Pack Design - Battery Design

With the module design we look at Mechanical, Electrical, Thermal, Safety and Control. In pack design we repeat that approach.

Mechanical

The mechanical integration and support of all sub-systems and components within the pack enclosure need to be considered. Also, the mechanical integration of the battery pack into the larger system has to be considered along with the requirements that that brings to the design.

Pack Enclosure

Defines the battery pack envelope, protects it from dust/water and is a mechanical barrier protecting the wider system.

Electrical

At pack level the individual modules or cell strings are joined into a single electrical system, and every joint in that system is a place where resistance, heat and failure risk can be designed in or out.

Voltage Architecture – how strings are arranged in series and parallel to hit the target pack voltage and capacity
Busbars – material and cross-section selection to carry pack current at an acceptable temperature rise
Contactors and precharge – switching the pack on and off safely, including how the precharge circuit limits inrush current into downstream capacitance
Fuses – protecting cell, module and pack-level circuits so a fault clears at the lowest tier possible
HVIL – keeping the system aware of connector and cover integrity throughout the pack
Creepage and Clearance – maintaining safe electrical spacing for the pack’s operating voltage and expected contamination/altitude

These choices interact with the mechanical and thermal layers rather than being decided in isolation: a busbar sized purely for resistive loss, for example, changes the local heat load the cooling system then has to manage.

Thermal

Early on you need to consider:

cooling and heating capability of the system
ambient operating conditions
- air temperature
- solar load
use cycles
- establish peak and continuous current
thermal requirement of Battery Disconnect Unit

Safety

It is important in any pack design for any application to outline a safety concept.

OsecoElfab DualGard breather and vent disc

Rupture Disc Integration for Overpressure Mitigation

Pressure equalization, commonly referred to as “breathing,” aims to maintain pressure balance within a battery enclosure due to environmental or elevation changes. Think of a plastic gas can in the garage in the summer months vs winter (it can be expanded or contracted in shape). However, pressure equalization alone does not mitigate overpressure risks. Rupture discs provide active pressure relief by opening at predetermined thresholds, exhausting excess pressure, and preventing catastrophic explosions.

Pack-level safety design has to assume a fault will eventually occur somewhere in the system, so the focus shifts from preventing every possible fault to containing the consequences when one happens.

Thermal Runaway and Thermal Propagation – stopping a single cell failure from spreading to its neighbours, covered in more depth on the Propagation Mitigation page
Arcing and DC Arc Flash Hazard – managing the risk of a sustained high-voltage arc during a fault, covered on the Arc-Management Methods page
Isolation monitoring – continuously checking that the HV system remains electrically isolated from the vehicle or system chassis
Protection coordination – arranging fuses and contactors so a local fault clears at the lowest tier rather than taking out the whole pack
Compliance with the propagation, enclosure and abuse-testing standards that apply in the target market

Control

The battery management system is what turns the electrical and thermal hardware above into a system that can look after itself. At pack level its main jobs are:

Estimating State of Charge and State of Health from the cell voltage, current and temperature data gathered by the BMS sensors
Cell balancing across strings, so the weakest cell in the pack doesn’t set the usable capacity for all of them
Driving contactor and precharge sequencing, and opening the pack safely if a fault, HVIL break, or isolation-resistance drop is detected
Running the thermal control loop – deciding when to heat, cool, or do neither, based on cell temperature and the planned duty cycle
Communicating pack status and limits to the vehicle or system controller and to the charger

Because the BMS sits across every other sub-system covered above, its design is usually finalised last — it needs the mechanical, electrical and thermal architecture settled first so it knows what sensors it has access to and what actuators it is actually controlling.