Charging capacity will decrease as it reaches its

Charging
procedure for lithium battery have three stages. The first phase is a
pre-conditioning stage, followed by constant-current stage and then
constant-voltage stage. When battery is deeply discharge, pre-conditioning
stage let battery to be charged with 0.1C until it is ready to receive full
current. This is to prevent the battery from overheat (1).
Most of the modern battery is equipped with switch which disallowed battery to
be discharged below its cut-off discharged voltage. Once the voltage of the battery
rises to a safe level, constant-current charging start where usually battery
will be charge with a constant current of 0.5C to 1C until it reaches its
cut-off charging voltage. Most lithium-ion have a 4.2V charging cut-off
voltage. However, during constant-current stage, battery will only be charged
up to 40%-70% of its maximum capacity. Using higher current for charging cause
voltage of the battery to increase faster. However, its capacity will decrease
as it reaches its cut-off charging voltage. Constant-voltage stage help battery
to be charged to its maximum capacity without overcharging the battery which
may damage the battery and cause explosion. In this stage, charging of the
battery continue until charging current slowly reduce to cut-off charging
current which is usually 50mA.

Charging
temperature for lithium battery should be in between 0°C and 45°C 1. Lithium battery perform better at elevated
temperature but at the expense of reducing life cycle. However, higher
temperature increases the possibility of cell’s cylinder to vent due to gas generation
within the battery, leading to thermal runaway. Some lithium batteries are specially
designs to have the capability to operate at extreme temperature. Lithium
battery used in oil drilling and high temperature environment can operate at maximum
temperature of 120C.  Battery subjected
to high temperature environment may loss its capacity. Another precaution that should
be made is lithium battery should not be charge below freezing point. Although
charging lithium battery below freezing point appear normal, plating will occur
at the anode. This effect is not reversible through cycling, making battery
more vulnerable towards stressful condition.

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Last
year project implement constant voltage charging method where current above
50mA are supplied to the battery while its voltage maintained at 4.2V. With
charging current of 100mA, this method required 56 hours to fully charge a
completely discharge battery. To minimized time required for battery to be
fully charge, combination of pre-conditioning charging, constant-current charging
and constant-voltage charging will be test in this design.  To
facilitate change of phases during charging process, PMIC is used. PMIC can
detect battery voltage and change the charging stages accordingly. Linear
Technology LT3652 is chosen as this IC is a step-down battery charger which
capable to receive input from 4.95V up to 32V and having an output up to 2A.
Therefore, plane power supply can be connected directly to this IC, eliminating
the need of buck converter module in PCB. Float voltage and charging can be
program externally by using resistor. Thermistor can be connected to NTC pin to
monitor battery temperature during charging. When LT3652 detect battery voltage
to be very low, it will automatically execute pre-conditioning charging where
charging current will be reduced to 15% of the charging current programmed.
This chip also equipped with auto-restart function where new cycle of charging
will be initiate when its detect battery voltage at feedback pin fall 2.5%  below float voltageDesired
operation requirement can be achieved by connecting batteries in series or
parallel connection. Connecting batteries in series increase its total terminal
voltage with the same capacity as a single battery while connecting batteries
in parallel give same terminal voltage as a single battery but with a higher
capacity. For some operation, batteries pack can be connected by combining both
series and parallel connection of the battery to have a higher nominal voltage
and boost in a capacity. Batteries with same parameter need to be used to
prevent imbalance in the pack. This imbalance will lead to weakest battery to
exhaust quicker compare to other batteries when connected to a load. This
particularly important when connecting batteries in series since its will
affect total terminal voltage of the pack. Besides than that, failure of a
single battery will cause total failure of the packs. During charge, weakest
battery will fill up and reach maximum voltage quicker compare to other
batteries. Prolong overcharge will lead to battery damage and increase the risk
of an explosion. In parallel connection, effect of imbalance is less severe.
Failure of a single battery will only cause reduce in total capacity of the
pack. However, parallel connected batteries are exposed to electrical shot due
to reverse polarization of the battery where fail battery will consume energy
from the remaining battery in parallel. Prolong exposure can lead to fire
hazard.Using
batteries pack require multiple safety precaution to reduce hazard cause by the
battery. Usually batteries pack are equipped with charge interrupt device (CID)
and positive temperature coefficient switches (PTC) 3
to govern current flow into the battery and pressure build up within the
battery. If overcurrent detected or excessive pressure are build up in the
battery, CID and PTC will interrupt charging process, preventing the battery
from damage. Packs equipped with serial connection may have a solid-state
switch equipped to bypasses damage cell to ensure continuity in current flow
despite at a lower total voltage while packs equipped with parallel connection
may equipped with a fuse at every single battery to disconnect damage cell when
short occurs.

For
Li-ion battery, its voltage should not exceed 4.2V during charge. This can be
governed by using PMIC which will be explained further in SECTION.  Two
Li-ion batteries connected in series will have a cut-off charging voltage of
8.4V. To prevent battery from overcharged due to imbalance in series
connection, we decide to stop the charging when the batteries voltage reaches
8V. Batteries
have been widely used in aviation industry since the beginning of plane
history. During earlier age of aviation, electricity was mostly used to ignite
the airplanes engine. As technology progress, more advanced airplanes and jet are
developed and equipped with a sophisticated electrical system. While engines
able to serve as generator and provide electricity, batteries are required to
provide electricity before the engine started and serve as a part of ignition system
in the engine. This battery also serves as backup power in the event of engine
failure. Battery used for these purposes are called main battery. In most of
the advanced airplanes, another battery usually installed to serve as auxiliary
power unit (APU). The fundamental
in batteries operation is converting chemical energy to the electrical energy.
This process occurs through electrochemical discharge of the reactant. However,
they are limited time in converting this energy since reactant can be depleted
and unable to supply electron.  Batteries
that unable to restore their reactant are called primary batteries. Some of the
batteries have the capability to restore their reactant to its initial state by
charging it through direct current. These batteries are called secondary batteries.
In airplanes, secondary batteries are widely used as it able to be used
multiple time through a cycle process of charge and discharge.The
chemistry of the battery used in airplanes are usually lead acid, nickel cadmium
or lithium. The battery used are big in nature since its consisted of multiple
cells connected in a series-parallel connection to give desired operation
requirement. Batteries are rated based on their nominal voltage and capacity.
Battery with 1000mAh capacity will have a runtime of 1 hour before completely
discharge when discharge with 1A. This discharge rate is equivalent to 1C. Subjecting
battery to a discharge rate of 2C means battery are discharge with 2A and will
have a runtime of 30 minutes before it is completely discharge. High rate cell can
be achieved by minimizing the internal resistance of the cell which highly
dependent on the internal construction of the cell itself.

The
use of lithium ion battery in aviation is quite popular due to energy density
that it can offer. Higher energy density means, more power can be supplied with
a smaller battery. Space required for battery installation is small as some of
the battery size can be similar to the size of car battery. Total weight of the
aircraft can be reduced leading to reduce in airplanes fuel consumption. Lithium
ion battery also have excellent life cycle, faster recharge time and require minimal
maintenance compared to nickel-cadmium battery. Boeing, an airplanes manufacturing
company have been knowingly used lithium ion battery in their airplanes, B787.
However, due to accident of thermal runaway of the B787 battery in 2013, concern
regarding the safety of using lithium ion battery in aviation arises, making
Airbus to drop the use of lithium ion in their A350 fleet. Immature technology
of lithium ion lead to further development in minimizing the risk of thermal
runaway in battery as year progress. Saft, a battery-maker company has succeeded
in developing more stable lithium-ion battery, making Airbus to deliver their first
A350 equipped with lithium ion battery in 2016.