Battery capacity monitoring goes beyond the voltmeter

Masoud Beheshti,Senior Product Manager,Portable Power Products,Texas Instruments Inc., Dallas

The next generation of notebook computers, cellular phones, personal digital assistants (PDAs), Internet appliances and portable audio players are continuously shrinking in size and form factor while gaining improved performance and features. These improvements force the design engineer to strike a balance between overall power consumption, processing speed and the battery capacity. This is, however, not a simple task. Higher processing speeds typically translate into higher power consumption. And, although we still witness small and evolutionary improvements in battery capacity and form factor, we are reaching a plateau in that area. If left unmanaged, these portable devices will turn into power-hungry appliances with less than desired run-times.

Proper power management is not possible without accurate and reliable information about the condition of the power source, the battery, and its remaining capacity. The traditional battery-capacity monitoring scheme is a voltmeter. The system simply monitors the battery voltage as a way to predict the remaining energy and run-time. Such a scheme, however, lacks the required accuracy for today's portable applications. Most of us are quite familiar (and frustrated) with a power bar on a cell phone that shows a full battery just before it goes down on the first attempt to make a call. There are several reasons for that:

• Battery capacity changes as a function of the discharge rate: So simply knowing the voltage is not enough. What is more important is how the battery actually reached that point. This is a daunting task for today's portable devices with variable and complex discharge profiles.
• Battery capacity changes as a function of temperature: looking at the cell voltage alone is also misleading without knowing the discharge temperature.
• Other factors such as self-discharge, aging and manufacturing quality also affect the remaining capacity.

The affects of temperature can be visualized when a lithium polymer cell is discharged at two temperatures using a digital wireless load profile. The cell voltage — which can vary from 4.1V to 2.9 V — is plotted on the X-axis. The change in capacity in amp-hours relative to the change in voltage is plotted on the Y-axis. The greatest change in capacity for the change in voltage at C is when the battery voltage is 3.7 V. This means if voltage is used to estimate the battery capacity, a very small change in battery voltage will greatly impact the capacity estimate at that temperature. A 100mV change in voltage from 3.7 V can affect the battery capacity estimate by over 50 percent at 25C. By the same token, discharging the battery at a cold 5C moves the mid-point of the battery capacity from 3.7 V to approximately 3.5 V. In other words, with a battery voltage of 3.5 V, the actual remaining capacity will vary from 10 percent to 50 percent depending on the ambient battery temperature.

The voltage-based method is also an inaccurate way to determine how full the battery is when it is being charged. For instance, during a typical charge cycle for lithium ion (li-ion) and lithium polymer (li-pol,) more than 70 percent of the charge time is spent in a "constant voltage" mode. This mode is responsible for replenishing more than 40 percent of the charge capacity. A voltage-based system cannot differentiate between a battery that is 60 percent full and one that is 75 percent full.

The coulomb mounting method provides the basis for more accurate and reliable capacity monitoring by measuring the charge, or coulomb, input to, and subsequently removed from, the battery. To accomplish this, the charge and discharge current is measured across a low-value series sense resistor between the negative terminal of the battery and the battery-pack ground contact. The voltage drop across the sense resistor is then integrated over time to provide an accurate representation of the state of the battery charge.

To achieve the highest level of accuracy, a voltage-to-frequency (VFC) converter is typically used for integration of the charge. The VFC provides continuous integration and can therefore capture variable and pulsed charge or discharge profiles. The charge and discharge activities are converted into counts and are accumulated over time to represent the charge and discharge flow into and out of the battery. Similar results can also be achieved with an oversampled sigma-delta analog-to-digital converter.

Different solutions

When it comes to battery-capacity monitoring, no one solution fits all applications. High-end portable devices, such as notebook computers, require a more sophisticated solution both in terms of the number of parameters monitored and calculated, and in terms of interface to the battery-pack electronics and system processor. The most common topology used today is the one described by the Smart Battery System Implementers Forum (SBS-IF), an industry consortium for smart-battery systems. Battery-capacity monitoring or gas gauge devices that conform to the SBS-IF requirements report a multitude of critical information to the system processor. This information is then used as the basis for managing the system power. Parameters reported include cell voltage, average and instantaneous current, temperature, remaining battery capacity, remaining time to empty with system alarms, relative and absolute battery state-of-charge, battery-specific and manufacturer-specific information. This data is communicated to the system processor over SMBUS lines.

A fuel gas gauge device is also responsible for other functions in the battery pack, including interface to the lithium ion protection device for pack safety, battery charge termination and possibly cell balancing to extend battery cycle life and capacity.

Smaller handheld devices such as cellular phones and PDAs typically do not require the sophisticated smart-battery gas gauges used in notebook PCs. However, these devices still require the same level of accuracy and repeatability from the capacity-monitoring device. These handhelds also present a different challenge as compared to notebook PCs. The standby power consumption levels in these devices are typically under 1milliAmp, significantly less than notebook PCs. This has the benefit of long standby times for the end-user, but requires much higher measurement accuracy at these low discharge levels.

To provide a cost-effective solution, most battery-capacity monitoring devices act as an analog front-end, capturing accurate charge, discharge, temperature, and voltage activities of the battery. This information is then passed on to the system processor. The system processor in turn converts the data into remaining system run-time information by implementing a gas-gauging algorithm.

Battery-capacity monitoring has become critical in portable electronic devices to improve the user experience with the system, and to provide reliable system operation. Because of the variation in battery capacity over the different temperature and discharge load profiles, systems should employ more advanced techniques beyond measuring the battery-pack voltage. These techniques usually include monitoring the charge and discharge current, battery temperature and battery voltage to provide a reliable estimate of the remaining battery capacity over a variety of use profiles. Cost-effective integrated solutions are becoming available to fulfill this growing need.

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