2.1 A general Battery Management System
The concept of the energy chain was explained in chapter 1. Essentially, the links in
this energy chain already reflect the basic parts of a BMS. In more general terms, the
charger can be called a Power Module (PM). This PM is capable of charging the
battery, but can also power the load directly. A general BMS consists of a PM, a
battery, a DC/DC converter and a load.
The intelligence in the BMS is included in monitor and control functions. As
described in chapter 1, the monitor functions involve the measurement of, for
example, battery voltage, charger status or load activity. The control functions act on
the charging and discharging of the battery on the basis of these measured variables.
Implementation of these monitor and control functions should ensure optimum use
of the battery and should prevent the risk of any damage being inflicted on the
battery.
The degree of sophistication of the BMS will depend on the functionality of the
monitor and control functions. In general, the higher this functionality, the better
care will be taken of the battery and the longer its life will be. The functionality
depends on several aspects:
• The cost of the portable product: In general, the additional cost of a BMS
should be kept low relative to the cost of the portable product. Hence, the
functionality of the monitor and control functions of a relatively cheap product
will generally be relatively low. As a consequence, the BMS will be relatively
simple. An example of the difference in BMS between a cheap and an
expensive shaver will be given in section 2.3.
• The features of the portable product: This is closely related to the product’s
cost. A high-end product will have more features than a low-end product. For
example, a high-end shaver with a ‘Minutes Left’ indication needs more BMS
intelligence than a low-end shaver with no signalling at all.
• The type of battery: Some types of batteries need more care than others. An
example of the influence on the complexity of the BMS when moving from one
battery technology to a combination of two battery technologies will be given in
section 2.3.
• The type of portable product: In some products, a battery will be charged and
discharged more often than in others. For example, a cellular phone might be
charged every day, whereas a shaver is charged only once every two or three weeks. The number of times a battery can be charged and discharged before it
wears out, together with the average time between subsequent charge cycles,
determines the lifetime of a battery in a device. So it is more important for this
number to be high in a cellular phone than in a shaver. This can be achieved by
making the BMS more intelligent. Therefore, a more sophisticated BMS is more
important in a cellular phone than in a shaver.