last several months have been tough for the EV industry, specifically the
2-wheeler segment with multiple instances of fire and even a case of explosion in
2 wheelers using Lithium ion (Li-ion) based battery packs.
reactions from media and “EV experts” have ranged from outright dismissal of
the potential of EVs for India’s de-carbonisation drive to blaming “sub-standard”
cells and materials. The objective of this article is to understand the issue
from first principles and discuss the ways in which this can be avoided through
design, quality and thorough testing so that the customer safety is given the
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.
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
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.
BATTERY PACK SAFELY
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
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
• 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
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
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
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
Role of simulation in designing the battery
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
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.
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.
are new electronic products like semiconductor switches and fast acting
electronic fuses that can provide increased safety.
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
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)
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