Fuel gauging for all: Page 6 of 7

August 19, 2016 // By Bakul Damle
Fuel gauging for all
As the pace of improvements in traditional consumer electronics such as smartphones and tablets is starting to plateau, many creative design engineers are focusing their attention on inventing the next big thing.

Servo mixing provides a fixed magnitude continuous error correction to the coulomb count, up or down, based on the direction of error from the OCV estimation. This allows differences between the coulomb count and OCV estimation to be corrected quickly. The resulting output from the mixing algorithm does not suffer accumulation drift from current measurement offset error and is more stable than a stand-alone OCV estimation algorithm (see figure 7).

Figure 7

This correction to the coulomb counter takes place continuously while the application is active as well as when it is in standby condition. In practical terms, this means that the coulomb counter corrections happen more than 200,000 times per day – in tiny steps that are almost invisible to the user. These corrections happen when the battery is under load, as well as at no-load condition, regardless of whether the cell is relaxed or not, and this is a significant advantage over other competing algorithms.

As the temperature and discharge rate of an application changes, the amount of charge available to the application also changes. The ModelGauge m5 algorithm distinguishes between remaining capacity of the cell and remaining capacity of the application, and reports both results to the user.

The algorithm periodically makes internal adjustments to cell model and application information to remove initial error and maintain accuracy as the cell ages. These adjustments always occur as small corrections to prevent instability of the system and prevent any noticeable jumps in the fuel gauge outputs. Learning occurs automatically without any input from the host. In addition to estimating the battery’s state of charge, the ICs observe the battery’s relaxation response and adjusts the dynamics of the voltage fuel gauge.

The ModelGauge m5 algorithm includes a feature that guarantees the fuel gauge output converges to 0% as the cell voltage approaches the empty voltage. As the cell voltage approaches the expected empty voltage, the IC smoothly adjusts the rate of change of SOC % so that the fuel gauge reports 0% at the exact time that the cell voltage reaches empty. This prevents unexpected shutdown or an early 0% SOC reported by the fuel gauge. This also provides an additional mechanism of desensitizing the SOC % error from any model mismatch errors.

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