Author: Linear Technology's Signal Conditioning product design manager Mike Kultgen
The electronic devices in hybrid electric vehicle batteries are the key to improving performance and safety. New technologies in the field of integrated circuit design enable battery pack designers to further improve the performance of lithium-ion batteries. Higher measurement accuracy, a stronger data link and active charge balancing of battery capacity all help achieve lower costs, longer driving cycles and faster charging.
The block diagram of a typical battery pack (Figure 1) consists of several sets of lithium-ion batteries connected in series. Their measurement and balancing are performed by a high-voltage analog integrated circuit. These analog front-end (AFE) ICs perform the difficult task of measuring the voltage, current, and temperature of each cell and pass data to the control circuit. The controller uses battery data to calculate the state of charge and health of the battery pack. The controller may command the front-end IC to charge or discharge certain batteries to maintain a balanced state of charge within the battery pack.
Figure 1: Block diagram of the battery pack
Higher accuracy means lower cost
The measurement accuracy of the analog front-end IC has a direct impact on system cost. Accurate measurements are required to achieve useful state of charge (SOC) calculations. In order to achieve a long life, the battery pack generally operates between 20% and 80% SOC. If there is a 5% uncertainty in the SOC calculation, then the size of the battery pack must be increased by 5%, which leads to a significant increase in the cost of the battery. Adding 5% capacity to a 16kW-hr battery pack requires about 360 Euros ($ 460). Improving the SOC calculation to achieve a 1% error means that each battery pack can save about 300 euros ($ 385).
Battery voltage measurement is a key element of the SOC algorithm. When measuring 3.3V LiFePO4 (Lithium Iron Phosphate) batteries, IC power supply and battery pack developers focus on the specification with a total measurement error of 1mV.
For laboratory equipment such as the Fluke-289 handheld multimeter priced at 480 euros ($ 615), it is common to measure voltages from 3.3V to 1mV. The AFE IC must provide the same performance at a cost of 1/100 and work continuously for 15 years in the automotive environment. Only a few IC technologies can achieve this goal.
Accuracy in the real world
What kind of IC technology is best for battery measurement? The answer can be obtained from the error analysis of Figure 2 (block diagram of a typical AFE IC). One of the 12 serial batteries is selected by the multiplexer (MUX) module. Store the battery voltage on a capacitor by closing the "S" switch. Open the "S" switch, then close the "T" switch. The voltage across the battery will be transferred to the ADC. This "flying capacitor" solution eliminates the large common-mode voltage of 33V at the top battery and maintains a differential voltage of 3.3V. An analog-to-digital converter (ADC) compares the battery voltage to its voltage reference and produces a digital result proportional to the ratio of VCELL to VREF.
Figure 2: Typical analog front-end IC
If the impedance of the switch is too large to charge the capacitor in a short sampling time, then MUX and flying capacitors may introduce measurement errors. A careful switched capacitor design can eliminate this error term.
The conversion from analog to digital by the ADC may also introduce errors due to device mismatch. Secondly, the combination of meticulous design and device fine-tuning can reduce the error caused by ADC.
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