When considering the use of lithium batteries in vehicles, you should examine the power-train block diagrams for series-hybrid, parallel-hybrid, purely electric, and other vehicle types. Fortunately, the lithium-battery pack looks much the same for any vehicle. The building block is a group of 100 to 200 2.5 to 3.9V, 4- to 40A-hr, series-connected cells. This dc-power source drives a 30- to 70-kW electric motor. The total pack voltage is high, so the average current is low for a given power level. Lower current requires smaller cables, less weight, and less cost. The pack should deliver 200A under peak conditions and be quickly rechargeable. In other words, the battery needs good power density as well as good energy density. Big systems, such as buses and tractor trailers, use as many as four 640V parallel packs.

The design problem with lithium-battery packs is balancing performance, economics, and safety. The two key variables are the battery-cell design and the cell-management electronics. For example, say that you want to build an EV that goes 100 miles per charge with a battery pack that lasts 10 years before you have to buy or rent a new one. To meet the 10-year, 3650-charge-cycle goal, you can use only a portion—say, 40%—of the cells’ capacity. To minimize vehicle cost, you want to use batteries with the fewest kilograms, and batteries are the most expensive components of the pack. To maximize performance, the cells must handle 200A peak charge and discharge currents. Above all, the chance of a rapid-oxidation event—that is, a fire—must be less than that for a gasoline-powered car.

Traditional lithium-cobalt cells, like those in laptop computers, have high energy density but tend toward thermal runaway when the separator material fails. Manufacturers are basing the new breed of lithium batteries on lithium-iron-phosphate, lithium-manganese, and lithium-titanate, which are thermally stable even when you puncture their packaging. Their prismatic form factor, which resembles a silver Pop-Tart, has low ESR (equivalent series resistance) to support high currents. They hold less energy than laptop lithium-cobalt cells but are still better than nickel-metal-hydride devices, and they last 10 to 15 years if you carefully monitor their charge and discharge levels.

Battery-monitoring systems now come into play because they monitor the battery’s state of charge, which in turn determines the battery’s cost and performance. If you know the battery’s state of charge, you can use more capacity from each cell, use fewer cells, and maximize the lifetimes of those cells. In a laptop computer, you perform this task by monitoring cell voltage and counting coulombs into and out of the stack of four to eight cells. Voltage, current, charge, temperature, and some math give a good indication of the state of charge. Unfortunately, you can’t count coulombs in a car because the battery is driving an electric motor, not a motherboard. The current spikes are 200A, and low-level idling follows those spikes.

You also have 96 to 200 cells in series, in groups of 10 or 12. These cells age at different rates, come from multiple lots, and vary in temperature. These factors mean that they have different capacities, and cells with the same number of coulombs could have different charge levels. For these reasons, battery-monitoring systems in cars focus on cell voltage. You must accurately measure the voltage of every cell and then use current and temperature measurements to tweak the readings for ESR and capacity changes. You keep a running estimation of each cell’s charge level. If some cells overcharge and others undercharge, you must adjust the level in each cell by bleeding off, or passively balancing, charge; another approach is to redistribute, or actively balance, the charge. When the cells reach a minimum state of charge, you are out of energy.

You need to figure out how accurately to measure the voltage. Start with the goal of knowing the state of charge within 1% over temperatures of –20 to +80°C. Keep in mind, however, that the data varies considerably among manufacturers and chemistries. The voltage changes approximately 200 mV from 30 to 70%, or 5 mV per percentage point, of the state of charge. A measurement range of 0 to 5V requires 0.1% total measurement accuracy. Translating that figure into data-acquisition specs requires a 12-bit ADC with 1-LSB (least-significant bit), or 0.02%, INL (integral nonlinearity), plus a voltage reference with 0.05% of initial accuracy and 5 ppm/°C of drift—that is, 0.02% for 40°C changes in temperature.

The data-acquisition system also must reject switching noise and high common-mode voltage.  Spreading the transient equally over the 100 cells means that the top cell has a 370V common-mode voltage, 100V common-mode transients, 1V differential transients, and a 3.7V-dc value. You need to measure the 3.7V-dc value with 5-mV accuracy.

The complete battery-monitoring system also requires cell balancing, data communications, and self-test features, which seriously complicate the schematic. The high component count makes the use of off-the-shelf approaches costly and unreliable. A modular-cell-measurement design, with one IC integrating most functions. The input multiplexer can tolerate 60V of common-mode voltage. Using switched-capacitor sampling techniques eliminates the CMRR limitation that most discrete designs face. The delta-sigma ADC is essentially ideal, leaving the reference voltage as the only component in the error budget. Without calibration, the LTC6802 achieves 0.12% room-temperature accuracy and 0.22% over a –40 to +85°C range. An initial factory calibration of the room-temperature error reduces the overall error to 0.1% over temperature. To gain more accuracy, you can add a low-drift external reference. Periodically measuring the output and using this information to adjust the cell measurements, along with an initial calibration, reduces the errors to 0.03%, which is the noise floor of the ADC, over a –20 to +70°C window.

By: DocMemory
Copyright © 2023 CST, Inc. All Rights Reserved