Let’s discuss the two most important blocks in a portable or wearable BMS: the battery charger and the battery fuel gauge.
A battery charger must monitor the state of the Li-ion battery at all times during the charging process to keep it within the safe operating area.
As we mentioned earlier, Li-ion is very sensitive to excursions outside its safe operation area, and a battery charger must follow a precise charging sequence. A deeply discharged battery, for example, can’t accept the full charging current without overheating, so the charging sequence begins with a pre-conditioning phase that applies a preprogrammed reduced current for a specified time. When the battery voltage rises to a safe level, the battery charger rapidly increases the voltage to a preset value while monitoring the IC temperature to ensure that it remains below the operating limit. This is the thermal-regulation phase.
During the current-regulation or “fast-charge” phase, the battery is charged at a preset constant current that’s a function of the battery capacity. The battery voltage gradually rises until it reaches 4.1 V or 4.2 V; the precise value depends on the Li-ion electrochemistry being used.
In the next phase, the charger switches to the constant-voltage phase to prevent overcharging. With a constant voltage, the current gradually declines as the battery charge increases. When current reaches the termination value, it ends the charging. As we’ve seen, with a wearable battery, it’s important to precisely control the termination point to attain the maximum battery capacity.
Several TI battery chargers are suitable for portable and wearable use. For the smallest batteries – in hearing aids, for example – the bq2510x (Fig. 4) is a great choice. It’s a highly integrated family of single-cell Li-ion and Li-Pol chargers that features a compact chip-scale IC package size of 1.6 by 0.9 mm; the total solution is as small as 2.1 by 2.2 mm. The device has a termination current of less than 1 mA to maximize the available capacity of small batteries and an extremely low leakage current of 75 nA (max).
|Figure 4.||The bq25100 can deliver up to 250 mA with accuracies of 1% for charge voltage
and 10% for charge current. The optional RC circuit improves current stability
for currents below 50 mA.
Figure 5 shows the charging profile for the bq2510x. The charger is very flexible, allowing the designer to program the fast-charge current and precharge/termination current. It can accept power from either a USB connection (100-mA limit) or dc adapter. The bq2510x includes system-level protection features such as input undervoltage lockout (UVLO), input overvoltage protection (OVP), and thermal regulation; it also monitors the battery-pack thermistor temperature via the TS pin.
|Figure 5.||The bq2510x family of battery chargers follows a precise charging profile to
ensure safe charging and maximum battery life.
Applications with larger batteries require larger charging currents. In these cases, the bq24040 can supply a charge current from 10 mA to 1 A with a termination current of 6 mA. This device has a solution size of 2.5 by 3.5 mm, and a range of features that includes charge status indication, as well as programmable pre-charge and termination rates. It’s a popular linear charger for low-power applications.
The charging voltage of a Li-ion cell depends on its battery chemistry. A lithium nickel-manganese-cobalt (NMC) cathode, for example, has a 4.2-V charging voltage, so the available voltage options are of great interest to designers. The bq24040 family has 4.2 V and 4.35 V available; the bq2510x offers 4.06 V, 4.2 V, 4.3 V, and 4.35 V, depending on part number.
The battery fuel gauge is the other key component in a wearable BMS. This component predicts the battery capacity and other characteristics of a single Li-based rechargeable cell. Battery capacity is expressed as state of charge (SoC). It’s a measure of the available energy in the battery, from 0% (empty) to 100% (fully charged). The fuel gauge also provides a battery state-of-health (SoH) estimation. This parameter is an indication of the current condition of the battery and its remaining useful life.
Texas Instruments’ proprietary Impedance Track SoC algorithm is a key part of TI’s high-accuracy battery fuel gauges. The algorithm distinguishes between three battery states: charge, relaxation (no load), and discharge. During charge and discharge, it measures the current flow and recalculates the change in depth of discharge (DOD) each second. When it detects a relaxation state, the algorithm measures the open-circuit voltage (OCV), corrects for any residual current flow, and then updates the DOD baseline value by linear interpolation from a chemistry-dependent lookup table.
The algorithm gives high-accuracy SoC predictions for the lifetime of the battery across a wide variety of operating conditions.
The bq27421-G1 (Fig. 6), which includes Impedance Track technology, is TI’s lowest-power fuel gauge for single-cell Li-ion batteries. Housed in a 9-ball, 0.5-mm-pitch chip-scale package, its 1.62- by 1.58-mm size is ideal for wearable applications. The device measures the charging and discharging of the battery by measuring the voltage across an internal 7-mΩ sense resistor, a technique called coulomb counting.
|Figure 6.||The bq27421-G1 battery fuel gauge provides SoC and SoH indications
with minimal microcontroller involvement.
The bq27421-G1 uses an integrated temperature sensor to estimate battery-cell temperature. Alternatively, the system microcontroller can provide temperature data.
This device also has several low-power modes for improved power efficiency: initialization, normal mode, sleep, hibernate, and shutdown. In shutdown mode, the bq27421-G1 consumes only 600 nA, which is negligible compared to the Li-ion cell self-discharge rate.
The bq27421-G1 offers ease of configuration and flexibility of battery-pack selection. The system microcontroller can obtain cell DOD information via the industry-standard I2C serial bus.
With an ever-increasing list of features, severely limited space for the battery, and a continual demand for longer run-time, wearables present a difficult challenge to the power-system designer. Making the most of the available capacity requires a system-level approach to power efficiency coupled with individual components that tightly control battery operation while consuming minimal power.
The Texas Instruments portfolio of single-cell battery management products offers multiple options for designers who want to develop compact designs that maximize run-time in a wearable application.
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