Your Guide to the Advantages of Pack-Side Fuel Gauging

With the increasing popularity of portable devices, lithium-ion (Li-ion) batteries have become ubiquitous. These batteries, however, have stringent safety requirements which typically require battery packs with integrated protectors.

The characteristics of Li-ion batteries also often warrant battery fuel gauges to accurately report state-of-charge (SOC) under various operating and environmental conditions. Depending on the type of applications, the system may be designed with a host-side fuel gauge (Figure 1) or a pack-side fuel gauge (Figure 2). A host-side fuel gauge resides on the host system and is connected to the application processor, while a pack-side fuel gauge resides on the battery pack and is connected to the Li-ion cell.

Figure 1. Host-side fuel-gauge implementation.

Host-side fuel gauges are useful when the battery pack is replaceable and lower SOC accuracy is acceptable for the application. Increasingly, however, devices are being designed with captive batteries with no option to replace them. In these cases, a pack-side fuel gauge may be suitable for the technical reasons as outlined below, or for logistical reasons. For example, there may be multiple cells being used by multiple battery pack makers for supply assurance, as each battery can carry its cell parameters inside the pack-side fuel gauge.

Figure 2. Pack-side fuel-gauge implementation.

In the pack-side approach, the proximity of the cells to the fuel gauge results in a number of unique advantages:

• The fuel gauge, battery protector, and even authentication can be integrated in a single, tiny solution.
• It improves the accuracy of SOC reporting by eliminating the effect of the connector resistance for voltage and current measurements.
• The fuel gauge typically includes a die temperature sensor to monitor the cell (if close to the IC) as well as protection FETs and can eliminate the need for a separate sensor for the cell temperature. Alternately, the fuel gauge can also measure temperature of a cell further away using a thermistor.
• The battery connector can be made smaller, as fewer connections and measurements need to be routed to the host side.

Fuel Gauge with Integrated Protector and SHA-256 Authentication

Maxim's MAX17301/11 fuel gauge with protector and SHA-256 authentication (Figure 3) is an example of a solution that simplifies pack-side fuel-gauge implementation. The IC integrates a 2-level protector and SHA-256 authentication to the industry's most accurate battery fuel gauge. MAX17301/11 offers the following features that are useful for pack-side fuel gauging:

• ModelGauge m5 algorithm that combines short-term accuracy and linearity of a coulomb counter with long-term stability of a voltage-based fuel gauge to provide industry-leading fuel gauge accuracy
• SHA-256 authentication to prevent use of unsafe counterfeit batteries
• Automatic compensation for cell aging, temperature, and discharge rate, with accurate SOC over a wide range of operating conditions
• Electronic serialization for traceability of battery packs

Figure 3. MAX17301/11 functional diagram

So, for your next Li-ion battery-based design, consider a pack-side fuel-gauge implementation. The battery—and the device user—might just thank you for this!