Dousing the concerns on EV fires

UNDERSTANDING THE BATTERY PACK

At the centre of the whole issue is the battery pack itself, which when overheated is said to cause fire. So, let’s first understand the battery pack – A complex confluence of mechanical, electrical, electronics and chemical sub-system working together to provide an environment friendly ride. The enclosure of the battery pack is the mechanical body that provides the pack with protection against mechanical shocks. This is made of specialised plastics or certain types of metals like Aluminium.

The electrical and electronics includes the boards, connectors, sensors, fuses, wires and software to manage the electronics. However, the bulk of battery weight (and cost) is due to the chemical component of the battery – the cells. Based on the voltage and current requirement, they are arranged in required series-parallel combination. It is the cells that generate heat and are prone to explosion under extreme conditions.

CELLS – THE SEAT OF HEAT GENERATION

As is clear, the heat is generated by the “chemical sub-system” of the battery packs – the cells. During running conditions, the cells are discharging and, in the process, get heated up. As the battery packs used in the 2 wheelers are air cooled and fully enclosed, the rate of heat generation many a times is more than dissipation, resulting in heat build-up. This is very true in the Indian condition, where peak temperature in excess of 40 deg is the norm for at least two months in a year. A high ambient temperature, along with heat generated due to charging or discharging, may cross the safe temperature threshold provided by the cell manufacturer. Is cell discharge the only reason for battery to heat up? No. There are other reasons like internal short circuit at cell level and pack level, water or dust ingress and above all, failure of the battery management system to detect abnormal conditions and cut-off the pack from supplying power to the vehicle.

OPERATING THE BATTERY PACK SAFELY

We would not like to speculate what caused the recent EV fires. We would, however, like to mention few steps that can greatly reduce and even eliminate the chances of EV fire. Our recommendations below are not either-or type, in fact, the more the suggestions given below are implemented, the better it is. What we propose is a multi- layer fail safe approach:

A robust battery Management System (BMS) – “What you cannot measure, you cannot control”. This maxim accurately depicts the role of BMS in EV battery packs also. We can’t stress the importance of a good BMS enough. 

The BMS, measures, monitors and controls the battery performance. It has in-built programs for detecting various types of abnormalities, including over-temperature and it shuts down the pack. Some of the functionalities of BMS include:

• Cell balancing - . An unbalanced cell, will become weaker with every charge/discharge cycle and at one point, will fail and may also lead to unsafe battery pack failure.  A good BMS has built-in cell balancing and helps to increase the life of the battery pack.

• Temperature dependent charging / discharging. – This is an especially important role the BMS can play in the Indian conditions. If the ambient temperature is high, say 45 degree C, then it is important to throttle the discharge current or charge current so as to operate the battery within its thermally permissible range. Failure to do so will result in thermal runaway and subsequent battery fires.

• Pre-charge and pre-discharge paths:  Some vehicles may need higher surge / current handling esp, when taking extra load or going over a gradient like a fly-over etc.  The pre-discharge path, limits the current and makes sure that excess current is not and limit the life of the battery.  Without this, the cells would be subjected to high initial charge/discharge current, which would degrade the battery.

Number and location of temperature sensors -This plays a crucial role in detecting the thermal performance and potential fire scenarios. A bunch of sensors in various locations of cell, scientifically determined through thermal simulations is one right way to go about. In addition, having sensors on the electronic switch called MOSFETs is also required. Based on the safe operating area of the switch and cell pack, the cut-off threshold can be programmed. Additional sensors for built-in redundancy can also be used. 

Cell level fuse– Though bit expensive, it is strongly recommended to use a cell level fuse – known as the current interruption device (CID) that is the saviour of last resort when an internal short circuit happens at the cell level. This becomes important as the BMS can only cut-off the pack as a whole from the vehicle, but when the short circuit happens, the BMS cannot effectively protect the cell.

Fire resistant grade of plastic enclosure

From an enclosure perspective, there is need for FR plastics that can prevent the fire from spreading or will prevent the start of fire.  Flame retardant plastics (FR-ABS, FR-PP) plastics also can help.  Use of Aluminium enclosures also help as they don’t catch fire easily (high threshold temperature to catch fire).

Breather / Pressure relief valve

What determines whether thermal run-away will lead to fire or explosion? It’s the pressure built up in the battery pack on account of thermal run-away. A proven technology to prevent explosion is by mounting a pressure relief value (or breather valve) so that any dangerous pressure built up is released before becoming catastrophic. Matching the characteristics of the valve with worst case pressure build-up scenario is a pre-requisite for right choice of the valve.

Role of simulation in designing the battery pack

There are powerful simulation tools available for electrical, chemical and thermal simulation of battery packs. A simulation based design, is not only economical for company developing the battery pack, but also gives the flexibility of testing the limits of design and iterating accordingly. For example, it is much more prudent to check the extent of impact of cell imbalance on battery life using simulation than physically. Similarly vibration, thermal and mechanical shock scenarios and its impact on key components can be better studied using simulation.

Other measures

There are new advanced materials such as gap fillers, thermally conductive adhesives, thermal pads, thermal conductive and electrically insulating material and encapsulants that can be used based on the thermal requirement.

Coolant cooled battery packs also can help, esp, in large format batteries. These are available as a standard design which can be used an outer enclosure for the battery pack which can be connected to a pump for pumping in the coolant.

There are new electronic products like semiconductor switches and fast acting electronic fuses that can provide increased safety.

Then there are simple fire alarm that can be incorporated in the design of EV or battery pack.  If the battery pack reaches a certain abnormal temperature, one can raise an alarm so that the rider is notified of the unsafe condition.  It may not be prevent the vehicle or battery pack, but can ensure passenger safety.

To summarise, the cardinal principles of an effective, efficient and safe performance of the battery packs are:

Design – Robust design (validated using simulation tools) with a high margin of safety factoring Indian climatic conditions.

Quality – Selection of components adhering to (or exceeding) the required quality standards.

Testing–Thorough testing in-house and at NABL accredited labs, at a component, sub-system and system level.

 (The views expressed by the author are personal)