Battery Charge Monitoring Considerations for Battery-Backed Storage Systems

Accurate battery fuel monitoring in battery backup systems requires special considerations. Using TI's battery fuel gauge with Impedance Track™ technology offers the distinct advantage of not requiring the battery pack to be fully discharged to perform an auto-learn (calculate the charge) as the battery ages. This article discusses different implementations and techniques to achieve a proper battery auto-learn cycle in backup applications. Additionally, we review a case study of capacity and impedance changes in an aging battery pack.

TI’s impedance tracking algorithm uses the battery’s voltage, current and impedance measurements to accurately calculate the remaining battery capacity and run time of the battery pack. The most accurate battery fuel gauging requires the correct selection of the specific battery chemistry. For the purposes of this article, there are six different categories of chemistry, with several options within each category.

The main issues in determining battery aging in a battery backup system are (1) the battery’s maximum chemical capacity (Qmax), which is expressed in milliamp-hours (mAh), and (2) the battery’s actual measured impedance (R_a table value), which will determine the true battery run time based on load and temperature.

Most notably, high temperatures will adversely affect Qmax and internal battery impedance. Charging and storing batteries at low voltages (between 3.9V and 4.1V for a standard 4.2-V battery) will extend their useful life, but at the expense of reduced run time.

Some previous battery fuel gauging technologies required that the battery be fully discharged to update capacity information. Impedance tracking eliminates this full discharge requirement and instead uses two relaxed voltage measurement points to update Qmax. In the default firmware, these voltage measurements are typically taken around a 40% change in the battery state of charge (SOC). With TI's improved firmware, this SOC range can be reduced to 10% for "shallow" discharges. Reducing the SOC range for the Qmax update affects the accuracy of the battery fuel gauge; the greater the SOC range used, the greater the accuracy.

Depending on the battery chemistry, it is necessary to perform two relaxation voltage measurements within the specified voltage range. An Excel® file of unqualified Qmax update voltage ranges based on battery chemistry can be found at http://www.ti.com/lit/zip/slua372.

Table 1 is an excerpt from this file. As shown in the table, if the chemistry ID is 0100, the Qmax-update voltage measurement is not allowed to be between 3737 and 3800mV because the voltage profile is flat for this SOC. This unqualified voltage range is based on measuring the battery voltage drop after a rest period of at least one hour. During discharge with a load greater than C/10, impedance measurements and updates are performed. (The “C rate” rating is based on the battery capacity. A 3s2p pack with a design capacity of 4400mAh would have a C/10 rate of 440mA. In this case, the safe rate is 500mA.)

To store the varying resistance at different SOC values, 15 grid points are used. If one grid point is recalculated, all subsequent grid points must be modified accordingly. Discharges over 500 seconds are required to avoid transient effects and distortion of the resistance values.

How to Start a Qmax Battery Automatic Learn Cycle
TI has evaluation software that displays the status and allows control of the “Impedance Tracking” fuel gauge parameters. After confirming that the battery voltage is outside the acceptable range, a “Reset” command can be sent to the fuel gauge, setting the R_DIS bit and clearing the VOK bit. Once the fuel gauge has made a correct OCV measurement, the R_DIS bit will be cleared. Now, the battery can be charged or discharged, which will set the VOK bit within a few seconds. With the firmware set up for a 10% shallow SOC change, the charge/discharge is allowed to change the SOC by at least 15%. After stopping the charge/discharge cycle, the battery is allowed to discharge (to a fully depleted state for up to 5 hours) outside the unqualified voltage range. The VOK bit should clear, indicating that a second valid OCV measurement has been taken and the Qmax update has been completed successfully. Table 1 Unqualified Qmax-Update Voltage Ranges Based on Battery Chemistry The following two examples describe different system implementations for a battery backup system. Example 1 Passive Battery Discharge In this configuration, the active current (~375 μA) of the fuel gauge chipset can be used to discharge the battery for a longer period of time. Depending on the capacity of the battery pack, this can be several months. The fuel gauge can be programmed to remain in active mode continuously by setting the SLEEP bit of the “Operation Cfg A” register to 0. Another method is to set /PRES GPI using a fixed bit (NR=0) set in the “Operation Cfg B” data flash register. With the firmware modified for shallow discharge (e.g. 20%) for Qmax updates, the battery pack is allowed to discharge to 75% of its capacity and then the battery is recharged to full capacity. The Qmax parameter is updated accordingly. Note that only the Qmax value is updated during this cycle and not the battery impedance (R_a table value). We assume that a few hours of sleep are allowed at the end of the charge to make a second relaxation voltage measurement. Example 2: Active Battery Discharge In this configuration, the system's discharge resistors can be used to actively discharge the battery. This should be controlled by a host processor either internal to the battery pack or external to the system. As mentioned earlier, the impedance grid point update requires a discharge current of more than 500 seconds C/10. Even if the 10% minimum discharge requirement applies to the Qmax update, the battery pack should ideally be discharged through two impedance grid point updates. These occur during the discharge at approximately 11% SOC intervals (i.e. 89%, 78%, 63%, 52%, etc.). In this case, a discharge from 100% to 75% capacity is sufficient. If the battery is stored at 80% SOC for durability reasons, two impedance grid point updates occur within 25% discharge. The correct Qmax update occurs only after two consecutive relaxed voltage measurements separated by charge or discharge (assuming both measurements are outside the disqualified voltage range for the specified chemistry ID). Therefore, after the battery pack is actively discharged to 75% of its capacity, several hours of rest are required, depending on the SOC. (Depending on the battery chemistry, half-charge state requires up to 3.5 hours, while fully discharged state requires up to 5 hours.) Case Study A 3s4p 8.8-Ah battery pack from Microsun Technologies, which has a number of LGDS218650 cells using the bq20z80 chipset manufactured in June 2006, was used to study the effects of long-term storage. The battery pack was stored at about 45% capacity at room temperature for two years without charge or discharge cycling. The important parameters are the Qmax variation and the cell impedance variation, as well as the accuracy of the remaining capacity and run time calculations. The estimated self-discharge of these cells is less than 4% per year. A constant resistive load of 3Ω was used to discharge the battery pack (equivalent to a discharge rate of approximately 3.5A). The Qmax variation and impedance value variation are shown in Table 2 (next page) and Figure 1, respectively. On average, Qmax decreased by 3% while the cell impedance increased by 35%. As with these cell variations, the initial discharge cycle after the two-year rest period was more than 99% accurate; in particular, a capacity of 67 mAh was reported when the termination voltage was reached (67 mAh/8819 Qmax = 0.00761, or 0.761% error). Figure 1 Variation of cell impedance over time Table 2 Qmax and cell impedance values ​​before and after discharge of the sample battery pack