This ain't our first summer - Building reliable and safe battery packs

Batteries are a huge store of energy, and that comes with an inherent risk of a fire. It doesn’t appear as scary but, there’s no guarantee that all battery packs are safe to use.

While Internal Combustion Engines have evolved over decades, energy dense battery packs are still the new kid on the block. It’s up to the EV manufacturers to speed up to the same or better quality standards much quicker. Any adverse incident has the potential to impact the industry as a whole.

Choosing the Cell

Safety cannot be an afterthought, but has to start right from the choice of cells that go into the battery pack. Manufacturers have to find the right balance between performance, cost, reliability and the ease of procurement.

The choice of a cell depends on the nature of application. After the choice of the type of cell, the daunting task of choosing the right supplier and grade comes in. At Ather, we work with suppliers who have been manufacturing cells for over a decade, and have proven quality records. We choose only the highest grade of cells from their vast portfolios, ensuring that every cell used is the best you can get, even if it comes at an extra cost. We evaluate cells over long periods of time, and gather sufficient data about them before we select them.

Source: Battery University

It’s important that cells are binned according to their grade and are checked for impedance and voltage parameters. This ensures that cells with a large delta of voltage do not go into the same pack, failing which it increases the risk of overcharging the cells if all cells aren’t balanced.

Picking the perfect cells that can go into a battery pack look like this:

In an ideal scenario, every cell and part comes out perfect, but in manufacturing, there are always imperfections. What matters is how high the tolerance for quality is, and how stringent the checks are, to minimize any adverse consequences

Design, packaging and structural integrity

The next step in a safe battery pack design is the packaging and structural integrity. The spacing between cells and the choice of materials are important to ensure that:

  1. If there’s a defect in one cell, the enclosure has to prevent thermal runaway propagation to other cells in the battery pack.

  2. The structures to hold cells in place have to withstand vibration/shocks from daily usage, as well as some rare scenarios like drops or crashes.

  3. The pack has to withstand Indian roads and seasons for at least a decade of use and any additional Ingress Protection (IP) ratings.

The battery enclosure is an aluminium casing, which is robust for automotive use and is great for dissipating heat and keeping the battery pack cool.

Battery Management System (BMS)

With great energy comes great responsibility, and the BMS or Battery Management System shoulders that responsibility in a battery pack. A BMS helps:

  1. Provide battery safety and longevity by managing charging, discharging and monitoring various parameters of the battery pack
  2. Share data about the functioning of the cells in the battery pack to other systems of a vehicle, including temperature, humidity, cell imbalance and calibration.

The BMS needs to be highly accurate and durable, since the vehicle determines power, charging and safety cut offs based on the data sent from the BMS. Every BMS needs to go through calibration and testing to ensure that the data is within acceptable error margins.

Testing

When the battery pack is under development, testing happens in 4 phases - Proof of Concept, Final design, Tooled-up design and Pre production. Safety testing happens at each of these phases.

  • Thermal runaway, short circuit, BMS functionality and cell safety tests are done at the proof of concept stage.
  • The battery packs and the components within the pack are put through over 150 tests which include abuse tests such as vibration, shock, thermal cycling, flammability, water immersion, corrosion, drop, and strain relief tests
  • Some of these tests are repeated with pre-production parts. The Indian testing standards are pretty liberal, and our teams go beyond that to not only meet the standards, but exceed them.

Quality checks

Apart from the standard quality checks that are done on larger components like the enclosure and structural parts of the battery pack, the BMS and cells have to undergo more stringent quality checks. False or inaccurate readings from the BMS can put the pack at a safety risk. If the readings are inaccurate or the calibration is off, there’s a chance that the cells can operate beyond an acceptable limit, which in turn can lead to safety hazards.

Hence all electronics such as the BMS that go into battery packs must undergo a check to verify that the readings from the BMS - current, voltage, and temperature are within acceptable tight limits of accuracy.

And finally, redundancy, redundancy, redundancy.

Components that are central to a battery’s safety systems such as temperature sensors, fuses and vents should have redundancies built in. Algorithms can fail, some sensors can malfunction etc, and while such failures are rare, building in redundancy will ensure the probability of failure is as low as possible. While this adds cost, it makes batteries more ‘fail safe’, a philosophy that’ll help manufacturers ensure that battery packs are reliable on road, and rebuild trust with consumers on the safety of battery packs used in automobiles.

16 Likes

Well explained

1 Like