Advantages of portable medical device energy supply

In order to help the performance of on-site rescue equipment, monitoring equipment and fixed medical equipment, the development of the healthcare industry was further promoted. However, in addition to portability, medical device manufacturers certainly hope to be able to create highly reliable devices, because people's lives are often suspended. It is annoying that a cell phone is broken, but if a portable heart monitor or infusion pump stops operating due to a depleted battery, the problems faced by end users - and patients - are much more serious.

A few years ago, medical professionals were unable to bring life-saving devices to the scene; because the technology of portable instruments was not yet mature. But today, a large number of monitoring instruments, ultrasound equipment and infusion pumps can be used in places away from the hospital - even the battlefield. Portable devices are more and more convenient to move. Due to the application of technologies such as lithium-ion batteries, a bulky defibrillator weighing up to 50 pounds can be replaced by a lighter, more compact, user-friendly device, without straining the muscles of medical staff.

Patient mobility has also become more and more important. Today's patients may be transferred from the radiology department to the intensive care unit, from the ambulance to the emergency room, or from an ambulance from one hospital to another. Similarly, the popularity of portable home appliances and mobile monitoring devices allows patients to stay where they like and not necessarily stay in medical facilities. Portable medical devices must truly be fully portable and provide the best services for patients.

The demand for smaller, lighter medical devices has also increased significantly, which has greatly inspired people's interest in higher energy density, smaller battery packs. Lithium-ion battery technology used in laptops and cell phones has made many breakthroughs, and medical device design engineers can make innovative use of them.

Compared with other traditional technologies, lithium ion batteries have many advantages in the application of portable medical devices. This includes higher energy density, lighter weight, longer cycle life, better battery capacity retention, and a wider temperature range.

Because of their unique chemical properties, the design limitations of lithium-ion technology differ from previous battery technologies such as NiMH, NiCd and SLA. At the same time, medical devices have more stringent operational requirements than consumer electronics in some respects; because reliability is very important, there is a need for powerful battery packs with accurate battery monitoring and reliable batteries.

This article outlines the design considerations for portable power systems, combining the requirements of medical devices and the characteristics of lithium-ion technology. And compared the characteristics and capacity of lithium-ion batteries and other chemical batteries.

The main advantage of lithium-ion battery technology is its significant increase in energy density. With the same volume and weight, lithium-ion batteries can store and release higher energy than other rechargeable batteries. Energy density is measured in two ways: volume and mass. Lithium ion technology can now provide a volumetric energy density of approximately 500 Wh/L and a mass energy density of 200 Wh/kg.

Lithium ions release more energy than other technologies, and they are smaller and lighter. Lithium-ion batteries have higher operating voltages than other rechargeable batteries, typically about 3.7V, while NiCd or NiMH batteries have 1.2V. This means that if you need to use more than one section of other batteries, only one lithium-ion battery is required to meet the requirements. The higher the battery energy density used in the design of portable instruments, the smaller the volume of the product and the better the portability. The reduction in battery pack size means that engineers can use extra space to add more new features to the same product.

Rechargeable battery capacity will continue to lose. This phenomenon is called self-discharge. However, if it is properly stored, most of its lost capacity can still be restored.

All batteries should be stored at room temperature (25°C or lower) to maintain maximum battery capacity. End-users must store SLA batteries at a low temperature and try to charge as close as possible to 100% of their capacity to maintain optimal performance. Sealed lead batteries have a self-discharge capacity of approximately 20% after being placed at 25°C for 6 months, but this value increases to approximately 30% after 6 months at 40°C. NiMH batteries should also follow similar recommendations to avoid long-term storage and deactivation of reactants. When NiCd and NiMH batteries were left at 25°C for 1 month, their self-discharge rate was about 20%, and then the rate of self-discharge rate slowed down significantly.

On the contrary, the best cycle life is obtained when the charge capacity of the lithium ion battery is 30-50%. Lithium-ion batteries have a self-discharge capacity of only 10% after storage for 6 months at 25°C.

When selecting the material for rate characteristics, the inrush current and maximum discharge rate of the terminal device should be taken into account. Discharging batteries or battery packs at high rates can cause voltage drops. If this is not taken into account in the design, the terminal device may be turned off due to insufficient voltage.

The continuous discharge rate of high-rate NiCd batteries can reach 2C (twice the rated capacity of the battery) or even higher, depending on the raw material of the battery and the internal impedance. The continuous discharge rate of many SLA batteries can reach 3C or even higher. Most of the continuous discharge rate of lithium-ion battery is only 1C, but the new battery using this technology, its continuous discharge rate is extremely high, up to 80A, sustainable for 30 seconds, in the competition with NiCd and SLA battery has a great Advantage.

Cycle Life The cycle life of a battery is the number of charge and discharge cycles the battery experiences before the battery capacity drops to a specified percentage of its original capacity. Lead-acid batteries have a cycle life of approximately 250 to 500 cycles, depending on the manufacturer's product quality and depth of discharge (discharge capacity up to 60% of rated capacity). NiCd, NiMH, and lithium-ion batteries typically withstand 500-700 charge and discharge cycles, and their capacity drops only to 80% of their rated capacity. Regardless of the chemistry used, the deeper the battery discharges, the less cycles users can use.

Charging Differences Lithium-ion batteries are charged differently than other batteries. The SLA battery is preferably charged at a constant voltage, typically 1/10 (C/10) of the rated capacity, with a charging time of 14-16 hours, or trickle charge or float charge, with a charge rate of C/20 to C/. 30. The termination of NiCd battery charging is recommended to use the -ΔV method, in which case the voltage of the charger reaches its peak value. Due to its heat generation characteristics, NiMH batteries require temperature detection during the charging process. ΔT/Δt is the preferred method. Specially made NiCd and NiMH batteries can be charged at C/2-C/3 for 4-6 hours. A very low-damping nickel battery is a fast-charged battery that can be charged at 1C for 1 hour. Finally, Li-Ion batteries are recommended for constant current/constant voltage charging (CC/CV).

Typically, lithium-ion battery-powered devices can charge 80-90% of their capacity from their original low energy state after charging 60-75 minutes at 4.1C to 4.1V. Other batteries, except for special batteries that can be charged with high currents, may require more time to charge to 80-90%. Lithium-ion batteries also need to be charged slowly for 4-5 hours to 4.2V to obtain the remaining 10-20% of power. There are two benefits to this charging method. The user can get nearly full charge in a very short time, and the actual voltage after the charge is completed will never exceed 4.2V.

It should be noted that if you only charge a lithium-ion battery to 4.1V instead of 4.2V, you can increase its cycle life; however, the amount of electricity it uses each time will decrease. In some medical devices, the battery is a backup device that is always charged to ensure it is always available. The chemical properties of lithium ions determine that they are not suitable for trickle charging; Lithium-ion batteries cannot be charged with a constant float charge. However, there are several ways to effectively reduce the possibility of overcharging a lithium-ion battery without damaging the battery or influencing the medical device. One of the methods is to ensure that the battery discharges at least 20% before triggering the battery to recharge, followed by standard charging. Lithium-ion technology significantly increases the energy density compared to SLA, and in most cases is sufficient to prevent the Li-Ion battery from fully charging.

Safety Circuits Each battery technology has its own set of safety considerations. The NiCd battery pack has some kind of current breaking device to prevent serious malfunctions, which is essential for excellent battery design. NiMH has a heat-generating chemical property. Therefore, the battery needs to be equipped with a heat-inducing device, which is connected with a charger to prevent overcharging. The battery pack itself also has a current breaking device. In lithium-ion batteries, lithium metal is generated in the event of an overvoltage. This means that safety circuits should be used in the battery to keep the battery voltage within a specific range during charge and discharge (see Figure 3).

Although SLA batteries generally do not require external safety components, many medical device manufacturers still insist on placing non-resettable fuses inside or around the battery. Since most SLA batteries have prominent positive and negative plates, if there is no fuse, when it is placed on the metal plate, it is very easy to short circuit, and the metal plate exists in a large number of health care equipment. These batteries may also be at risk of other short circuits. If a short circuit occurs, the device may explode. Lithium-ion battery packs have a low risk of short-circuiting, and safety circuits are primarily used to protect the battery.

Adding safety circuits to the battery increases the cost of the device and consumes more space. Designers must realize that these are the trade-offs that will be considered in the selection of the battery. In general, despite the existence of safety circuits, lithium-ion batteries can still reduce the size of the battery pack, reduce its weight, and release more energy.

The more power-monitoring medical device manufacturers use lithium-ion technology, battery management features are becoming more common in the industry. The power monitoring device can provide some information for the end user, such as the expected usage time of the battery. The introduction of management features, to a large extent, clarified the battery power assessment and implementation of the charging scheme.

For battery management, designers using lithium-ion batteries have a variety of options. For example, some Li-Ion battery fuel gauge devices contain informational features that report the number of charge-discharge cycles that have passed. This kind of information plays an important role in some important medical devices. There are two basic methods of fuel gauge monitoring: voltage-based and coulomb counting. The accuracy of the solution that combines the two technologies is as high as 99%.

High-temperature-resistant lithium-ion batteries perform better than other batteries at high temperatures ranging from 40° to 45°C. SLA and NiMH batteries do not work well in high heat environments. This has become a limiting factor in its use in first aid kits because at this time, users cannot keep their portable equipment in a low temperature environment.

Conclusion When selecting the best power solution for a portable device, its total cost and overall performance must be evaluated. The high-voltage characteristics of lithium-ion technology can reduce battery usage, thereby reducing the cost of the battery pack, making it roughly equivalent to a battery using nickel technology. In addition, lithium-ion battery suppliers continue to use new materials to reduce battery costs.

Lithium-ion batteries have the potential advantages of small size, light weight, high energy, long cycle life, good durability, high voltage, and good heat resistance. Medical electronics manufacturers can use these features to broaden the product market and ultimately bring benefits to consumers, healthcare professionals, and patients.