The main advantage of lithium-ion battery technology lies in its significantly improved energy density. For the same volume and weight, lithium-ion batteries can store and release more energy than other rechargeable batteries. Energy density is measured in both volumetric and gravimetric ways. Lithium-ion technology can currently offer a volumetric energy density of nearly 500 Wh/L and a gravimetric energy density of 200 Wh/kg (see Figure 1).
Compared to other technologies, lithium-ion batteries release more energy and are smaller and lighter. Lithium-ion batteries operate at a higher voltage than other rechargeable batteries, typically around 3.7V , compared to 1.2V for NiCd or NiMH batteries. This means that when multiple other batteries are needed, only one lithium-ion battery is required. Higher battery energy density in portable instrument designs results in smaller product size and better portability. Smaller battery packs allow engineers to utilize the extra space to add more new features to the same product (see Figure 2).
Self-discharge
Rechargeable batteries will continuously lose capacity. This phenomenon is called self-discharge. However, if stored properly, most of the lost capacity can be recovered.
All batteries should be stored at room temperature (25°C or lower) to maintain maximum capacity. End users must store SLA batteries at low temperatures and charge them to approximately 100% capacity each time to maintain optimal performance. Sealed lead batteries have approximately 20% self-discharge capacity after 6 months at 25°C; however, this value increases to approximately 30% after 6 months at 40°C. NiMH batteries should follow similar recommendations to avoid reactant deactivation due to prolonged storage. NiCd and NiMH batteries have approximately 20% self-discharge rate after 1 month at 25°C, after which the rate of increase in self-discharge slows significantly.
Conversely, lithium-ion batteries achieve their optimal cycle life when stored at 30-50% charge. The self-discharge capacity of a lithium-ion battery stored at 25°C for 6 months is only 10%.
Ratio characteristics
When selecting materials, the inrush current and maximum discharge rate of the terminal device should be taken into account. High-rate discharge of batteries or battery packs can cause voltage drops. If this is not considered in the design, the terminal device may shut down due to insufficient voltage.
High-rate NiCd batteries can achieve continuous discharge rates of 2C (twice the battery's rated capacity) or even higher, depending on the battery materials and internal impedance. Many SLA batteries can achieve continuous discharge rates of 3C or even higher. Most lithium-ion batteries have a continuous discharge rate of only 1C, but new batteries using this technology have extremely high continuous discharge rates, reaching 80A and lasting for 30 seconds, giving them a significant advantage in competition with NiCd and SLA batteries.
Cycle life
Battery cycle life is the number of charge-discharge cycles a battery undergoes before its capacity drops to a specified percentage of its original capacity. Lead-acid batteries typically have a cycle life of 250 to 500 cycles, depending on the manufacturer's product quality and the depth of discharge (discharging capacity up to 60% of rated capacity). NiCd, NiMH, and lithium-ion batteries generally withstand 500-700 charge-discharge cycles, with their capacity dropping to only 80% of rated capacity. Regardless of the chemistry used, the deeper the battery's discharge, the fewer usable cycles the user will have.
Charging differences
Lithium-ion batteries require different charging methods than other batteries. SLA batteries are best charged using constant voltage charging (C/10), typically at 1/10 of the rated capacity, for 14-16 hours. Alternatively, trickle charging or float charging can be used at rates of C/20 to C/30. For NiCd batteries, charging is recommended to terminate at -ΔV, at which point the charger voltage reaches its peak. Due to the heat generated by NiMH batteries, temperature monitoring is required during charging, with ΔT/Δt being the preferred method. Specialized fast-charging NiCd and NiMH batteries can be charged for 4-6 hours at C/2-C/3 rates. Ultra-low damping nickel batteries, a type of fast-charging battery, can be charged for 1 hour at a 1C rate. Finally, constant current/constant voltage charging (CC/CV) is recommended for lithium-ion batteries.
Typically, devices powered by lithium-ion batteries can reach 80-90% charge from their initial low energy state after being charged at a 1C rate for 60-75 minutes to 4.1V . Other batteries, except for specially designed batteries that can be charged with high current, may require more time to reach 80-90%. Lithium-ion batteries also need to be slow-charged for 4-5 hours to 4.2V to obtain the remaining 10-20% charge. This charging method has two advantages: users can obtain near-full charge in a very short time, and the actual voltage after charging will never exceed 4.2V .
It's important to note that charging lithium-ion batteries to 4.1V instead of 4.2V can extend their cycle life, but the usable capacity per charge will decrease. In some medical devices, the battery serves as a backup, always kept charged for immediate availability. The chemistry of lithium ions makes them unsuitable for trickle charging; lithium-ion batteries cannot be charged using constant float charging. However, several methods can effectively reduce the likelihood of overcharging lithium-ion batteries without damaging the battery or affecting the medical device. One method is to ensure the battery is discharged to at least 20% before triggering a standard charge. Lithium-ion technology significantly improves energy density compared to SLA, which is sufficient in most cases to prevent lithium-ion batteries from being fully charged.
Safety circuit
Each battery technology has its own set of safety considerations. NiCd battery packs incorporate some form of current interruption device to prevent serious malfunctions, which is essential for good battery design. NiMH has a thermally heating chemical property, so the battery must be equipped with a thermal sensing device connected to the charger to prevent overcharging, and the battery pack itself also has a current interruption device. In lithium-ion battery packs, lithium metal is produced when overvoltage occurs. This means that safety circuits should be used in the battery to maintain the battery voltage within a specific range during charging and discharging (see Figure 3).
While SLA batteries generally do not require external safety components, many medical device manufacturers still insist on placing the non-resettable fuse inside or around the battery. Because most SLA batteries have protruding positive and negative plates, they are prone to short circuits when placed on metal plates, which are ubiquitous in healthcare devices, without a fuse. These batteries also pose other short-circuit hazards. In the event of a short circuit, the device could potentially explode. Lithium-ion battery packs pose a lower risk of short circuits; their safety circuits primarily protect the battery itself.
Adding safety circuitry to batteries increases the cost of the device and requires more space. Designers must recognize that these are trade-offs to be considered during battery selection. Overall, despite the presence of safety circuitry, lithium-ion batteries can still reduce battery pack size and weight while releasing more energy.
Battery monitoring
More and more medical device manufacturers are adopting lithium-ion technology, and battery management features are becoming increasingly common in the industry. Battery monitoring devices can provide end users with information such as estimated battery life. The introduction of management features has significantly clarified battery capacity assessment and the implementation of charging plans.
In terms of battery management, designers using lithium-ion batteries have a variety of options. For example, some lithium-ion battery power monitoring devices contain information features that can report the number of charge-discharge cycles that have been completed. This information plays an important role in some critical medical devices. There are two basic methods for power monitoring: voltage-based and coulomb counting. Solutions that combine both technologies achieve an accuracy of up to 99%.
High temperature resistance
Lithium-ion batteries outperform other batteries at temperatures of 40°C-45°C. SLA and NiMH batteries fail to function properly in high-heat environments. This limits their use in emergency tools, as users cannot preserve their portable devices in low-temperature conditions.
in conclusion
When selecting the optimal power solution for portable devices, its total cost and overall performance must be evaluated. The high-voltage characteristics of lithium-ion technology can reduce the amount of battery used, thereby lowering the cost of battery packs to roughly the same level as those using nickel-based technology. Furthermore, lithium-ion battery suppliers are continuously using new materials to reduce battery costs.
Lithium-ion batteries possess significant advantages due to their small size, light weight, high energy density, long cycle life, good durability, high voltage, and good heat resistance. Medical electronics manufacturers can leverage these characteristics to expand their product markets and ultimately bring therapeutic benefits to consumers, healthcare professionals, and patients.
Consumer electronics and many other industries are increasingly emphasizing product mobility, and medical device manufacturers are no exception. This trend has improved the performance of field rescue equipment, monitoring devices, and fixed medical equipment, thereby driving the development of the healthcare industry. However, in addition to portability, medical device manufacturers certainly want to create highly reliable devices, as people's lives are often hanging by a thread. A broken cell phone is certainly annoying, but if a portable cardiac monitor or infusion pump stops working due to a dead battery, the end user—the patient—faces a much more serious problem.
A few years ago, medical professionals couldn't bring life-saving equipment to the field because portable devices weren't yet technologically advanced. Today, however, a plethora of monitoring instruments, ultrasound equipment, and infusion pumps can be used far from hospitals—even on the battlefield. Portability is increasingly convenient. Thanks to technologies like lithium-ion batteries, bulky 50-pound defibrillators can be replaced by lighter, more compact, user-friendly devices that don't strain the muscles of healthcare workers.
Patient mobility is becoming increasingly important. Today, patients may be transferred from the radiology department to the intensive care unit, from an ambulance to the emergency room, or by ambulance from one hospital to another. Similarly, the widespread availability of portable home-use devices and mobile monitoring equipment allows patients to stay where they prefer, rather than necessarily remaining in a medical facility. Portable medical devices must truly achieve full portability to provide optimal service to patients.
The demand for smaller, lighter medical devices has increased significantly, greatly stimulating interest in higher energy density and smaller battery packs. Lithium-ion battery technology used in laptops and mobile phones has already seen many breakthroughs, which medical device design engineers can leverage for innovative applications.
Compared to other traditional technologies, lithium-ion batteries offer many advantages for use in portable medical devices. These include higher energy density, lighter weight, longer cycle life, better capacity retention, and a wider operating temperature range.
Due to its unique chemical properties, lithium-ion technology imposes different design limitations compared to previous battery technologies such as nickel-metal hydride (NiMH), nickel-cadmium (NiCd), and sealed lead-acid (SLA). Furthermore, medical devices have more stringent operational requirements in some aspects than consumer electronics; because reliability is paramount, powerful battery packs with accurate charge monitoring and reliable operation are necessary.
This article outlines key considerations for designing portable power systems, taking into account the requirements of medical devices and the characteristics of lithium-ion technology. It also compares the characteristics and capacities of lithium-ion batteries with those of other chemical batteries.
Energy density and voltage
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