Page 141 - Lindens Handbook of Batteries

5.18 PRINCIPLES OF OPERATION

Temperature Control. As with any battery system, high temperature will cause irreversible dam-
age. With lithium-ion and polymer cells, temperature can alter how the cells should be charged or
discharged. Guidelines from industry organizations limit charge currents when temperatures exceed
suggested thresholds, while cell manufacturers often have similar limits for discharge currents
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at low temperatures. Internal battery temperature, for most applications, should be kept below
75°C. Temperature cutoff, with a trip of 70°C and reset temperature in the range of 45 to 55°C, is
routinely used. Temperatures in excess of 100°C could result in permanent cell damage. For this,
permanent type fuses are used, typically set for 104°C with a tolerance of +/– 5°C. Temperature in
lithium rechargeable chemistries is more difficult to detect compared to nickel-based chemistries’,
which exhibit a more linear trend. Internal cell temperatures are difficult to detect, and once a large
rise occurs, it may be too late to take effective action. Thermal runaway can occur at temperatures
as low as 130°C.

Short-Circuit Protection. Normally, current limits are incorporated into the protection circuits
located in the battery pack or device. These circuits monitor the current in or out of the battery cells
via a very low value series sense resistance placed in series with the power path. These circuits must
be operating continuously and respond quickly to open a power MOSFET or similar device to inter-
rupt the current. Short-circuit protection on discharge as well as overcurrent protection on charge
(from a faulty charger) are often employed in lithium battery packs. As a backup, a PTC device
or fuse is placed in series with the battery pack. It is advisable to place the PTC between the pack
assembly and the output of the battery. By placing it at this point, the PTC will not interfere with
the operation of the upper or lower voltage detection of the electronic control circuit. However, for
some high-rate devices, such as power tools and electric vehicles, a PTC is not utilized since short
duration peak currents must be tolerated. In these devices, the electronic overcurrent monitors may
have multiple detection thresholds that are able to respond to not only the magnitude of the current,
but also the time duration of the current.

5.6 ELECTRONIC ENERGY MANAGEMENT AND
CONTROL SYSTEMS

In the mid-1990s, an important development in rechargeable battery technology was the introduction
of the use of electronic microprocessors to optimize the performance of the battery pack, control
charge and discharge, enhance safety, and provide the user with information on the condition of
the battery. This microprocessor function can be incorporated into the battery (the “smart battery”),
into the battery charger, or into the host battery-using equipment. Since their introduction, such
smart battery electronics have become more sophisticated, more integrated, more precise, and less
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costly.
Although generically referred to as a “smart battery,” there are varying degrees of smart or intel-
ligent battery systems. Specific examples detailed later include Smart Battery System (SBS SMBus)
products that conform to a set of specifications for interoperability. 11,12
But batteries of all sizes can incorporate electronic controls for enhanced performance, safety,
and reliability. Guidelines for the proper design and use of rechargeable battery packs have expanded
since some high-profile accidents in recent years. These guidelines suggest the use of electronics
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for maintaining the operation of the battery cells within safe limits for particular applications such
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as cellular phones and laptop computers.
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Electronics in a battery pack can range from basic protection functions that prevent or mitigate
abusive conditions to sophisticated measurement, calculation, and communication engines that pro-
vide protection, monitoring, and communications to a host or end-user device. Such equipment is
commonly found in battery packs using lithium rechargeable chemistries, such as notebook comput-
ers and smartphones, but it is also being utilized with other chemistries in handheld two-way radios,
power tools, and hybrid-electric vehicles.