Bạn đang xem bản rút gọn của tài liệu. Xem và tải ngay bản đầy đủ của tài liệu tại đây (19.64 MB, 415 trang )
Fig. 2.1 Set-up of a lithium-ion battery (shown is the discharging process)
These cell materials are used to produce cylindrical, prismatic and pouch cells,
the design of these cells is described in detail in Chapter 9.
Depending on the application, a single battery cell is used or several cells are
connected in series in a module. Also, a parallel connection is possible, dependent
on the required capacity. Several connected modules form a battery system for automotive applications (as an example, see Fig. 2.2).
For controlling purposes, automotive battery systems are equipped with a battery
management system. This system performs cell monitoring functions and uses
sensor technology to monitor cell voltages and temperatures. It also monitors the
current and enables the switching on and off of the battery system. The battery
management system is furthermore used to control the temperature management
(cooling or heating) of the battery system.
Fig. 2.2 Set-up of a battery system for automotive applications (left battery module; right battery
The advantages of lithium-ion batteries and the systems derived thereof are: high
specific energy, high specific power, high efficiency during charging and dischar
ging as well as low self-discharge rates.
The standard charging process for lithium-ion batteries is CC-CV (constant current/
constant voltage): First, the battery is charged to a certain maximum voltage with a constant current (CC). Then, it is charged with a constant voltage (CV) and a decreasing
current. The charging process ends after a predetermined time has elapsed or when a
certain current value has been reached. Depending on the materials used, lithium-ion batteries can be charged up to different determined maximum voltages, but not any further.
Overcharging the batteries causes deterioration reactions from a certain voltage
on. These deterioration reactions might differ in their intensity, depending on the
employed safety measures. The charging currents with which a battery can be maximally charged are also dependent on the design and the temperature.
2.5Definitions (capacity, electric energy, power,
Typical parameters for batteries are nominal capacity, electric energy and power. They
are used to characterize a battery cell or system and are therefore discussed here.
Capacity describes the amount of electric charge a power source can deliver under
specific discharge conditions. It depends on the discharging current, the cut-off voltage,
the temperature, and the type and amount of active materials. The unit is Ah.
The energy of a battery or a rechargeable battery is calculated as the product of
capacity and average discharge voltage. The unit is Wh. Specific energy refers to the
mass of the rechargeable battery and its unit is Wh/kg. Energy density refers to the
volume of the rechargeable battery and its unit is Wh/l.
Power is calculated as the product of current and voltage, for instance during
discharging. The unit is W.
The efficiency of lithium-ion batteries is very high, usually above 95 %. Efficiency
is the energy released during discharging divided by the energy stored during charging.
Safety of lithium-ion batteries
Fig. 2.3 shows, for an example of an automotive lithium-ion battery system, that
the chemical, electrical, mechanical, and functional safety characteristics play an
important role in product safety. The chemical safety is defined by the battery cell's
design, for instance by the choice of active materials and the set-up. The electrical
safety is achieved by the isolation of the battery system’s cables, housing, and subcomponents. The mechanical safety depends on the respective design, for example
the use of a special crash box. The functional safety is guaranteed by monitoring
2 Lithium-ion battery overview17