Battery Pack 101

What is a 'battery pack?'

I have been asked this question many times. Often the term 'battery' and 'battery pack' is being used interchangeably. I thought this could be an interesting topic to write about.
We will try to cover the following topics in this blog post:

• What is a battery pack?
• What components/systems does a battery pack contain?
• Block level diagram of a typical battery pack
• Battery Management System

*I will specifically talk about Lithium-ion battery packs. some of the concepts may not hold true for other battery chemistries like Lead-acid, NiMH, NiCd etc.  

Let's begin with what's a battery pack?

Figure 1

As shown in the above diagram, The term 'cell' refers to a single energy storage device.  When one or more cells are connected in a series-parallel connection, a whole assembly is called a 'battery' or 'battery pack'. As said before, a battery pack may consist of one or more cells. The number may vary from single cell to several thousand cells as shown in figure 1. For example, cellular phones, tablets have a single cell in their battery pack. Power tools generally have 2-4 cells in series and several cells in parallel. The number of cells in series-parallel increases based upon power and energy requirements of an application.

Power = Voltage x Current

When higher power is required, voltage and current need to be higher. It is not always possible to increase current to meet the power requirements (higher the current, higher power loss). Hence in order to increase the voltage, more and more cells are added in series.

Crudely speaking, a generalized battery pack would look something like figure 2

Figure 2

As now we are well versed with the term 'battery pack' let's go ahead and see what are the components of the battery pack and what are their respective functions.

Battery packs can be made with a variety of shapes, types, and complexities depending upon applications. Roughly speaking, a battery pack system can be divided in the following manner.

Figure 3: battery pack system level components

As can be seen in figure 3, a battery pack consists of three subsystems.

1. Electrical
2. Firmware/software
3. Mechanical

The electrical system consists mainly of hardware (electrical circuits and harness).  Which is responsible for monitoring various battery parameters like cell voltages, current, the temperature of a battery pack etc. In some cases, the electrical system also balances the charge within cells in the case of unequal distribution of electrical charge among cells inside a battery pack. (click here to know more about cell charge imbalance). The electrical system responsible for such monitoring and balancing operations is collectively called 'Battery Management System' or 'BMS'. In some applications, battery-pack itself has an inbuilt protection switch (we will see more about this point later). This switch is responsible for power control to-and-from battery pack.

Software or firmware resides in the microprocessor of a battery pack (refer figure 5). It is the brain of a battery pack. The most challenging and important function of firmware is to keep a track of how much energy (or charge) is remaining inside a battery. In very simple terms, remaining charge aka 'State Of Charge' (SOC) is calculated as:

Where 'q.dt' is total charge flowing to-and-from the battery (in simple words, integration of current flowing to-and-from battery over the time period with input and output currents having opposite signs). There is one more term called 'State Of Health' or SOH which is also calculated by battery pack firmware. This indicated how much battery has degraded over the time. Calculation of SOC and SOH is not as state forward as it seems. Perhaps I will post a different blog dedicated to this topic!

The last but not the least, the mechanical component of a battery pack:

Again, depending upon the application, the mechanical structure of a battery pack can vary from just a simple laminate case to meticulously designed structure consisting of thousands of parts. For example, a cell phone battery pack has a very simple mechanical design (figure 4). Whereas EV battery pack contains a large number of parts for structural strength, cooling etc.

Now we will have a look at very basic battery pack:

Figure 4: basic battery pack

As shown in figure 4, a cell phone battery pack has a very simple mechanical structure (just a lamination to protect from short-circuiting). The electrical system consists of a few basic building blocks. We will see the function of each component one-by-one

• Power Control: it is a switch (most of the times a semiconductor FET) which cuts off battery pack from load or charger if there is an anomaly
• Protection IC: This IC monitors the cell parameters and detects 
• Over/under voltage (overcharging, over discharging)
• Overcurrent 
• Over/under temperature (over temperature may lead to battery explosion! similarly very low temperature is also not good for battery)
• communication with cell-phone microprocessor (provide basic info like temperature,  current etc)
• There is a grey block on '-' (or GND) line of the battery pack (in figure 4). It is a current sensor. It measures the total current flowing to/out of battery
• A red box after the 'power control switch' indicates a protection fuse. This is the last line of defense for the battery pack. If the power control switch or protection IC goes bad due to some reasons, This fuse will automatically blow if very high current flows through.

As said before, the battery pack shown in figure 4 is a very basic one. It does not have the brain to compute State of charge or state of health. It only protects the battery from getting blown!
Following video has very good info regarding 'how battery protection works?'

Block level diagram of a bit more advanced battery pack is as shown in figure 5.

Figure 5: advanced battery pack block diagram

If we compare the battery pack shown in figure 4 and figure 5, there are few additions. For example, the protection IC in figure 4 is replaced by two different blocks in figure 5, monitoring and microprocessor IC.
Battery packs shown in figure 5 is typically used in bit sophisticated power tools, electric scooters, light electric vehicles (golf carts for example) and home UPS. The number of cells in series typically varies from 4 to 16 giving voltages of 12V to 68V.
As I said previously, LiB is not as fault tolerant as other chemistries. It has to be monitored very carefully while in operation and at rest. If there are '#n' number of cells (or cell-blocks) in series, we need to monitor cell-block voltages of each and every cell block! As the number of cell blocks increases, using separate Analog to Digital (ADC) for every cell voltage is an impractical solution.  There are many special purpose battery monitoring ICs available in the market to make our job easier. To list a few, companies like Texas Instruments, Linear Tech. (Analog Devices), Intersil, Lapis/ROHM semiconductor etc have a very good portfolio for this type of ICs. Rest of the functioning of these type of advanced level battery packs is similar to what we have already discussed for basic battery packs.

Battery pack design is very intricate and interesting topic to work with. I wrote this blog just to give a brief idea about what's inside a battery pack. There are tones of resourceful webpages to dive deeper into the battery pack and BMS technology. I will list a few:

• http://www.ti.com/power-management/battery-management/support-training.html
• https://en.wikipedia.org/wiki/Battery_management_system
• Very interesting experiment with the use of wireless technology for EV BMS:
  https://www.analog.com/media/en/technical-documentation/technical-articles/S65-Wireless_BMS_EN.pdf

If you feel I should write in detail about specific topics, please let me know in comments. Feel free to post any questions. I will try my best to answer them!

--Harshad