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If you are developing an IoT device, chances are that you’ll need a battery to power your connected object. Batteries come in all sorts of shapes and chemistries and not all batteries will be fit for purpose. One way you can find out if a battery matches your application’s profile is by carefully reviewing the datasheet against your design requirements. How much energy does my device consume? What battery capacity do I need to make sure my device works for the whole intended duration of its life? How will the battery react across the range of operating conditions I am intending to subject it to¬ —electrically and environmentally? How physically big is the battery and how much space do I need to plan in my design to accommodate it? These are some of the answers that you can derive from the datasheet.
However, this technical document may seem a bit daunting to a first time user.
This article intends to explain and clarify in plain English the most relevant specifications that you may find in a primary battery datasheet, how to analyze the battery’s spec against your use case, and how to compare battery performance…
First and foremost, and to be able to properly analyze a battery against your use case, you’ll initially need to determine:
These are the most important parameters as they have a direct effect on the battery’s behavior and life expectation.
Indeed, 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 you’ll find in the datasheet several graphs that model the battery’s behavior over different temperatures and according to various current drains (on page 2 in the example below). You need to look at these graphs first – and not at the written spec that can only give you fragmented information - to make sure that the battery will be able to withstand your applications needs.
Knowing the temperature’s profile is important in determining the voltage response of a battery. The temperature in the field has an effect on electrochemical efficiency: higher temperatures have a tendency 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 that the battery system, becomes less powerful at cold temperature (lower voltage and power). 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 that can help you with that is the “Voltage plateau vs. current and temperature (at mid-discharge)”
How to read the graph?
E.g.: Your application will be deployed inside a house and will therefore be exposed to temperatures averaging 20°C (orange line). There is 50mA peak current in my device consumption profile and its cut-off voltage is 2,5 V. The line is well above the cut-off level (3.4 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 3,5 V, this battery won’t work. Our application engineers can then recommend you an alternative that matches your needs.
Now let’s compare the performances of this battery to another one featuring the same specs. At 20°C and with a peak current consumption of 50 mA, there isn’t any data. Moreover, for a lower level of current i.e. 2 mA, the level of voltage differs (3.6 V for Saft, 3.4 V for the competitor).
The battery nominal capacity corresponds to the amount of energy that the battery can nominally deliver from fully charged, under a certain set of nominal discharge conditions: for lithium thionyl chloride bobbin systems, 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” 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 the graph?
The battery’s capacity is measured in Ampere-hours (Ah) on the X axis and is the product of the current consumption x the hours to discharge the battery down to 2.0 V.
Ah= Current X Hours to Discharge down to 2.0 V.
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, the battery has a maximal capacity of 2.6 Ah at a discharge current of 1 mA, at 20°C. With a higher discharge current, of 100 mA, the capacity falls to 1.15 Ah. By increasing the discharge current by 100, the overall capacity of the battery has fallen by nearly 66%.
So if your device needs 35 mA to function and your cut-off voltage is 3 V, your maximum capacity will be 1.5 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.
If we compare the discharge profile of two similar batteries, here is what we get for a device that needs say 20 mA to function with a 2.5 V cut off voltage:
The performances are pretty similar but you might as well go for the longest operation time.
As you know by now, 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. Now that you have verified that the battery has the capacity to deliver the required power over it’s whole lifetime, you’ll need to make sure that it does so at your given temperature range. That’s where the third graphic comes into the picture: "Capacity vs. current at various temperatures".
How to read the 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 contrarily to the previous graph, the curve is not linear and starts slightly lower (for current drains lower than 1 mA) until it reaches its peak at nominal current drain, which is 2 mA for this LS 14500 cell. This data 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! The more stable the line, the greatest battery capacity.
In this 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, its capacity will degrade over time. In that case, we can augment the capacity by putting two or more cells in a battery pack.
At 55°C, the self-discharge is more important so your capacity will be lower, unless your discharge current is higher than 0.3 mA and lower than 20 mA.
If your application requires a 10 mA current at 70°C then, with its capacity of 2.3 Ah, it could be the perfect battery for your device.
Now let’s compare the performances of this battery to one of our competitor’s. We can see in this second graph that at -20°C, the curve is unstable and the capacity will degrade much quicker than the Saft battery above. However, we can expect similar performances at 20°C or at 60°C.
If you’ve got this far, you should now be able to read a battery’s datasheet like a pro! You might be wondering about the information that is on page 1 of the datasheet and why we haven’t mentioned anything about that?
The electrical values that are 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 the datasheet’s and therefore, the data on the table won’t accurately reflect your usage. 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. Hence, the need to double-check with the battery manufacturer that the battery you shortlisted is adapted to your needs. A test in real conditions is also recommended to check the battery behavior in your device. That’s it for now. But if after reading this article you are still confused about how to select the right source of energy for your device, you can head over to our Smart IoT selector Tool, an online tool that helps you —in just 7 steps— discover which batteries match your use case, how much space you need to leave in your product design to accommodate them, an average estimation of their lifetime, and their price level. You can even play with the parameters of your application to find out in real time their impact on your battery choice.
And if you have any question or need any additional information about the battery’s datasheet, feel free to get in touch, we will be happy to help.
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