the charging unit.

Current sensing devices

A BMS commonly relies on a device to sense the current flowing through the battery pack. This is used to monitor the safety of the pack (to detect charge or discharge currents that are too high), and also to calculate the SoC (State of Charge) of the pack. One way to estimate SoC is by monitoring the current flowing through the pack over time — this technique is called Coulomb counting.

One such device used for this purpose is a Hall Effect sensor. This sensor is placed around one of the battery pack wires and measures the magnetic field induced by the current flowing through the pack. One of its biggest advantages is the galvanic isolation between the sensor and the battery circuit. Another benefit is that the high current flowing through the battery does not pass through the sensor itself, which means it doesn’t heat up from the current. However, Hall Effect sensors with high current accuracy tend to be on the more expensive side. If extremely precise SoC measurement is not required, a less accurate Hall Effect sensor can still be a very attractive and cost-effective solution.

Hall effect sensor 
Source: https://www.accuenergy.com/products/hak-hall-effect-dc-current-sensors/

Another device used for current measurement is a shunt resistor. This resistor is placed on the negative side of the battery and creates a voltage drop. Knowing the resistance value and measuring the voltage drop across it allows you to calculate the current flowing through the battery pack. A shunt resistor can provide accurate current measurements, but its resistance value and physical size must be carefully chosen to ensure measurement accuracy and sufficient heat dissipation. Because the shunt is placed in series with the battery pack, all of the pack’s current flows through it, generating heat that must be managed appropriately.

Shunt resistor 
Source: https://nl.hkfc-industrial.com/product/shunt-resistors-sr/

Temperature management

When current flows through battery cells, they begin to heat up due to their internal resistance and electrochemical reactions. The higher the current, the more heat is generated. Battery cells have defined lower and upper operational temperature limits. Depending on the State of Charge (SoC) and the temperature, the maximum allowable charge or discharge current may vary. If a cell is operating near its temperature limits, the BMS should reduce the current to prevent cell damage.

To avoid reducing current capacity unnecessarily, the temperature of the cells must be managed carefully.

If the cells are near their lower temperature limit, a heating device can be used to raise their temperature. For example, a heating pad can be placed underneath the battery cells and controlled by the BMS. When the BMS detects low cell temperatures, it can activate the heating pad to bring the cells back to an optimal operational range.

During intensive use, heat buildup toward the upper operational limit must be avoided to preserve battery performance and lifespan. One way to improve thermal performance is through careful layout design of the battery pack. When all battery cells are placed tightly together, the pack forms a large thermal mass. Outer cells cool faster due to exposure to air, while inner cells retain heat longer.

Distributing the cells in such a way that each one has contact with free air or an effective cooling surface reduces this thermal imbalance. This ensures more uniform cooling and more stable performance under demanding conditions.

Conclusion

This high-level article covered only a few components of HV battery packs — such as cells, BMS, and temperature control. Equally critical components like contactors, fuses, and busbars also require careful consideration.

Designing a battery pack is only the beginning. Rigorous testing is required to uncover overlooked issues and ensure safe, long-term operation. For example: Can a BMS still balance cells effectively after significant cell aging? Does the system stay within safe thermal limits during prolonged use?

Testing validates design assumptions and builds trust in performance — making it as essential as the design itself.

Sources

Lukas Esprit

R&D engineer at Engibex