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DESIGN & PRODUCTS ANALOG & MIXED SIGNAL Gauging the high accuracy 60V gas gauge By Christopher Gobok Many of us can relate to battery-powered devices which display how much power or run time a device currently has, especially because we have been surrounded by a multitude of gadgets at the home front. From electric shavers to tablets, we rely on all sorts of battery indicators to help determine if and how these devices will be used. In time, we become somewhat familiar with each device’s level of accuracy and know how much confidence to place in a device reporting, for example, 10% power left. In higher power multicell applications, situations can be more critical if users are found without adequate power, such as the case with ebikes, battery backup systems, power tools or medical instrumentation. A spare battery pack may not always be available or continuous operation for a specific length of time may be required, which allows us to appreciate accurate battery gas gauging (aka fuel gauging) or assessing how much charge a battery or battery pack has at any point in time. Battery gas gauging is just one of many functions typically found in a smart multicell battery system in addition to charging, protection and cell balancing circuitry. Regardless of function, battery systems present a unique set of design challenges simply because batteries are always changing in electrical nature. For example, a battery’s maximum capacity (also known as state-of-health or SOH) and self-discharge rate always decrease over time, while charge and discharge rates vary over temperature. Well-designed battery systems continuously address as many of these parametric shifts as possible in order to provide end users with consistently accurate battery performance criteria, such as charging time, estimated power or expected battery lifetime (or number of charges left). Simply put, accurate battery gas gauging requires an accurate battery gas gauge IC and a relevant battery-specific model to ultimately provide systems with the most coveted parameter in battery gas gauging – state-of-charge (SOC), or the present battery capacity as a percentage of maximum capacity. While there are battery gas gauges in the market that integrate battery models and algorithms in order to provide direct SOC estimations, peeling back the onion reveals that these devices tend to oversimplify SOC estimation at the tragic expense of accuracy. Moreover, these devices usually only work with particular battery chemistries and require additional external components to interface with high voltages. Enter Linear Technology’s LTC2944 depicted in figure 1 – a simple, 60V battery gas gauge that intentionally provides the bare essentials for accurate, single or multicell, battery gas gauging. Count on counting coulombs Current studies have shown that precise coulomb counting, voltage, current and temperature are prerequisites for accurate SOC estimation, which so far have resulted in a minimum of 5% error. These parameters allow us to pinpoint where along a charge or discharge curve a battery is at, where coulomb counting not only reinforces voltage readings but also helps differentiate any flat regions of a curve – figure 2 shows typical discharge curves for different battery chemistries. Fig. 1: LTC2944 60V battery gas gauge. Counting coulombs helps dodge the example situation of a device misleadingly reporting 75% SOC for a long period of time and then suddenly dropping down to 15% SOC, which is what tends to happen in devices that only measure voltage to assess SOC. To count coulombs, users initialize a coulomb counter to a known battery capacity when the battery is fully charged and then count down when discharging coulombs or count up when charging coulombs (to account for partial charging). The beauty of this scheme is that battery chemistry does not have to be known. Because the LTC2944 integrates a coulomb counter, this device can be easily copied-and-pasted across multiple designs, agnostic of battery chemistry. Take a look at how the LTC2944 counts coulombs in figure 3. Remember, charge is the time integral of current. The LTC2944 measures charge with up to 99% accuracy by monitoring the voltage developed across a sense resistor with a ±50mV sense voltage range, where the differential voltage is applied to an auto-zeroed differential analog integrator to infer charge. When the integrator output ramps to the high and low reference levels (REFHI and REFLO), the switches toggle to reverse the ramp direction. Control circuitry then observes the condition of the switches and the ramp direction to determine polarity. Next, a programmable prescaler allows users to increase integration time by a factor of 1 to 4096. With each underflow or overflow of the prescaler, the accumulated charge register (ACR) is finally incremented or decremented by one count. It is worth noting that the analog integrator used in the LTC2944’s coulomb counter introduces minimal differential offset voltage and, therefore, minimizes the effect on total charge error. Many coulomb counting Christopher Gobok is Senior Product Marketing Engineer at Linear Technology Corporation – www.linear.com Fig. 2: Typical discharge curves for different battery chemistries 28 Electronic Engineering Times Europe March 2017 www.electronics-eetimes.com


EETE MAR 2017
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