Lithium-Ion Battery Overview

What is a lithium-ion battery? What are its considerations? Battery life is an important factor in many applications today. For implantable medical devices, patients need to be confident that the batteries will give them a long time between needing to be recharged, known as the recharge interval.

Almost as important, the battery's usable capacity and the time between charges will gradually decrease over its lifetime, determining how many years it will take to replace the battery. This determines how long the battery lasts and how many useful charge/discharge cycles it can last. When choosing a battery, longevity can be a key purchasing criterion - once a battery reaches the end of its useful life, it will need to be replaced, which will involve some kind of surgical procedure in the implanted device.

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Typically, a battery's operating time can be reduced by 20% before it needs to be recharged, which is considered a problem with capacity reduction. As a result, the service life of a rechargeable battery is often defined as the number of charge and discharge cycles before the capacity drops to 80% of its original value. It is important for designers of medical devices to have accurate information about the various rechargeable batteries available on the market. They need to be confident that when they look at batteries from different manufacturers, they are comparing the same parameters and that the numbers on the datasheet reflect real-life behavior.

In this article, we’ll look at how designers can ensure they have the right information on which to base their decisions. We will review what factors affect the service life of lithium-ion batteries. These factors are especially important because lithium-ion batteries are more susceptible to changes than other technologies and their performance is greatly affected by testing, use and storage conditions. .

How battery life is affected

As with any rechargeable battery, how quickly a user needs to find a charging point depends on several factors.

First, there are environmental factors such as temperature and vibration. Environmental factors have a large impact on battery life, with minimal degradation at temperatures around 25°C (which is generally considered for lithium-ion batteries). This also means that thermal management may be important in certain applications, such as electric vehicles, to ensure that the heat generated by charging or discharging does not make the battery temperature too high.

But for batteries in implantable medical devices, these factors typically don't have a significant impact. This is because the medical device remains at a constant temperature of approximately 37°C after implantation, with minimal shock or vibration. For medical devices, the main influence on the charging interval is the so-called "operational factor". These include the rate of charge and discharge and the percentage of full capacity the battery is charged or discharged to. Storage is also important - once a battery is installed in a device and put into use, what percentage of its charge is retained on long-term storage affects its behavior.

Charging voltage is key

In a lithium-ion battery, the potential difference between the positive and negative electrodes increases as the battery charges and energy enters the battery. As the battery discharges with use, it decreases and consumes energy. This means that the voltage that can be measured between the terminals is a reliable indication of how fully charged the battery is, and therefore how much energy can be left in it. For example, this voltage measurement allows your smartphone or laptop to determine the percentage of battery power remaining and then use that to estimate how long it will take before the battery uses up all its power.

For medical applications, lithium-ion batteries are typically rated at 3.6V or 3.7V. In practice, standard procedure is to charge the battery to a maximum of about 4.1V and discharge it to a low of 2.7V. The maximum voltage at which charging is stopped is called the end-of-charge voltage (EoCV) level. But what happens if we change these parameters? Instead, if we only discharge the battery to a minimum voltage above 2.7V, it will only discharge part of its charge. For example, if we stop draining the battery when it reaches 3V, it could mean that the battery is only discharged to 40% or 50% of its capacity.

This low-voltage point defines what is called the depth of discharge (DoD). Therefore, fully discharging a battery is considered 100% DoD, while we can measure life using a smaller percentage of DoD. Another change that may affect lifetime is lowering the upper voltage endpoint from 4.1V to a lower value. These changes are caused by different chemical reactions taking place in the lithium-ion battery, which, for example, degrade the electrolyte or deposit insoluble compounds on the anode, reducing its efficiency.

These voltage changes make a surprisingly large difference in practice. If we slightly change the upper and lower limits of the voltage, the number of charge and discharge cycles in the battery life can only be reduced to 20% of the previous one, or even less. Although we are only discussing small changes in charge and voltage here, it is important to remember that rechargeable batteries may eventually be completely discharged in the field. For example, a patient may simply forget to charge the battery at the correct time, causing its voltage to drop too low, or the patient may have stored the battery for an extended period of time.

This is a different issue than the lifetime changes we just discussed, but for many lithium-ion batteries, this complete discharge can cause damage that greatly reduces their usability. EnerSys® solves this problem with its Zero-Volt™ technology, which ensures the battery can still operate at peak capacity even after the battery is discharged to zero volts (Figures 1 and 2).

Test Results

When we tested our own Quallion® batteries, we demonstrated excellent low capacity fade performance while cycling the battery at a Depth of Discharge (DoD) value of 100% (as low as 20%). As a result, the loss of battery capacity is minimized even after multiple charge and discharge cycles. Capacity fading performance can even be further improved during electrical cycling by reducing the end-of-charge voltage (EoCV). Modifying the EoCV from the maximum recommended value of 4.1V to a lower value of 4.0V will increase the battery's usable capacity.

When we look at 100% DoD, which is a common use case in medical applications, we check the cycle life of the battery when it reaches 80% of its initial retained capacity. Most medical applications specify this duty cycle life value, and the required value will vary depending on the intended use of the medical device. Typically, medical applications require batteries to satisfy 500 to 1,000 cycles at 100% DoD cycling conditions while maintaining 80% of the battery's initial retained capacity. The Qualllion® chemistry used in its medical batteries exceeds these cycle requirements with a retained capacity of 80% or greater.

in conclusion

Even small differences in operating conditions (such as charge and discharge voltage) can have a significant impact on battery life or service life. This means device designers should ensure they compare batteries in a similar manner and should check the test conditions specified by the battery manufacturer on their data sheets. The above is the relevant analysis of lithium-ion batteries, I hope it can help you.