Lessons learned in the coordination of lithium-ion battery charging and control

It is possible to design lithium-ion (Li-ion) battery systems with built-in functions to operate safely and efficiently with standard telecom rectifiers, and indeed this is a mandatory requirement for any manufacturer wishing to sell Li-ion batteries for existing applications with legacy charging equipment. From a charging control standpoint the basic needs are a means for limiting recharge current and a means for avoiding damaging or unsafe high voltages. Furthermore, these functions must be built into the battery along with adequate controls to provide for fail-safe operation. Saft's experience has been that these charging needs can be met without problem in systems with standard telecom rectifier configurations. Inevitably, however, there are attempts to 'push the envelope,' taking Li-ion batteries designed for those standard systems and applying them with other charging sources and in other system configurations that do not conform to the norm. In some of these cases we have encountered incompatibilities and have had to work with the system integrator and/or adapt our control strategies accordingly. Examples include: systems with rectifiers in which software-controlled current limiting was implemented; ac power systems in which there was a high level of dc ripple; systems in which the charging source requires a minimum dc bus voltage in order to be energized; and deep-cycling systems requiring additional coordination of parallel string operation. This paper discusses some basic methods of Li-ion charging control and describes the lessons learned by Saft in nonstandard rectifier configurations. Typical communication, alarm management and control strategies are also discussed, including specific characteristics of Saft's latest Li-ion telecom products. Saft's position is that operators will not be properly served by these products unless communications and control are standardized in the industry and adopted by all rectifier and Liion battery manufacturers. Possible avenues for achieving this standardization are discussed.