Design and Development of Battery Monitor for e-bikes

Design and development of battery monitor
for
e-bikes
Executive Summary:
In electric vehicles power battery management system is the most important task,
which is necessarily carried out to prevent the battery over-charging and over-
discharging. The performances of the batteries are mainly decreased due to the
aging, elevated temperature and cycling process due to the exchange of ions.
Lithium ion batteries is the most used battery component and the battery that
promotes high capacity, safety and long life is the lithium iron phosphate (LiFePO4)
and the charging method mostly adapted is the current charge- constant voltage
method (CC-CV). Therefore in the monitoring process, temperature, total current
and voltage are regularly monitored. The cycle count of the batteries is determined
by the depth of discharge (DoD) and longer the DoD minimizes the lasting period of
the batteries.
The battery monitoring system is estimated to reach US$8,053.489 million by the
year 2024 with the peak of 27.20% when compared to the year 2018, which was
around US$1,901.168 million. The entire process undertaken by the battery
monitoring system involves with estimating the state and thereby adjusting the
environment. One of the major drivers involves the rising focus on e-vehicles with
the BMS market. The government takes initiative in developing the e-vehicles in
order to control the air pollution, thereby expanding the growth of BMS.
BMS supplier market has been sectioned based on the battery type, components,
geography and end users. Moreover, hardware segments in BMS market holds with
the fast sampling process, high-resolution recording capabilities and effortless
installation. The components also includes with the sensors, data recorders and
controllers. Among the other countries, North America holds the global monitoring
market with the progressive technologies combined with the data centers
expanding investments. This work deals with the study of Battery Monitoring
System (BMS) economics and supplier in North America along with the
enhancement in the quality. This work deals with the enhancement made with the
BMS along the techniques adapted to improve the technology. Detailed analysis had
also been carried out with the various temperature sensors implemented for the
system.

1. Introduction:
Electric bikes are usually composed of lithium ion batteries with the nominal voltage
of 3.7 V and a current of about 2000 mAh to 3000 mAh. Li-ion batteries are mostly
preferred when compared to the lead-acid batteries due to their high energy
density. The batteries in these applications are connected either in series or parallel
manner so that they have to tendency to promote high voltage. But, in order to
maintain its sustainability with the long life-time condition, each cell in the pack
should be in the same energy condition. Either the charging or discharging process
should be stopped completely when the threshold of the energy conditions are
reached (overheat, overvoltage or overcurrent). Additionally, a balance should be
maintained among the battery cells to keep up with the same voltage level. The
battery performance will also be reduced if any one cell within the battery drops out
of voltage leaving the remaining other cell under a safe situation. This problem
could be avoided by implementing a balancing circuit, which is of two types: active
method and passive method. The passive method is extremely cheap that releases
its energy from the cell to the resistors until the voltage drops the minimum level
(smallest voltage of the cell). However, this method is not highly efficient. On the
other hand, the active method does the same process, thereby equalizing the
voltage for all the cells. This method is very efficient but it is very expensive as the
components involved in this method are highly complex.
Enhancement made among the battery system makes the people to adapt the e-
vehicles that are more closely adapted to the traditional vehicles and also have
several impacts and advantages. Moreover, the emission of CO2 is highly reduced
thereby protecting our environment and surroundings. In most of the countries,
lead-acid batteries are highly adapted in the E-bikes. This should be replaced by
Lithium ion batteries, which can increase the vehicle performance. On the other
hand, contamination could also be avoided at a high rate.
The outline of the section is as follows: Section 2 deals with the background of BMS
system, section 3 deals with the economics of BMS system, Section 4 deals with the
study to advance the performance of battery life followed by section 5 deals with
the BMS supplier in North America along with the temperature sensor to be
implemented in the BMS system and at last, section 6 deals with the conclusion.
2. Battery monitoring system:
The most widely used batteries among the e-vehicles includes the Lithium ion
batteries due to their exclusive power rating, energy density and

charging/discharging coefficient in the pulsed energy flow systems. The problem
among the Li batteries is their ability to overcharge that could result in short span
and also at the worst cases it could also damages the functioning of the battery
systems. Therefore, it is required to have a proper Battery Management system
(BMS) thus maintaining a reliable and safe operation for the battery systems [11].
The primary functionality of the BMS system is to provide with a amount of energy
that has been utilized by the load, in order to know the regular status of the system.
Since the internal resistance and the capacitance of the battery changes with the
time, this is determined to be a difficult task. The series-connected battery cells
could cause certain unbalancing issues that could be a problem in extending the
lifetime of the system, which should also be considered in the BMS system. This
could also minimize the battery usable capacity as the low charged cell obtains the
end of the discharge, though certain amount of energy are stored in the other
battery cells. Lithium ion batteries have a strict voltage limits that still limits the
self-recovery of the charge imbalance and it also declines with time. If a single cell
from the battery stack has reached the upper limit then it stops the charging
process thus making other cells of the battery uncharged. Assuming the cells to
have the same capacity (only a few percent of mismatch could be limited), cells
that is of varying discharge rate could cause charge imbalance [12]. This could be
dealt and obtained by the temperature gradient implemented along with the
battery stacks. Therefore, charge equalization technique should be adapted by the
BMS system to manage and restore the balanced state. Based on the number of cell
arranged in the stack, the voltage could be determined. Number of cells in a stack
could be obtained by [13]:
Number of cells= Runtime ( Hr) × Ibattery ( Amps)
battery capacity
Where, I battery is the average current of a battery.
1.1 SOC and SOH Estimation:
Li-ion batteries had been widely adapted in hybrid electric cars, scooter and electric
bikes applications. As the advancement in the technologies arises, batteries also
requires in communicating among the other components to enlarge their working
range and also its speed. One of the metric used to denote the energy within the
battery is the state of charge (SOC) as it is used to estimate the amount of energy
utilized from the initial state and it also indicates the time the battery will be lost.
On the other hand, state of health (SOH) of a battery determines the degradation
level of the battery due to its aging factor. SOC could be obtained by estimating the
ratio among the battery capacity and the amount of energy stored in a battery, but
this condition could not be known accurately due their sensitivity and other noise
impact factors. In [1] [2], indirect methods of determining the battery SOC had been
developed. For example, the open circuit voltage had been determined in the SOC
computation since the open circuit voltage as well as the SOC has a linear

relationship for both the Li-ion and lead acid batteries. But at certain situation, it
becomes harder to obtain the open circuit voltage measurement as it shows a flat
region when the SOC values are larger.
SOC could be obtained by the formula,
SOC ( t )=SOC ( t0 )−∫ I ( t ) dt
CM
Cm=(charge passed )
( SOC2−SOC1)
SOH= CM
CN
× 100 %
Where, SOC(t0) is the charge or the initial discharge state
I(t)- working current
CM- current battery capacity
CN- new capacity brand of the battery
SOC1 and SOC2- State of charge before and after the battery discharge obtained by
the OCV (open circuit voltage) curve
Battery power is the most required factor in tracking the moving objects especially
in vehicles with motors such as car, train, lorries and boats [3]. In case of electric
bikes, they have an on-board battery, which could be connected to the monitoring
system. This way could be adapted in tracking a vehicle without compromising the
quality of the data required for the battery life [4]. As mentioned earlier, LiFePo4
batteries have several advantages when compared to the lead-acid, nickel metal
hydride and lithium ion batteries [5] as they enhance the safety and also promotes
the environment protection. This power battery management system along with the
extended life cycle of the battery pack is detailed in [6]. The architecture for the
management system had been proposed in [7] that consist of single-cell detection
equalizer with n-blocks and a touch screen controller module. The transmission of
data is carried out through the CAN network among the touch screen as well as the
single-cell detection equalizer modules. The BMS system is carried out in four main
sectors such as thermal management module, charging control module, single-cell
detection equalization charging system and finally the touch screen controller. The
Schematic diagram of a typical BMS system is shown in Figure 1.