Power Electronics

Multi-Stage Charge Timer Enhances Li-Ion Battery Safety and Functionality, Complies with IEEE Std. 1725-2006

A multi-stage charge timer provides charge safety and battery fault detection benefits to Li-Ion charging.  It also provides application flexibility by terminating charging when the charger output current is shared with an operating load.  The multi-stage timer tops off the battery without resorting to perpetual float charging.  It provides fault detection and charging safety, and enhances battery capacity and service life.

Lithium Ion batteries are widely used in handheld devices, due to high volumetric and gravimetric energy density. But benefits come at the cost of charging complexity and risks compared with NiMH batteries. Integrated chargers with timers typically limit total charge time for safety. But standards organizations recommend separately timing each phase of the CC/CV charge process, which can permit different timer responses at each step of the process. Rather than a cost/safety tradeoff, an integrated multi-stage timer will enhance safety and add functional value to your product at little or no additional cost. Benefits include greater flexibility in charger/battery/load topology, more robust charge termination, and preservation of capacity as the battery ages.

Consider the system shown in Fig. 1. The system load (for example, a mobile phone) is represented as a resistor. For the case where the load current while charging is small compared to the maximum charge current (ILOAD << ICHARGE/10), a conventional Lithium-Ion (Li-Ion) battery charging algorithm progresses through four charging stages as shown in Fig. 2, with the stages indicated by symbols “(a)” through “(d)’.

Charge current is commonly specified relative to battery capacity, with the charging rate “C” defined as 1A for each 1Ahr of capacity.

When deeply discharged, the battery must be “precharged” at a low charge current IPREQ, typically specified to be 0.1×C to 0.2×C, until the battery voltage rises above its precharge voltage threshold, VTPREQ, typically 2.8V for a single cell Li-Ion battery. (If the battery voltage is already greater than VTPREQ when charging begins, this stage is skipped.) The precharge stage (denoted (a) in Fig. 2), when it occurs, can be brief -- a few seconds to perhaps a few minutes.

Fast Charging

When VBAT > VTPREQ, charge current may be increased to the battery’s specified fast-charge current, IFQ. A typical battery manufacturer-specified maximum charge current is 0.7×C to 1.0×C. The fast charging current stage (b) is also referred to as the Constant Current (CC) stage, because the charge current is constant, independent of battery voltage.

During CC charging, as the battery’s state of charge increases, the battery voltage rises until it reaches a specified charge voltage, VCV = 4.2V typically. At this point, charge control begins the Constant Voltage (CV) stage (c), in which the battery voltage is regulated to VCV. While CV charging, as the battery state of charge increases further, the battery will accept progressively less current. When the CV stage current into the battery decreases to its specified termination current threshold, ITERM, the battery is determined to be fully charged. Typically, ITERM = 0.1×C.

Following termination due to IBAT < ITERM, the charge controller may indicate end-of-charge to the user or system processor, and either turn off the charger or continue CV charging for a fixed time interval, then turn off the charger. This extended CV charging period is known as the Top-Off (TO) stage (d).

The Multistage Timer

Prolonged time spent in some charging stages may indicate a faulty or damaged battery. The IEEE Standard for Rechargeable Batteries for Cellular Telephones, IEEE Std. 1725-2006, a precursor to UL2575, recommends a multi-stage timer, in Section 7.3.5 entitled “Timer Fault.” This standard states, “If the host is in a charging state for a time period exceeding the system specification, the host shall stop charging. The system should support a maximum charging time-out function for each stage in the normal charge process, post-precharge.”

To time each stage individually, the timer must detect a well defined transition from one stage to the next. If each charging stage completes prior to its timeout, the timer advances to the next stage. But if the allowed time for any stage is exceeded, an event termed stage timeout, the timer must initiate stage-appropriate action.

Precharge Stage Timer

A damaged or defective battery may be unable to accept any charge at all. The precharge stage timer is a key means of detecting this fault. The precharge timer stage begins when power is applied to the charger or when the charger is first enabled. If the battery voltage does not exceed the precharge threshold, VTPREQ, prior to precharge timeout, a defective battery must be assumed, a precharge fault is indicated, and further charging should cease. Typically, a device’s user interface will then indicate that the battery must be replaced. A typical precharge timeout period can range from 15 minutes to one hour.

When the battery voltage exceeds the precharge threshold, the timer begins timing the Constant Current stage. The CC stage ends when the battery voltage has risen to VCV, and the regulation mode switches to Constant Voltage regulation. If the control mode does not switch to CV regulation within the CC stage timer period, a CC stage timeout occurs. This is considered a fault condition. Again, a defective battery must be assumed, a fault is indicated, and charging should stop.

The expected duration of the CC stage can vary greatly, depending on the battery’s initial state-of-charge and the fast charge current. If charging at a 1C rate, the CC stage may last one or two hours. But a lesser charge current may be available to the charger, requiring a lower IFQ. For example, a device may contain a large capacity 1.5Ahr battery, but may be required to charge its battery from a low-current source, such as a USB host in low-power mode at 100mA, which is a C/15 fast charge current. If charging a large battery with a very low current, CC stage can be as long as 10 to 15 hours, or more. This should not indicate a fault condition.

A good default value for CC stage timeout is three hours when charging most batteries at the specified maximum charge current. But a means of programming a longer CC stage timeout should be provided for low-current charging of large batteries to prevent a false CC-stage timer fault. For example, in the multi-stage timer of the Semtech SC824 single cell Li-Ion charger, the CC stage timeout defaults to three hours, but can be programmed from 2 hours to 15 hours or more with an optional external resistor.

CV Stage Timer

When the battery voltage has risen to VCV, the CC stage ends and the CV stage begins. Upon reaching CV stage it is assumed that the battery is not faulty. While in CV regulation (VBAT held at VCV), the battery will accept less current as its state-of-charge rises. When the current into the battery decreases to its specified termination current, typically 0.1C, the battery is determined by the charge controller to be fully charged. When this occurs, end-of-charge is indicated (by LED or status bit), and the charger is either turned off or advances to top-off charging.

But system operational conditions may prevent termination by current, even though the battery may be fully charged. Consider the case of Fig. 1 in which the load current is larger, possibly exceeding the programmed termination current, thus keeping the charger output current above ITERM up to and beyond CV-stage timeout. This does not mean that the battery charge current still exceeds ITERM, only that the sum of the charge current and load current exceeds ITERM. If the charger has been CV-regulating its output for a sufficient period, the battery can be judged fully charged regardless of the charger output current.

The battery state of charge immediately upon switching to CV charging depends on the CC stage charge current. A comparatively low CC stage current will raise the battery voltage to VCV at a later time, but at a higher state of charge than with a high CC stage current. Thus CC charging at the highest specified current will require the most time at constant voltage to completely charge the battery, somewhat variably but typically 1.5 to 2 hours after reaching VCV. This period is largely independent of battery capacity. Since the state of charge at the beginning of the CV stage cannot be reliably determined without knowing the fast charge current, battery capacity, battery chemistry, etc., the CV stage timeout period of a stand-alone general purpose charger can ensure complete charging of the battery only by assuming the highest permitted CC-stage current. Therefore, a fixed duration CV stage timeout of three hours is recommended.

The CV stage timer will terminate charging by current or by time, whichever occurs first. CV stage timeout is not a fault, but rather an alternative termination criterion. Termination by time is illustrated in Fig. 3.

Top-off Stage Timer

The end of the CV stage indicates that the battery has completed the specified charge procedure and is ready for use. Turning off the charger at this time may result in a small relaxation voltage drop, slowly over time, and also immediately due to ITERM×R voltage drop across the battery series resistance when the small ITERM charge current is removed. As the battery ages, repeated charge cycles gradually reduce capacity and increase series resistance. As the battery series resistance increases, the true state of charge at termination by current decreases because of the increasing ITERM×R voltage drop, exaggerating the inevitable loss of capacity.

A small recovery of capacity is obtained by applying VCV to the battery terminals a short time longer. Top-off charging holds the battery terminal voltage at VCV until top-off timeout, typically 30 minutes to one hour in duration, completing charging at a current usually lower than ITERM and thus reducing the I×R voltage drop across the battery series resistance. After top-off timeout, the battery should be monitored for discharge. For example, the Semtech SC824 top-off stage times-out in 45 minutes, at which time the charger is turned off and enters the monitor state. If the battery voltage drops below VReQ = VCV - 100mV, a recharge cycle is initiated. Because the battery’s charge state is already high, the SC824 will step through its normal stage completion events quickly without timeouts, finally executing the full 45 minute top-off charge, and again turning itself off.

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