Page 142 - Lindens Handbook of Batteries

BATTERY DESIGN 5.19

Some of the features and benefits of embedding advanced electronics into the battery pack, sys-
tem or charger include:
• Charge control: Battery electronics can monitor the battery during charge and assist in con-
trolling the charge rate and charge termination, such as time, Vmax, -deltaV, deltaTemp, and
deltaTemp/deltatime, to cut off the charge or to switch to a lower charge rate or another charge
method. Constant current to constant voltage charge can be controlled and options can be incor-
porated into the electronics for pulse charging, ‘‘reflex’’ charging (a brief periodic discharge pulse
during charge), or other appropriate control features, such as pre-charging cells at a lower charge
rate when at low capacity levels or low temperatures. Finally, charge protection from overcur-
rent and overvoltage conditions can also be included and tailored for the particular end products
requirements. Charge control can occur within the battery pack or the charger.
• Discharge control: Discharge control is also provided to regulate such items as discharge rate, end-
of-life cutoff voltage (to prevent overdischarge), cell equalization (balance) and assist with thermal
management. Individual cells, as well as the entire battery pack, can be monitored for voltage or
temperature during discharge and direct action taken to alter the discharge current or inform the host
device to terminate or slow the discharge. Pack current can also be monitored to detect overcurrent
and short-circuit conditions to prevent damaging the cells. Discharge control can be placed inside the
battery pack or the host end-user equipment using current control devices such as power MOSFET
switches.
• Cell balancing: Cell balancing can be used to improve the performance of multicell battery
packs by maintaining all the series cell elements in voltage balance with one another. Maintaining
cell balance therefore increases the usable capacity of the battery pack and improves cycle life.
Cell imbalance is often the cause of poor battery pack life in multicell series packs, typically
those greater than four series cells. Even if the individual cells are well matched when the battery
pack is assembled, cells will diverge with time due to slight differences in capacity, self-discharge
rate, etc. Temperature gradients across a large multicell battery pack will also alter the rate of
divergence such that cell balancing or equalization is often required in applications such as
electric and hybrid-electric vehicles. Some chemistries are easier to rebalance using overcharge
techniques, while most lithium rechargeable chemistries require alternate approaches such as
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bypass balancing or charge transfer balancing.
• Communications: Communicating the battery information to the end-user or host device can be
accomplished by both simple and sophisticated methods, depending on the requirements. Basic
measurement data, such as voltage, temperature, and current, can be relayed to the host device
for use in charge or discharge control or to calculate battery state-of-charge (SOC). If the battery
pack contains on-board calculation capability, then more information can be calculated locally
and communicated to the end-user or host device. Information such as battery SOC information
estimates the remaining battery capacity by factoring in such variables as the discharge rate and
time, temperature, self-discharge, cell impedance, past history, charge rate, and charge duration.
The SOC, remaining capacity, or run-time can be displayed locally on the battery pack by a
sequence of illuminated LEDs or a LCD display. Detailed data can also be directly communi-
cated to the host device via a standard communications link such as the Inter-Integrated Circuit
(I2C) bus or the derivative System Management Bus (SMBus). This communications option
provides significantly more information than can be displayed via a local method and is often
utilized by the end-device for use in a more detailed graphic form, such as on a laptop computer’s
main screen. Alternatively, single-wire data communications (DQ-bus, HDQ, or others) or simple
level-based analog threshold signals can also be used to communicate that the battery is operating
outside of normal limits and that external action should be taken by the charger or the host end-
use device.
• Historical information: Battery data is collected during the life of the battery and can be used to
make changes to the operating algorithm to maintain optimum performance. Other information
can also be collected beyond initial manufacturing information (date of manufacture, chemistry,
configuration), including detailed battery history, cycle count, and other such data, which can