Charge monitoring for electric vehicle applications requires a different skill set than charge control for a cellular handset. Precision is the key to measuring voltages, currents and temperatures safely.
High voltage battery charging and management solutions focus on different applications, battery chemistries and usage profiles. Each battery type will have its own characteristic charging and discharge characteristics ― reflecting a variety of variables including voltage, current, temperature and chemical responses to battery depletion. Though they are both dependent on lithium, multi-cell batteries for electric cars will reflect entirely different charging characteristics than the single-cell batteries used for mobile handsets. It is important to recognize important characteristics to maintain battery performance and safety. This article examines some of the charging envelopes for different battery types, and identifies some of the integrated circuits which can be harnessed to maintain particular charging profiles.
The dominant rechargeable battery chemistries include lithium-ion (or lithium-polymers), lithium-iron phosphate (LiFePO4), Nickle-Metal-Hydride (and Nickle-Cadmium), and Lead Acid. Additional formulations requiring specialized charging profiles include supercapacitors and multi-chemistries. Among rechargeable batteries, lithium compounds have the highest energy density. They are now widely used in consumer products like mobile phones and laptop computers. (Increasingly, power tools, traditionally users of NiMH batteries, are using lithium.) Lead-Acid has typically supported 12-volt car batteries, though rechargeable batteries (Li-Ion and NiMH) have grown their footprint in cars and electric vehicles.
In high-voltage applications, like electric cars, the voltage monitoring and charging control functions of a battery management IC are typically separated. The monitoring functions are embedded in the battery, while the charging control is external. In applications requiring lower voltages, the charging control and monitoring functions are combined in a smart battery pack. While the automotive and many industrial battery monitoring circuits will take a hands-off approach to charging, the lower-voltage battery monitors will facilitate charging (in conjunction with an outboard AC/DC converter or charging cradle). The battery chargers contain their own DC voltage regulators, and power management IC suppliers will specify the regulator type supplying charging currents and voltages— linear or switch-mode — on their data sheets and application drawings.
Whether they simply monitor parameters or exert some sort of control over the charging function, the battery management ICs must be extremely accurate in their ability to assess the condition of the battery — its voltage, current draw and temperature. Engineers must be alert to what the monitoring circuits report. Consider Texas Instruments bq24130, a charging controller with 600-kHz synchronous switch-mode voltage regulator inside. With a 4-A output (and built in 20 A MOSFETs), it will charge a fairly large battery (or supercapacitor). The accuracy of charge voltage regulation is better than ±0.5% and charge current regulation is to within ±4%. Battery drain current is less than 15 µA (with its external adapter removed. Applications for this one include Tablet PC, Netbook and Ultra-Mobile Computers, Portable Data Capture Terminals and Medical Diagnostics Equipment
Electric vehicles (EVs) and hybrid electric vehicles (HEVs) are using lithium batteries consisting of multiple (literally hundreds) of cells. The battery cells are placed in series to create high voltage capability; in parallel to enable higher currents — creating an elaborate matrix of cells. (An electric vehicle like the Tesla Roadster will absorb more than 900 amperes on acceleration. What Tesla promotes as its “secret sauce” is its battery technology — 300 miles on a single recharge).
On a multi-cell battery, the variables like voltages, currents, temperatures, and the number of charging cycles (aging factors) are carefully monitored when the battery is charged, and discharged in use. If one cell draws more current on charging than its neighbors, you have the possibility of a hot spot in the serial or parallel cell string. If it draws significantly less current than its neighbors, you have the possibility of developing a dead cell ― and that threatens the life of the entire string. For these applications, the durability of the battery is assessed by monitoring each cell. A multi-cell monitoring IC lie Linear Technology Corp.’s LTC6811 measures separate cell voltages up to 5V, in a multiplexed sequence, with a total measurement error of less than 1.2mV. All 12 cells can be measured in 290µs (though lower data acquisition rates can be selected for system-wide noise reduction). (See Figure 1).
Figure 1: Electric vehicles (EVs) and hybrid electric vehicles (HEVs) are using lithium batteries consisting of multiple (literally hundreds) of cells. The LTC 6811 battery management chip measures cell voltages up to 5 V, in a multiplexed sequence, with a total measurement error of less than 1.2 mV. All cells can be measured in 290 μs. (Source: Linear Technology Corp.)
It’s a precision monitor, Linear Technology insists, not a charging controller — though there are passive circuit elements (like a switched capacitor) on chip that will perform small adjustments in the capacity of each cell. While the 6811’s talk to a variety of battery chemistries, the charging profile (like Tesla’s “secret sauce”) resides with the battery or automotive supplier.
Multiple LTC6811 devices can be connected in series, permitting simultaneous cell monitoring of long, high voltage battery strings. Each cell in fact is individually addressable by a serial controller. Other features include an onboard 5-V regulator, five general purpose I/O lines and a sleep mode, in which current consumption is only 6 µA. Applications, apart from EVs, include backup battery systems, grid energy storage, high power portable equipment
Other electric car battery monitoring ICs include a device under development at NXP. The MC33771 is a Li-Ion battery cell controller IC. While it is designed for automotive applications such as HEVs, EVs, the MC33771 will also support high voltage industrial applications (like uninterruptable power supplies). The batteries that back up UPS systems are typically lead-acid rather than lithium, but precision power monitoring is an increasingly important requirement for data centers and compute servers.
On chip ADC converters enable precision measurements on the differential cell voltages and currents as well as enabling coulomb counting (fuel gaging) and temperature measurements. The MC33771 can attach to the company’s MPC5xxx 32-bit MCU families.
Lithium ion battery charging
The battery monitors and controllers are generally combined in space constrained applications like mobile handsets, PDAs, audio, and GPS navigation devices. The applications also include portable medical devices, digital still cameras and digital video cameras. A majority of this equipment uses single-cell Li-Ion batteries. The use of these rechargeable batteries has changed over the years: Increasingly, they are charged within the devices they power (rather than in a separate battery cradle/charger). An AC-to-DC is attached to the phone via a micro-USB connector (or a Lightning Connector for Apple products). The adapters have been standardized among cellphone suppliers, taking an 110-Vac input, outputting 5 V and up to 2 A current (10 W) for charging a single cell battery to it 4.2-V capacity.
Caution must be observed with lithium-based batteries. The lithium charge controllers MUST monitor voltage, current and temperature — as well as retain some awareness of “history,” how many times the battery has been charged and discharged, and whether any shifts in current, voltage or temperature occurred along the way. In much simplified terms, the charging profile for a lithium-ion battery uses a period of constant voltage and another period of constant current. (Lithium iron phosphate batteries differ from lithium-ion. They are charged the same way, though the constant voltage phase is limited to 3.65 V rather than 4.2.)
The basic Li-Ion algorithm requires you to charge at constant current at a low temperature until the battery voltage reaches 4.2, and hold the voltage at 4.2 volts until the charge current drops off. The capacity of the battery cell 4.2 V is actually about one-half of its full capacity. The charging needs to continue until the current drops. Protection circuitry between the battery and the charger can be programmed to stop the charge if the voltage gets too high (or if the battery gets too hot). One advantage to charging at lower voltages is that this will help preserve the cell’s cycle life.
As discussed with automotive applications, multi-cell lithium ion battery packs should have a method of keeping the cell balanced and preventing them from being discharged too steeply. This can accomplished with a safety circuit which monitors the charge and discharge of the pack ― and stays alert to overcharging. Safety circuits generally guard against charging under adverse temperature conditions, and unsupportable undercharging and overcharging conditions. Overcharge protection stops the charging process where cell voltage per cell rises above 4.30 V. Excessive discharge protection will stop battery drain (disconnect it from the circuit) when battery cell voltage falls below 2.3 V.
Japan Electronics and Information Technology Industries Association (JEITA, a voluntary group) was formed in reaction to the battery fires that effected computer batteries in the mid-to-late 1990s.The JEITA standard was intended to improve safety by defining the charging envelope for lithium-based batteries (see Figures 2a, 2b,2c).
The JEITA standard was intended to improve safety by defining the charging envelope for lithium-based batteries
Older batteries were charged at constant voltage and constant current, which provoked a temperature rise from 0 to 45°C. To improve the safety of charging Li-ion batteries, JEITA and the Battery Association of Japan released safety guidelines on April 20, 2007. Their guidelines stressed the need to avoid high charge currents and high charge voltages outside of a particular temperature range. Temperature monitoring is critical. If, for example, the cell temperature reaches 175°C with a cell voltage of 4.3 V, thermal runaway (battery fires) could occur. The recommendations said that lithium battery cells could be charged at temperatures up to 60°C with a reduced charging voltage. https://www.ti.com/lit/an/slyt365/slyt365.pdf
Though the specification was released over 10 years ago, manufactures claiming JEITA compliance include Texas Instruments (with its bq2405x single-cell controllers), and Maxim Integrated (with its MAX77301). The MAX77301 has what Maxim calls a Smart Power Selector, which chooses between the internal battery and an external USB port adaptor for any application it runs. Adapter power not used by the system is channeled toward charging the battery. Geared toward Bluetooth headsets, PDAs, and MP3 players, the charging controller automatically detects adapter types. It accepts Input currents up to 1,500 mA and outputs charging currents up to 900 mA. Shutdown current (preserving battery life) is only 6 µA.
Texas Instruments’ similar bq2405x family comes in a variety of flavors. One family member is a 1-A, single-cell Li-Ion and Li-Pol battery charger. It too, includes automatic adapter and USB detection. It boasts high charge voltage accuracy (1%) and Charge Current Accuracy (10%) operating from either a USB port or AC adapter. The USB input current is selectable (from 100 mA or 500 mA) high-input voltage range with input overvoltage
TI’s bq24050 single-cell linear battery charger was designed to meet the JEITA specifications for handheld devices. It reduces the charge current by half when the cell temperature is between 0°C and 10°C, and reduces the charge voltage to 4.06 V when the cell temperature is between 45°C and 60°C.
Figure 3: A typical application circuit with the bq24050 linear charger.
Figure 3 shows a typical application circuit with the bq24050 linear charger. The charger monitors the battery’s cell temperature via the thermistor (TS) pin and adjusts the charge current and voltage when the monitored temperature reaches a dangerous threshold.
By Stephan Ohr, Consultant, Semiconductor Industry Analyst
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