Electrochemical accumulator batteries can be used in electrical and hybrid transport systems or embedded systems, for example.
An electrochemical accumulator usually has a nominal voltage of the following orders of magnitude: 1.2 V for NiMH type batteries, 3.3 V for iron-phosphate-based lithium-ion, LiFePO4, technology, and 3.7 V for cobalt-oxide-based lithium-ion type technology.
For most systems to be powered, these nominal voltages are too low. To obtain the proper voltage level, power, and capacity, it is known to arrange several accumulators in stages. The number of stages and the arrangement of accumulators within a stage varies vary according to the desired voltage, current, and capacity for the battery. The association of several accumulators is called an accumulator battery.
The charging of an accumulator results in an increase in the voltage across its terminals. Each accumulator technology has its own distinct charge profile, defined for example by changes in the voltage of an accumulator over time for a given charge current.
An accumulator is charged when, under a given current, it has reached a nominal voltage level defined by its electrochemical process. If charging is interrupted before this voltage is reached, the accumulator is not fully charged. The accumulator is also charged when charging has lasted a predetermined duration, or when the charging current, while the accumulator is maintained at constant voltage, has reached a minimum threshold value.
Because of manufacturing variations, accumulators have different characteristics in practice. Because of heterogeneous wear of the battery's accumulators, these differences, which are relatively small when the battery is new, grow with time. Variations still remain even when accumulators from the same manufacturing batch are associated in a battery.
The operating range of a cobalt-oxide-based Li-ion type accumulator is typically between 2.7 V and 4.2 V. Use outside this range can irreversibly deteriorate the battery's accumulators. A voltage below the operating range deteriorates the cell. Overcharging can lead to the destruction of an accumulator or accelerated wear. In some cases, thermal runaway phenomena can cause the accumulator to explode.
It is therefore useful to have control devices to monitor the voltage level of each stage. Each control device communicates with a central unit to supply it with the measured voltage level for its stage. The central unit generates charge or discharge interruption commands for each stage according to the voltage levels that it receives.
The charging of all the stages is interrupted, for example, when the most charged stage reaches an upper limit of the operating range. The voltage of the least charged stage is thus equal to a voltage below the upper limit.
The central unit also controls the discharge interruption of the battery when the least charged stage reaches a lower limit of the operating range.
As the measurements and the interruption decisions are carried out remotely, communication between the control devices and the central unit is essential to ensure the integrity of the battery.