Yes! The batteries you’ve ordered and that will give your IoT solution 8 years lifetime in the field have just arrived!
As a meticulous solution developer, you’ll want to check the quality of the incoming materials. So you set about your tests:
But Oh, disappointment, disillusion! Not only is the performance vastly different from what is indicated on the battery’s datasheet, but the capacity of the battery is well under the 8 years expected for your application!
Before you pick up the phone to complain to the after sales, you should read this article because this type of test is not representative of the battery’s behavior in the field. Here is why, and here is how to move forward with your incoming inspection test to get the real picture.
In a nutshell:
First of all you need to remember that battery cells are designed for a specific usage. Depending on their construction – bobbin or spiral – and their electrochemical system (we are more specifically dealing with lithium primary systems here), they would feature different behavior. For Lithium Thionyl Chloride systems, bobbin cells are optimized for low discharge currents, and for operations of up to 20 years, whereas spiral cells are designed for higher discharge currents, and for operations up to approximately 10 years. Conversely, Lithium Manganese Dioxide systems only come with a spiral design and are therefore optimized for high current discharge.
The surface of ions exchange for spiral cells is more important than that of bobbin cells, which is enabling higher current drains.
Low power electronics, deep sleep mode, improvement on components’ leakage currents are broadly adopted trends in the IoT world. Lithium Thionyle Chloride (Li-SOCl2) bobbin cells have been widely optimized to match the needs of these low power applications. So, when using a bobbin cell, a discharge under high current won’t be representative at all of the behavior of the battery, particularly within a low power application.
Let’s look at the datasheet to understand the principle (if you’d like some explanations about how to read a datasheet, head over here):
The impact of a high discharge current on the available capacity is shown on the “Capacity vs current at various temperatures” graph.
On this example of an LS 14500 bobbin Li-SOCl2 battery, we can clearly see that at 20°C, the cell doesn’t restore the same capacity under 20 mA or more, as under a continuous current drain of 1 mA.
Under a discharge current of 1 mA, the maximal capacity would be of 2.6 Ah at 20°C but with a higher discharge current drain, of say 20 mA, the restored capacity falls to 2.1 Ah. So by increasing the discharge current by 20, the overall capacity of the battery has fallen by nearly 20%.
Whereas this applies to any battery, this is even truer for Li-SOCl2 bobbin cells (Saft LS range), which are optimized for low discharge currents and long lifetime, typically for discharge currents in the range of a few mA. Thus, high current drains above 10 mA will lower the cell’s efficiency (with potentially some carbon saturation phenomenon) and the restored capacity will not be anywhere close to the nominal capacity (also called rated capacity). The results you would find by applying such capacity tests might be spread over a wide spectrum of values, the datasheet showing the most “typical” ones. Thus, applying a high rate continuous current to check the potential restored capacity of a cell doesn’t really make sense.
And there are more reasons for having a wide spread of results when discharging a cell with high currents.
Some of our customers have been occasionally puzzled by discharge curves under fast discharge rates, because the cell was not in the upright position during the test… Indeed, voltage stability after mid-discharge may also be impacted by cell orientation in fast continuous discharge conditions. This phenomenon is called “grassing effect”. When the contact between the carbon pores and the electrolyte droplets is temporarily lost, the voltage quickly goes down. When it is recovered the moment after (electrolyte goes back inside the pores), the voltage goes up. It is also a consequence of electrolyte starvation within the pores of the carbon support at the end of a high rate continuous discharge.
Again, these phenomena are only noticeable for bobbin cells under rapid discharge, when not used in an upright position. A low average discharge rate and long recovery time between high pulses will enable the battery to work fine whatever its orientation. This is why incoming tests in rapid discharge may lead to results considered as not acceptable, even though they are not representative or even relevant for the application in its field life conditions.
Cell orientation effects with a bobbin Li-SOCl2 cell such as Saft's LS cell-series, is a phenomenon only noticeable under a continuous discharge load combined with a high discharge rate (i.e accelerated discharge). It will never come into play and reduce the battery’s lifetime when the battery is used normally.
Some customers might also experience some failure while discharging a bobbin Li-SOCl2 cell if the discharge rate is too elevated. Indeed there exist a phenomenon called “cathode blocking” or “cathode limitation” phenomenon. During the electrochemical reaction leading to the creation of an electrons’ flow, the products of the electrochemical reaction (LiCl crystals) are stored in the carbon mass. These crystals need so go through pores that have to be opened to let them through: if the current drain is very high, it creates a massive flow of these discharge products coming to the carbon mass surface. If the discharge is continuous, the carbon mass surface can quickly get saturated. And when that happens, the chemical reaction simply stops even if there are some remaining active materials. Which means that the available capacity can’t be fully restored.
The pictures below illustrate this phenomena by showing the carbon mass surface of a bobbin Li-SOCl2 cells, at different continuous discharge rates: we can observe that the LiCl crystals are bigger when the rate increases, leading to a surface saturation of the carbon mass.
Discharge rate 30 mA Discharge rate 100 mA Discharge rate 200 mA
And that’s not all! Having good results under high and continuous discharge rates does not even guarantee that your cell will deliver the expected performance for your IoT application.
Good results in rapid discharge do not guarantee good results in the application, particularly for IoT applications, and for applications with pulse currents in general.
You might be tempted to test lithium primary batteries with high continuous currents during incoming inspection to make sure they will sustain the pulse current afterwards. The thing is: batteries can sustain pulsed requests very well, even after some time in the field, but they might restore low or poor capacity under a high rate continuous discharge. Conversely, you might actually end up getting good results when doing such an accelerated discharge: some battery manufacturers indeed optimize their chemistry exactly for these purposes.
A battery powering an IoT application is expected to have the same performance over its whole field life. So looking at the voltage response capability of a fresh cell doesn’t mean that the battery will reach the same voltage response under high pulse currents after ageing. Indeed, fresh cells performances ¬—particularly in voltage readings— do not reflect aged cells performance. Furthermore, cells’ behavior under high continuous current drains differs strongly from a cell’s response to IoT devices consumption profiles, where IoT devices are put into sleep mode most of the time and then reactivated to transmit data for a few seconds, thus generating a current burst.
The results that you can observe on the graphs below come from more than one year of testing. The voltage readings of Saft cells after a one month storage period at 70°C are above 2.4 V, while competitors are showing voltage readings below 1 V, not compatible with an expected voltage response to power an IoT solution for multiple years.
So what can we learn from these curves?
Instead of doing testing which is only 50% relevant, our best bit of advice would be to audit your battery manufacturer to verify their quality control processes. You can then make sure that they are checking the batteries frequently and that quality is consistent.
You can also ask them for a conformity certificate: battery certification services test the safety and quality of batteries and ensure compliance with relevant rules and regulations.
And if you’d rather test for yourself, you can perform an OCV (Open circuit Voltage) test, which consists of connecting a voltmeter to the positive and negative terminals and measuring the terminal post voltage with no loads or a CCV (Closed Circuit Voltage) test which consists of closing the circuit through a resistance (56 Ohm for a LS 14500 for example) and measuring after a short delay (2 s) the voltage on the load. A minimum criterion can be set for cell voltage recorded at 2 seconds, to sort out defective cells in a very efficient way. This is much more rapid and relevant than a discharge test!
If you’d like to find out more about incoming inspection, feel free to email us at energizeIoT@saftbatteries.com. We’ll be happy to guide you.
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