Page 143 - Lindens Handbook of Batteries

5.20 PRINCIPLES OF OPERATION

provide a complete accounting of the battery’s usage over time. Data such as maximum tempera-
ture, time at temperature, time at voltage, and similar parameters can also be utilized to determine
battery aging. This data can be retained even if a battery failure occurs so that warranty returns
can be properly evaluated and analyzed.
• Customization: The continued advancement of electronics technology allows the monitoring
electronics to be custom tailored to the chemistry of the battery as well as the specific require-
ments of the cell manufacturer and host end-device. This improves the accuracy, safety, and
reliability of the battery pack as well as the performance of the device using the battery. “Smart”
battery electronics today range from simple protection circuits with limited communications to
sophisticated all-in-one circuits that provide protection, fuel-gauging, balancing, charge control,
history, and adaptive algorithms that compensate for the battery as it ages.
There are several main elements to consider in the design of batteries containing electronics.
These are:

1. Monitoring and measurement. There are multiple parameters that can be measured directly by
battery pack electronics to provide the basic information about the battery pack. These include
cell and pack voltage, pack current, cell and/or pack temperature, and time.
The accuracy of the measurements should fit the requirements of the chemistry and the intend-
ed application: Precise fuel-gauging and charge control often require accurate measurements while
protection from abusive conditions may require less stringent monitoring. For high-reliability gas-
gauging of remaining capacity and run time, it is important that these measurements be made as
accurately as possible to provide the best data for the control algorithms and predictive functions.
In some cases, measurements are not required, only simple monitoring is needed. Signals are
only generated when the monitored values (cell voltages, pack currents) exceed preset thresholds—
otherwise, the exact measured values are not required. Simple comparator-based monitoring is often
sufficient when protecting the battery from operation outside desired limits.
Combinations of certain measurements can provide additional information, such as a cell’s DC
resistance or impedance for example, when voltage and current measurements are synchronous.
Voltage measurements can be critical as the charge control and termination depend on the
battery voltage, which, for some chemistries, should be accurate to 50 mV or better at the cell
level. An inaccurate measurement could result in under- or overcharge, which could lead to short
service life or damage to the battery. In the case of the lithium rechargeable cells, overcharge
could be a safety hazard. Similarly, on discharge, terminating the discharge prematurely results
in a shortened service life, while overdischarge could result in damage to the cells.
Errors in the measurement of current affect not only the calculation of capacity and the state-of-
charge “gas gauge,” but influence the termination of charge and discharge, as the termination volt-
ages may vary depending on the current, particularly in high-current devices such as power tools.
Complicating this measurement is the fact that current is not a constant value during discharge;
there are often multiple device power modes and high current pulses as short as milliseconds.
Likewise, temperature is an important parameter, as the performance of batteries is highly
temperature dependent, and exposure to high temperatures can cause irreversible damage to the
cells. Temperature gradients across the battery pack that cause the cells to reach different tem-
peratures will create cell imbalance, which limits battery life.
The key points are to select monitoring electronics that match the application requirements. A
nickel battery pack for a power tool may only require pack-level temperature and voltage moni-
toring, while the same application using rechargeable lithium may require individual cell voltage
monitoring and limited temperature monitoring.
2. Calculation. Having calculation ability in the battery pack allows the system to adjust to the
environment or usage conditions and permits safer operation and better performance. The level of
calculation required can vary from a predefined logic state machine to a more versatile processor
that can factor more variables when making adjustments.
For example, consider the precharge function typically used with lithium rechargeable chem-
istries. When the cell voltage or temperature is below preset thresholds, a separate charge path