Making Batteries Smarter
These days it seems like everything is becoming a “smart” device. Even the batteries that power smart devices are becoming “smart.” A conventional battery simply exposes positive and return terminals. You’re left to figure out how to use the battery. In contrast, a smart battery looks after itself, tells you what its status is, and even lets you control what it’s doing. Sounds great, doesn’t it? But do you really need a smart battery? And if you do need a smart battery, what’s actually involved in making one?
Why do you need smart batteries?
Imagine if your phone didn’t have a way of letting you know how much battery life was left. Isn’t it convenient when you can quickly determine how much remaining capacity is left on any portable device?
Smart batteries not only give you a method of determining remaining battery life but also provide an advanced level of protection. A smart battery can ensure the cells are never exposed to unsafe voltage levels, unsafe charge or discharge currents, or use in extreme high or low temperatures. For this reason, smart batteries are commonly associated with Li-ion cells. The unique charge and discharge characteristics of Li-ion chemistry based batteries pose a fire danger. A properly implemented smart battery can prevent any user error from being the cause of such a fire.
However, smart batteries are not for every application. If your device won’t benefit from utilizing a Li-ion chemistry cell, and you won’t be monitoring the battery’s state of charge, then there’s no advantage to using a smart battery for your application. Your classic TV remote or clock are great examples of applications where you would stick to a standard AA battery, and not worry about using a smart battery.
What's inside a smart battery?
So you’ve decided to use a smart battery. What are you actually getting?
A smart battery is a battery pack that integrates battery cells with a Battery Management System (BMS). The BMS typically includes protection circuitry for the cells and fuel-gauging circuitry to monitor the State of Health (SOH) of the pack. The smart battery has a data communication interface to allow a host device to query the battery for SOH information and possibly even to change the battery parameters.
Knowing that much about a smart battery is probably enough to allow you to buy an off-the-shelf (OTS) smart battery and to understand its datasheet. But if you’re interested in building your own smart battery or just want a deeper understanding of what’s inside, you probably need to know a little more about what the BMS is and how it works. So let’s look at the details.
The architecture of a smart battery varies slightly depending on the needs of the application. Some parameters that play into selecting a smart battery architecture are: cell count, power requirements, and required capabilities (i.e., cell balancing support, state-of-charge indicators, etc.). The number of Integrated Circuits (ICs) that make up the BMS will also vary, typically from one to three, depending on these same parameters. Here’s what a common smart battery architecture looks like:
The BMS part of the smart battery pack includes the following key components:
- Charge and Discharge FETs that enable/disable the connection from the cell stack to the external connector, or to the system load. The Battery Management Controller will signal the AFE to disable these FETs if any fault condition (under voltage, over current, etc.) is observed.
- An Analog Front End (AFE) that monitors voltages at each cell terminal in the stack and the voltage across the sense resistor. The AFE also controls the Charge and Discharge FETs.
- A Battery Management Controller (BMC) that interfaces with the AFE, performs monitoring and capacity management functions (see below), and stores lifetime history information. For lower cell-count packs you can often find the AFE and the Battery Management Controller in a single IC.
- A sense resistor (typically less than 5mΩ) that measures current, which allows the BMS to perform coulomb counting and thereby track the battery’s state of charge.
- A secondary protection IC, if the application calls for having an additional layer of hardware protection. A permanent fuse will accompany this IC and permanently disable the pack if a certain unsafe voltage is reached on the cell stack.
- State-of-Charge (SoC) indicators and a push button. Most BMC components will offer this feature so you can easily push a button and light up a few LEDs and see how much capacity is left in your pack. This is an optional part of the BMS that may or may not be populated based on your application needs.
The Battery Management Controller monitors current usage and voltage levels through the AFE. If any voltage, current, or temperature levels exceed pre-configured settings in the controller, the controller will signal back to the AFE to disable the FETs and allow the cells to disconnect from the load.
An additional function of the Battery Management Controller is to continuously integrate the amount of current used by the system and to predict the remaining capacity of the cells. This is commonly known as “fuel gauging” or “gas gauging.” BMC manufacturers (TI, LT, Maxim IC, etc.) have different names and acronyms for their proprietary algorithms to predict remaining battery capacity, and each has its own advantages and disadvantages. These algorithms all have a unique method of taking the parameters monitored by the AFE and developing a model for the current state of the battery cell(s).
How much will it cost to make my battery smart?
Sure you can buy an OTS smart battery, but what if you are working in a small volume and can’t afford the space a canned OTS smart battery solution will consume? How about if you want to put those cool state-of-charge indicators and buttons in just the right place. The answer is… roll your own! Starting from the smart battery architecture I described above, you can pick your components, design your circuit, and integrate your cells. However, keep in mind that adding smarts to your battery will cost you more than the price of the hardware. The additional space and weight of integrating a BMS into your battery pack may impact the user experience of your portable device. In addition, if your battery powered application is already scraping every micro-amp of inefficiency out of your system, you may not be able to afford the idle power consumption of a BMS. These “costs” must be considered as well.
Depending on the cell count, power requirements, and required capabilities, you can get BMS components alone (no cells) for prices that vary from approximately $5 to $30, in quantities of 1,000 or more.
Space and Weight
Space and weight “cost” is difficult to generalize and will heavily depend on the design requirements. However, with today’s fabrication technology, the ICs and discrete components contained in the BMS are likely to be extremely small when compared to the cells. For example, a two-cell, low-power smart battery pack will use ICs and FETs that are small and easy to tuck away in an end-cap with the connector. A larger cell-count high-power smart battery pack will require larger FETs and a higher pin-count AFE, but will still be relatively small compared to the cell volume. Make sure to be cognizant of your design parameters when selecting components and laying out your PCB; these are a few prime places to reduce weight.
All battery management ICs are designed to have very low quiescent current and the ability to enter sleep modes and consume even lower power. Power consumption will vary greatly based on the in-line components you select (FETs, current sense resistor, or permanent fuse. if implemented), but typical battery management IC quiescent current consumptions are under 1mA in an active state and under 100uA in a sleep state.
If you’re planning on using Li-ion battery cells in your device, you could certainly benefit by integrating a BMS. You’ll get all the protection features you need for a safe design, in addition to fuel-gauging ability and a communication interface to monitor the battery’s state of health. Although off-the-shelf solutions do exist, battery packs are often a system component that needs to be optimized and tailored for your specific application. Designing your own BMS allows you to select components to suit your application’s needs and fit into your available space, which is typically a critical requirement for most battery packs.
Although most smart battery designs are focused on Li-ion cells at the moment, Battery Management Systems will continue to evolve as more research is done on cell chemistries. In fact, it’s possible the next generation of smart batteries will combine multiple chemistries into a “software-defined battery” that can dynamically use different battery properties depending on the changing application needs. That could open up a whole new range of interesting product design possibilities.