Design News, May 2013

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Source: Electrochem Solutions Inc. Figure 3: A complete Li-ion battery pack consists of much more than cells; it has a circuit board, electrical and thermal insulation, contacts, and overall enclosure. The biggest challenge to upgrading from SLA to Li-ion is in the charging-cycle management and protocol. SLA batteries can be charged in one of several ways, all fairly simple and inexpensive to implement: by trickle charging at their self-discharge rate; by placing them across a constant-voltage DC source ("floating"); by using a constant current or voltage which tapers down as the cell voltage increases and the battery reaches full charge; or by using a constant voltage and then switching to a constant current. More advanced SLA chargers use a multiphase approach starting with bulk charging to a pre-set voltage, then switching to constant voltage, and finally going to a float phase. In contrast, there is only one generally accepted way to charge Li-ion batteries. First, a constant current is used, at a rate equal to or lower than the maximum charge-rate current specified for that battery type.When the maximum voltage is reached, the charging circuit switches to a constant-voltage mode, and stays in that mode until the charge-termination criteria established by the battery maker is met. In general, this is when the battery has reached maximum charging voltage and the declining value of the charge current falls below a specified value; typically, this is between 1/30th and 1/10th of the battery's maximum charge rate. While this charging regime may seem similar to the constant current/constant voltage once used for some SLA batteries, it is not similar enough to guarantee proper charge while also ensuring safety, capacity, and lifecycle performance targets will be met. In addition, Li-ion batteries need protection circuitry to stop charging if there is any overheating due to internal cell problems or excessive discharge rates. Upgrading: New Cells, Electronics, Partnership While standard, off-the-shelf Li-ion battery packs are common in high-volume consumer applications such as lapDesign News | MAY 2013 | w w w. d e s i g n n e w s . c o m –41– magenta cyan yellow black ES244625_DN1305_041.pgs 05.02.2013 07:03 UBM

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