In part 1 of this blog series, I looked at the importance of battery protection in lithium-ion batteries. In this installment, I’ll cover the other two elements necessary in order to create a safe and healthy battery pack: A monitor, which helps in grabbing diagnostics of each cell in the stack and protect as well, and a gauge which helps in intelligently calculating the state of charge and health of the battery.

Monitors (AFEs)

Given that protection is the primary function of all battery-pack electronics, it is common to see battery-pack devices equipped with some level of protection. For example, a battery monitor (analog front end [AFE]) is designed to do all of these below or a mix:

  • Measure individual cell voltages, pack current and pack temperature (through a separate temperature sensor).
  • Drive an integrated LDO for powering additional components, such as a gauge or microcontroller (MCU).
  • Drive a cell-balancing network to make sure that each cell in the battery stack is equal in charge.
  • Drive charge (CHG) and discharge (DSG) FETs to protect the battery pack.
  • Provide a mix of hardware protections including voltage and current protection.

As you can see, a monitor provides key functions for a battery pack. These functions make a monitor extremely important to have; however, it is important to note that not all applications need a standalone monitor. Some lower-cell-count applications (less than five cells) don't use a monitor; instead, an all-in-one gauge (protector, monitor and gauge) like the bq40z50-R1 includes monitoring functions. Also, all monitors need some form of host, MCU or gauge to control them and receive communications such as cell measurements.


Now let’s talk about what a gauge really is and how you can go about choosing one. A gauge is meant to … gauge. In other words, its job is to estimate or determine the magnitude, amount or volume of something. In a battery pack, a gauge calculates 2 things: state of charge (SOC) and state of health (SOH). SOC refers to how much charge is left inside the battery to run the system, while SOH is the measurement of how old the battery is and therefore needs to be replaced. The gauge will then know when to alert the system of both of these things, so the user can take corrective action on this. When it comes time to choose a gauge, you can take one of three paths, selecting:

  • An all-in-one gauge (equipped with a monitor (AFE), and some kind of protection)
  • A discrete gauge that does not perform any monitoring or protection.
  • An MCU.

Discrete gauges are devices that have special firmware that allows them to perform gauging activities. Choosing an MCU rather than a gauge will depend on what experience you have with developing any sort of gauging algorithm, or if you rather have something that is accurate, readily available and easy to start with. TI has an extensive portfolio of different algorithms with their own levels of sophistication and accuracy. Our Impedance Track™ algorithm, for example, is capable of delivering very precise accuracy with an error of 1% .

One last thing to know is what role a discrete gauge plays with a monitor inside a battery pack. In such cases the gauge acts as the brains in which it will control a monitor and do things like set protection thresholds or drive the protection FETs.

To wrap up this series, let’s review. All battery packs have three basic functions: protection, monitoring and gauging. Battery packs can comprise either a single-chip solution (like the bq40z50-R1) or a multichip solution; each has its own trade-offs. The protection function in a battery pack might be a stand-alone device, or contained inside a gauge or monitor. Hardware and firmware protection can work together for redundancy purposes. Monitors are dependent on a host for communication purposes, and a battery pack needs either a gauge or an MCU with some type of gauging algorithm.

Additional resources:

Read part one of this blog: Get to know your battery pack: Part 1