Developing a quality product matters.

That’s why developers of IoT devices are meticulous in making sure the components of their products are as expected – and as required.

When it comes to batteries, there are a number of quick checks that are often performed during incoming inspection:

• Checking the box for damage and proper battery packing (e.g., no short circuits).
• A visual inspection of a selection of batteries for any signs of leaking or buckling.
• Checking the capacity of a selection of batteries in the batch.

This third check can cause concern.

During the device’s prototyping phase, and from discussions with the manufacturer, the batteries will have been chosen with a specific capacity in mind. Because developers don’t have a lifetime (ten years, perhaps) to wait – they run a continuous high rate discharge test to get results quickly.

More often than not – the performance and capacity are well below what was indicated on the battery’s datasheet.

This type of test, though, is not representative of the battery’s behavior in the field.

Here we explain why that is, and how you can properly inspect your primary batteries to make sure they’re what you expected.

Why an accelerated discharge is unhelpful

Some cells are not designed for high rate continuous discharge

Batteries come with different technologies and constructions – each designed for specific uses.

Depending on their construction – bobbin or spiral – and their electrochemical system (we are dealing with lithium primary systems here), they exhibit different behaviors. For Lithium Thionyl Chloride systems, bobbin cells are optimized for low discharge currents and an operational life of up to 20 years. 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 enables higher current drains.

A high rate discharge doesn’t lead to the same restored capacity

Most IoT devices are optimized for their energy need with low power electronics, deep sleep mode, and improvement on components’ leakage currents broadly adopted.

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 of the behavior of the battery, particularly within a low power application. As a result, 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 cell’s orientation can affect the results spread under high rate continuous discharge

Sometimes the reason people see puzzling discharge curves under high rate discharge tests is very simple – the cell was not in the upright position during the test.

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 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 unacceptable, even though they are not representative of the application in its field life conditions.

A high rate discharge may lead to a cathode blocking phenomenon

With an elevated discharge rate during testing, a battery (particularly a bobbin Li-SOCl2 cell) might experience a phenomenon called “cathode blocking” or “cathode limitation”.

During the electrochemical reaction that leads to the creation of an electron’s flow, the products of the electrochemical reaction (LiCl crystals) are stored in the carbon mass. These crystals need 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 be quickly saturated and, when that happens, the chemical reaction simply stops (even if there are some remaining active materials).

All this means that the available capacity can’t be fully restored.

For testing purposes, this means we don’t see the true behavior of the cell over the lifespan.

The pictures below illustrate this phenomenon by showing the carbon mass surface of a bobbin Li-SOCl2 cell at different continuous discharge rates. You can see that the LiCl crystals are bigger when the rate increases, leading to a surface saturation of the carbon mass.

A continuous discharge isn’t representative of the cells’ behavior under the application’s pulse profile

Some developers may be tempted to test lithium primary batteries with high continuous currents during incoming inspection to make sure they will sustain the required pulse current throughout the lifespan.

Primary batteries can sustain pulsed requests very well, even after some time in the field, making them ideal for IoT applications like meters or sensors. But they might restore low or poor capacity under a high rate continuous discharge and so testing at a high rate will yield ‘poor’ results.

A fresh cell behaves differently to an aged cell

For example, looking at the voltage response capability of a fresh cell may be misleading. A battery may not reach the same voltage response under high pulse currents after ageing.

A battery powering an IoT application is expected to have the same performance over its whole field life. Their consumption profile will include many thousands of bursts with devices often primarily in sleep mode, and activated intermittently to transmit data for a few seconds.

This means they age differently too.

That means a battery use in IoT devices will behave very differently to one exposed to a continuous high current drain – and day one performance needs to be replicated many years later after experiencing thousands of such activations.

How to properly inspect incoming batteries

Instead of doing testing which is only part-relevant, our best 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.

If you still want to test for yourself, you can perform an OCV (Open Circuit Voltage) test or a CCV (Closed Circuit Voltage) test.

• An OCV (Open circuit Voltage) test involves connecting a voltmeter to the positive and negative terminals and measuring the terminal post voltage with no loads.
• A CCV (Closed Circuit Voltage) test consists of closing the circuit through a resistance (56 Ohm for a LS 14500 for example) and measuring the voltage on the load after a short delay (2 seconds). A minimum criterion can be set for cell voltage recorded at 2 seconds to sort out defective cells in a very efficient way.

Both are much more rapid and relevant ways of assessing primary battery quality than a discharge test.

If you’d like to find out more about incoming inspection, feel free to email us at energizeIoT@saft.com.

We’ll be happy to guide you.

*This is an updated version of an article first published in February 2021.