Battery life is a critical factor in many applications today. For implantable medical devices, patients need to be confident that the battery will provide them with a long charging interval, known as the charging interval.
Almost equally important, a battery's usable capacity and the time between charging and discharging will gradually decrease over its lifespan, determining how many years it will take to need replacement. This determines the battery's lifespan and how many useful charge/discharge cycles it can be used for. Lifespan can be a key purchasing criterion when choosing a battery—once the battery reaches the end of its lifespan, it needs to be replaced, which would involve some kind of surgery in an implanted device.
Introduction
Typically, a 20% reduction in usable time before needing recharging is considered the point at which capacity reduction becomes a problem. Therefore, the lifespan of a rechargeable battery is usually defined as the number of charge/discharge cycles before its capacity drops to 80% of its original value.
For designers of medical devices, obtaining accurate information about the different rechargeable batteries available on the market is crucial. When reviewing batteries from different manufacturers, they need to ensure they are comparing the same parameters and that the figures on the datasheets reflect potential real-world behavior.
In this article, we will examine how designers can ensure they have the right information to guide their decisions. We will review which factors affect the lifespan of lithium-ion batteries, which is particularly important because lithium-ion batteries are more susceptible to change than other technologies, and their performance is greatly affected by testing, use, and usage conditions. Storage.
How is battery life affected?
How often a user needs to find a charging point for any rechargeable battery depends on a variety of factors.
First, there are environmental factors, such as temperature and vibration. Environmental factors have a significant impact on battery life, with minimal degradation occurring at around 25°C, which is considered a typical characteristic of lithium-ion batteries. This also means that thermal management can be important in some applications, such as electric vehicles, to ensure that the heat generated during charging or discharging does not cause the battery temperature to rise excessively.
However, these factors typically do not have a significant impact on batteries in implantable medical devices. This is because, after implantation, the medical device is maintained at a constant temperature of approximately 37°C, and experiences minimal shock or vibration.
For medical devices, the main factors affecting charging intervals boil down to so-called "operational factors." These include the charging and discharging rates and the percentage of full capacity the battery is charged and discharged to. Storage is also important—the percentage of charge a battery retains during long-term storage affects its behavior after it is installed in a device and put into use.
Charging voltage is key
In a lithium-ion battery, the potential difference between the positive and negative electrodes increases as the battery is charged, and energy is injected into the battery. This potential difference then decreases as the battery discharges during use, consuming energy.
This means that the voltage that can be measured at the terminals is a reliable indicator of whether the battery is fully charged, and therefore how much energy is left in the battery. For example, such a voltage measurement is how your smartphone or laptop determines the percentage of its remaining battery life, and then can estimate how much time you have left before the battery loses all its power.
For medical applications, lithium-ion batteries are typically rated at 3.6 V or 3.7 V. In practice, the standard procedure is to charge the battery to a maximum of approximately 4.1 V and then discharge it to a low voltage point of 2.7 V. The maximum voltage at which charging stops is called the charge termination voltage (EoCV) level.
But what happens if we change these parameters? Instead, if the battery only discharges to a minimum voltage above 2.7V, it will only discharge a portion of its capacity. For example, if we stop operating the battery when the voltage reaches 3V, this could mean the battery has only discharged to 40% or 50% of its capacity.
This low voltage point defines what's known as the depth of discharge (DoD). Therefore, a fully discharged battery is considered to be at 100% DoD, and we can measure lifespan using a smaller DoD percentage. Another change that could potentially impact lifespan is lowering the upper voltage threshold from 4.1 V to a lower value.
These changes are caused by different chemical reactions occurring in lithium-ion batteries, such as the degradation of the electrolyte or the deposition of insoluble compounds on the anode, which reduces its efficiency.
These voltage variations produce surprisingly large differences in practice. If we change the upper and lower voltage limits, even slightly, the number of charge/discharge cycles during battery life can only be reduced to 20% or even less of what it used to be.
Although we are only discussing minute changes in charge and voltage here, keep in mind that rechargeable batteries can eventually discharge completely in the field. For example, a patient might simply forget to charge the battery at the correct time, causing its voltage to drop too low, or the battery might be stored for an extended period of time.
This is a different issue from the lifespan variation we just discussed, but for many lithium-ion batteries, such a complete discharge can be damaging, significantly reducing their usability. EnerSys has addressed this issue with its zero-voltage technology, which ensures that the battery continues to operate at peak capacity even after being discharged to 0V.
test
When we tested our own Quarlion batteries, we demonstrated excellent low capacity degradation performance while cycling the batteries from 100% to 20% DoD. This means that even after many charge/discharge cycles, the loss of battery capacity will be minimized.
By reducing the EoCV, capacity degradation performance can be further improved during electrical cycling. Changing the EoCV from the maximum recommended value of 4.1 V to a lower value of 4.0 V will increase the battery's usable capacity.
When we look at 100% DoD (a common use case in medical applications), we examine the battery's cycle life when it reaches 80% of its initial retained capacity. Most medical applications specify this operating cycle life value, and the required value varies depending on the intended use of the medical device.
Typically, medical applications require batteries to withstand 500 to 1,000 cycles at 100% DoD while retaining 80% of their initial capacity. The Quarlion chemistry used in their medical batteries exceeds these cycle requirements, achieving 80% or better capacity retention.
in conclusion
Even small differences in operating conditions (such as charging and discharging voltages) can have a significant impact on battery life or lifespan. This means that device designers should ensure they compare batteries in a similar manner and should check the test conditions specified by the battery manufacturer on their datasheet.
