Your cell phone, your laptop, and that rechargeable product you received as a gift last year; how long should one expect the batteries in these devices to last? Most of us take notice of decreased battery performance in these gadgets within a couple of years. And fortunately, we’ll have replaced them with newer models before we ever see what happens five years later.
Now imagine a battery that must deliver consistent performance for up to ten years. A battery that doesn’t spend its life on an office desk or tucked in a pocket, but outside in northern winter cold and southern summer heat. A battery with a price tag so high, that if it failed, the necessary replacement cost would render the residual value of the entire product worthless. This battery, of course, is the modern EV/HEV battery.
Yes, after over a century of promise the era of electrified vehicles has arrived! Well, with a catch of course. You see, the latest Lithium-ion technology powering these vehicles is right on the edge of being able to deliver all that customers expect. If you want to enjoy this ride, be sure to operate it in its sweet spot of performance, manage its thermal needs, and respect its electrochemical nature (the devil in doing that, of course, is in the details).
So now the switch has been thrown and the race is on. Today carmakers are in a dash to integrate electrification into as many vehicles as possible. Either as battery-only electric vehicles, various forms of hybrids, or at least start-stop options. And when an automaker integrates technology, the focus becomes systems and subsystems. Where is the value, what is the cost, is it robust, and is it safe?
As a full-service automotive semiconductor supplier, Renesas has responded: By developing the latest silicon with BMS features like the simultaneous sampling of cell voltages to simplify and improve the efficiency of control algorithms. By integrating cell charge balancing switches to lower solution costs. By designing cell sensing topologies that offer robust survival of “hot plug” and EMC transients. And, by providing support for ASIL and functional safety analysis. All the types of things that system designers and OEMs require to realize a new vehicle. Keep up with the latest by visiting our Automotive Battery Management page.
Now back to that devil and the details conundrum. All these system-level efforts are just that; they make the system more viable. But what does it really take to make sure our shiny new EV battery sees its tenth birthday? Well, a BMS can practically only measure three real-time parameters: cell voltage, cell temperature, and the common battery current that runs through all the cells connected in series. Everything else related to battery control is ultimately calculated as a function of these values. And of the three, none is more important than cell voltage.
Why cell voltage? Because cell voltage provides the clearest indication of the electrochemical state of a cell. Yes, temperature is important, but it is mostly used to adjust the way cell voltage is interpreted. Measuring current and coulomb counting is useful, but its calculations are combined with or bounded by limits in cell voltage. In other words, if current and temperature are sound and touch, cell voltage is sight.
Which brings us to accuracy - how closely does the measured voltage represent what is really on the cell’s terminals (initial accuracy). Then when a BMS IC is soldered to a PCB, the resulting stress affects its’ accuracy (post-soldering accuracy). And how about managing a battery during an entire drive cycle? Since cell voltage is used to determine battery capacity, any variation in BMS IC accuracy with changing temperatures or applied voltage will result in inaccurate state of charge (SoC) or “gas gauge” functionality. Thus, Renesas has focused on designing BMS ICs like the ISL78714 that lead the industry in minimal post-soldering effects, low temperature drift, and low sensitivity to applied voltage.
And lest we forget, our battery will still be on the road in ten years or more. If we expect to be able to track the condition of the battery over its life (its State of Health, or SoH) the BMS IC will have to have very stable accuracy over the same period. And because a BMS can’t distinguish between changes in cell voltages that are a result of shifting BMS IC accuracy, as opposed to actual changes in the cells, Renesas has led the industry in characterizing and modeling lifetime accuracy of its BMS ICs for customer vehicle life profiles.
So how does Renesas predict the future of a customer’s BMS accuracy? By using long-term drift performance data obtained from actual laboratory testing at 25 °C, and accelerated life testing, Renesas has developed a mathematical process for calculating the ISL78714 life-time shift in accuracy. Thus, a customer only needs to follow the recommended PCB layout guidelines and soldering reflow profiles, and provide an expected vehicle life profile, to receive results like the 15-year life plotted below (the IC’s board-level accuracy and long-term drift characteristics are both logarithmic and predictable).
In summary, the age of electrified vehicles has arrived. Productionized systems and higher volumes are fueling the demand for more feature-rich BMS IC solutions. Renesas is responding, but with an acute awareness that without sufficient voltage accuracy, higher integration is for naught.