Everything you need to know about reading a battery’s datasheet
Everything you need to know about reading a battery’s datasheet
Choosing the right battery for your device is a crucial part of the design process. Batteries come in various shapes and chemistries, with different benefits to each. One way of finding out if a battery matches your application’s profile is to review the datasheet against your design requirements – but how do you read these technical documents?
Here we explore datasheets, examining what we can learn from them, how to analyze the battery’s specifications against your application’s profile, and how to compare battery performance.
Knowing your application’s profile
To be able to properly analyze a battery against your use case, you’ll first need to determine:
- The temperature range at which your device operates (the temperature profile of the application).
- The cut-off voltage of the application.
- The maximum current of your application.
These are the most important parameters as they have a direct effect on the battery’s behavior and life expectation. We’ll delve deeper into these below.
A battery is not a constant voltage generator. Its behavior evolves over the battery’s lifetime depending on various parameters such as the temperature in the field, or the current drain. This is why the datasheet includes several graphs that model the battery’s behavior over different temperatures and according to various current drains, To make sure the battery will be able to meet your application’s needs look at these graphs first – and not at the written specifications (which only gives you fragmented information).
Ensure that the battery can deliver the required power across the device’s range of operating temperatures
Knowing the temperature’s profile is important in determining the voltage response of a battery.
The temperature in the field influences electrochemical efficiency: higher temperatures tend to increase self discharge, a phenomenon in batteries in which internal chemical reactions reduce the available energy of the battery without any connection between the electrodes or any external circuit.
Conversely, lower temperatures tend to reduce the rate of self-discharge and preserve the initial energy stored in the battery, but the inner active materials are less reactive at cold temperature so the battery system becomes less powerful. In both cases the voltage response during a pulse will drop over the battery’s lifetime.
You need to make sure that the battery can maintain a voltage above the cut-off voltage of the application over the entire temperature range of its intended use. The graph to help you is “Voltage plateau vs. current and temperature (at mid-discharge)”
How to read this graph
- Select your temperature range (from the key in the middle) and your maximum peak current (on the x-axis)
- Check your cut-off voltage on the y-axis. If the coordinates of the temperature and the discharge current go below your cut-off voltage, the battery won’t be able to power your device correctly.
For example: Your application is for residential use and will therefore be exposed to temperatures averaging 20°C (dark blue line). There is 50mA peak current in the device consumption profile and its cut-off voltage is 2,5 V. The line is well above the cut-off level (3.6 V), which means that the battery will be able to power your application over its whole lifetime. However, if your application cut-off voltage is at 3.7 V, this battery won’t work.
Ensure that the battery has the capacity to deliver the required power over its lifetime
The battery nominal capacity corresponds to the amount of energy that the battery can nominally deliver when fully charged, under a certain set of nominal discharge conditions.
For lithium thionyl chloride bobbin systems like the LS 14500 cell we are using throughout this article, it is at 20°C to 25°C and at a certain current rate, generally a few mA discharged down to 2 V.
As the rate of discharge goes up (above 10 mA), the capacity of the battery goes down. Consequently, you’ll need to make sure the selected battery can deliver the required energy to power your device for its entire lifetime.
You’ll find this answer in the “Typical discharge at 20°C under various rates” graph that shows how the cell loses power under various discharge conditions. But be warned, one can’t model every temperature on this graph so if your operating temperatures are outside of the given range, you’ll need to double check the information with the battery manufacturer
How to read this graph
The battery’s capacity is measured in Ampere-hours (Ah) on the x-axis and is the product of the current consumption multiplied by the hours to discharge the battery down to 2.0 V. (Ah = Current X Hours to Discharge down to 2.0 V.)
But then it’s really to put on hold until we get progress with the team.
The rate of discharge—at which a battery goes from a full charge to the cut off voltage—is measured in Amperes (A) or in this case, in mA, in the graph.
For Li-SOCl2 bobbin cells, which are optimized for discharge currents in the range of a few mA, the higher the discharge current, the quicker the discharge and the lower the overall capacity (Ah).
In this graph from the LS 14500 datasheet, the battery has a maximal capacity of 2.6 Ah at a discharge current of 1 mA (when at 20°C). With a higher discharge current, of 32 mA, the capacity falls to 1.42 Ah. By increasing the discharge current by this amount, the overall capacity of the battery has fallen by over 40%.
For example: Your device needs 32 mA to function and your cut-off voltage is 3 V, your maximum capacity will be 1.32 Ah and you will lose what is left of the battery’s capacity. But if you lower your cut off voltage to 2 V, you would then use your battery longer, thus optimizing its use. That’s why our application engineers often recommend lowering the cut off voltage of your application to its minimum to broaden your choice of batteries.
Ensure that the battery has the capacity to deliver the required energy during its lifetime
The performance of a battery drops at low temperatures whilst high temperatures improve performances but increase self-discharge and thus shorten the battery’s life.
Once you have verified that the battery has the capacity to deliver the required power over its whole lifetime, you’ll need to make sure that it does so at your given temperature range. That’s where the third graph comes into the picture: “Capacity vs. current at various temperatures”.
How to read this graph
In this graph, the battery’s available capacity measured in Ampere-hours (Ah) is indicated on the y-axis. The rate of discharge is indicated in milliamperes (mA) on the x-axis. The temperature is indicated by the curve in the middle of the graph.
Again, you can see that for a Li-SOCl2 bobbin cell, at all temperatures, the higher the current drain (mA), the quicker the discharge and the lower the overall capacity (Ah). But contrary to the previous graph, the curve starts slightly lower (for current drains lower than 1 mA) until it reaches its peak at nominal current drain. This data (where available) before the maximum available capacity corresponds to the capacity loss due to self-discharge.
What you need to look at here is the shape of the curve of your application’s given temperature:
- Does it correspond to the current need of your application?
- Is it stable over a wide range of discharge currents or does it go down quickly?
The more stable the line, the more stable the system and the greater the battery capacity.
For example: we can see that a 20°C temperature will offer the greatest capacity and one of the lowest self-discharge (the beginning of the curve is nearly flat). The battery would be ideal for an application necessitating a current between 0,3 mA and 7 mA. However, if the application needs more current, particularly over 10 mA, its capacity will degrade over time. In that case, we can augment the capacity by putting two or more cells in a battery pack.
Understanding the specifications on the first page
You should now be able to read a battery’s datasheet like a pro. But what about the information presenting on the first page of a datasheet?
The electrical values indicated in the table are typical values related to fresh cells, and discharged in very specific conditions. However, it’s unlikely that your operating conditions will be the same as these specific conditions and therefore, the data on the table won’t accurately reflect your usage. That’s why the graphs on the second page are so vital for correctly choosing a battery for your application.
Additionally, some batteries are subjected to electrochemical transitory phenomenon that cannot always be modeled on a datasheet as they depend on what the battery is being used for. For example, primary Lithium cells based on a liquid cathode technology develop an chemical phenomenon called passivation that protects the cell from self-discharge but can also cause voltage delays and drops.
That’s why we recommend double-checking with the battery manufacturer and undertaking a test in real-world conditions to check the battery behavior in your device.
Watch our video for more information on how primary lithium batteries work
Find out more
If, after reading this article, you are still confused about how to select the right source of energy for an IoT device, you can use our Smart IoT selector Tool that guides you through seven steps to determine which batteries match your application.
You can even play with the parameters of your application to find out in real time their impact on your battery choice. By downloading a report from the Smart Selector, you will have a full lifetime estimation for the performance of each of the batteries that match your project.