Characteristics and considerations of portable medical power system design

A few years ago, medical professionals couldn’t bring life-saving equipment to the field; the technology wasn’t there yet for portable devices. But today, a host of monitoring instruments, ultrasound equipment, and infusion pumps can be used far from the hospital—even on the battlefield. Portable devices are becoming more mobile. Thanks to technologies such as lithium-ion batteries, bulky 50-pound defibrillators can be replaced with lighter, more compact, user-friendly units that won’t strain the muscles of medical personnel.

Patient mobility is also becoming increasingly 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 one hospital to another by ambulance. Similarly, the popularity of portable home instruments and mobile monitoring equipment allows patients to stay where they like, rather than necessarily staying in a medical institution. Portable medical devices must truly be fully portable to provide the best service to patients.

对更小型、更轻便的医疗器械的需求也因此显著增加，这大大激发了人们对更高能量密度、更小巧的电池组的兴趣。笔记本电脑和手机使用的锂离子电池技术已经有了许多突破，医疗设备设计工程师可以对此加以创新利用。

Compared with other traditional technologies, lithium-ion batteries have many advantages in portable medical device applications, including higher energy density, lighter weight, longer cycle life, better battery capacity retention characteristics, and a wider applicable temperature range.

Due to its unique chemistry, lithium-ion technology imposes different design constraints than previous battery technologies such as nickel metal hydride (NiMH), nickel cadmium (NiCd), and sealed lead acid (SLA). At the same time, medical devices have more stringent operating requirements than consumer electronics in some respects; since reliability is very important, a powerful battery pack with accurate capacity monitoring and reliable cells is required.

1 Energy density and voltage

Figure 1. The energy density of lithium ions is much higher than that of nickel compounds.

锂离子电池技术的主要优势在于其能量密度的显著提高。相同体积和重量时，锂离子电池可储存和释放的能量比其它充电电池更高。能量密度以体积和质量两种方式测量。锂离子技术现可以提供近500Wh/L的体积能量密度和200Wh/kg的质量能量密度（见图1）。

Compared to other technologies, lithium-ion can deliver more energy in a smaller, lighter package. 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 a single lithium-ion battery can be used where multiple other batteries are needed. The higher the energy density of batteries used in portable instrument designs, the smaller the product and the more portable it is. The smaller the battery pack, the more space engineers can use to add new features to the same product (see Figure 2).

2 Self-discharge

Figure 2. Lithium-ion batteries come in cylindrical and prismatic shapes, allowing them to be manufactured in a variety of sizes and capacities.

Rechargeable batteries lose capacity over time. This phenomenon is called self-discharge. However, if stored properly, most of the lost capacity can be recovered.

所有电池均应在室温（25°C 或更低）下储存，以保持最大的电池容量。终端用户须将SLA电池放在低温下储存，并尽可能每次充电量接近其容量的100%，以保持最佳性能。密封铅电池在25°C下放置6个月后自放电容量约为20%；但40°C放置6个月后该值则增加到约30%。NiMH电池也应遵循类似的建议，避免长期储存使反应物失活。NiCd 和NiMH电池在25°C下放置1个月，其自放电率约为20%，随后自放电率的增速显著减慢。

In contrast, the best cycle life is achieved when lithium-ion batteries are stored at 30-50% charge. The self-discharge capacity of lithium-ion batteries stored at 25°C for 6 months is only 10%.

3. Rate characteristics

When selecting materials, the inrush current and maximum discharge rate of the end device should be considered. Discharging a cell or battery at a high rate will cause a voltage drop. If this aspect is not taken into account during design, the end device may shut down 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 battery materials and internal impedance. Many SLA batteries can reach a continuous discharge rate of 3C or even higher. Most lithium-ion batteries have a continuous discharge rate of only 1C, but new batteries using this technology have an extremely high continuous discharge rate of 80A, which can last for 30 seconds, giving them a great advantage in competing with NiCd and SLA batteries.

4 Cycle life

The cycle life of a battery is the number of charge and discharge cycles that a battery can go through before its capacity drops to a specified percentage of its original capacity. Lead-acid batteries have a cycle life of about 250 to 500 times, depending on the manufacturer's product quality and the depth of discharge (discharge capacity up to 60% of rated capacity). NiCd, NiMH and Li-ion batteries can typically withstand 500-700 charge and discharge cycles before their capacity drops to only 80% of rated capacity. Regardless of the chemistry used, the deeper the battery is discharged, the fewer cycles the user can use.

5 Charging Differences

Lithium-ion batteries are charged differently than other batteries. SLA batteries are best charged at constant voltage, usually at 1/10 of rated capacity (C/10), for 14-16 hours, or trickle or float charging, at rates of C/20 to C/30. The recommended termination of NiCd battery charging is -△V, when the charger voltage reaches its peak. NiMH batteries require temperature detection during charging due to their heat generation characteristics, with ΔT/Δt being the preferred method. Special fast-charging NiCd and NiMH batteries can be charged at rates of C/2-C/3 for 4-6 hours. Ultra-low damping nickel batteries, as a type of fast-charging battery, can be charged at 1C for 1 hour. Finally, constant current/constant voltage charging (CC/CV) is recommended for lithium-ion batteries.

Typically, a Li-ion battery-powered device can reach 80-90% of its original low energy state after charging at 1C rate for 60-75 minutes to 4.1V. Other batteries, except for special batteries that can be charged with high current, may take more time to charge to 80-90%. Li-ion batteries also need to be slowly charged for 4-5 hours to 4.2V to obtain the remaining 10-20% of the power. This charging method has two advantages. The user can obtain nearly full power in a very short time, and the actual voltage after charging will never exceed 4.2V.

须注意的是：如果仅将锂离子电池充至4.1V而非4.2V，可延长其循环寿命；但其每次可用的电量将会下降。在某些医疗器械中电池是一种后备装置，它始终保持充电的状态，以保证随时可用。锂离子的化学性质决定其不适合采用涓流充电；锂离子电池不能采用恒定浮充充电。但有几种方法可以在不损害电池或影响医疗器械的基础上，有效降低锂离子电池过度充电的可能性。方法之一是在触发电池再次充电前确保电池放电量至少为20%，随后进行标准充电。锂离子技术与SLA相比显著提高了能量密度，在大多数情况下足以防止锂离子电池电量完全充满。

6 Safety circuit

Figure 3. Electronic safety devices are required in the design of lithium-ion battery packs. The battery pack also contains fuel monitoring devices and charging circuits.

Each battery technology has its own set of safety considerations. NiCd battery packs have some sort of current disconnect device to prevent catastrophic failure, which is essential to good battery design. NiMH has a heat-generating chemical nature, so the battery needs to have a heat sensing device in it, which is linked to the charger to prevent overcharging, and the battery pack itself also has a current disconnect device. In lithium-ion battery packs, 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 elements, many medical device manufacturers still insist on placing non-resettable fuses in or around the battery. Since most SLA batteries have protruding positive and negative plates, without a fuse, they can easily short-circuit when placed on metal plates, which are abundant in healthcare equipment. These batteries may also present other short-circuit hazards. If a short circuit occurs, the device has the potential to explode. Lithium-ion battery packs are less likely to short-circuit, and the safety circuit is mainly used to protect the battery.

Adding safety circuits to the battery increases the cost of the device and consumes more space. Designers must be aware that these are trade-offs that will be considered in the battery selection process. Overall, despite the presence of safety circuits, lithium-ion batteries can still reduce the size and weight of the battery pack and can deliver more energy.

7 Power Monitoring

As more medical device manufacturers adopt lithium-ion technology, battery management features are becoming more common in the industry. Battery monitoring devices can provide end users with information such as the estimated battery life. The introduction of management features has largely clarified the execution of battery charge assessment and charging plans.

Designers using lithium-ion batteries have several options when it comes to battery management. For example, some lithium-ion battery fuel-gauging devices include information features that report the number of charge and discharge cycles that have passed. This information plays an important role in some critical medical devices. There are two basic methods of fuel-gauging: voltage-based and coulomb counting. Solutions that combine the two technologies can have an accuracy of up to 99%.

8 High temperature resistance

Lithium-ion batteries outperform other batteries at high temperatures of 40°-45°C. SLA and NiMH batteries do not function properly in high heat environments. This becomes a limiting factor for their use in first aid kits, as users cannot keep their portable devices in low temperatures.

in conclusion

This article summarizes the design considerations for portable power systems based on the requirements of medical devices and the characteristics of lithium-ion technology. It also compares the characteristics and capacity of lithium-ion batteries with other battery chemistries.

When choosing the best power solution for portable devices, it is important to evaluate its total cost and overall performance. The high voltage characteristics of lithium-ion technology can reduce the number of batteries used, thereby reducing the cost of battery packs to roughly the same level as batteries using nickel technology. In addition, lithium-ion battery suppliers continue to use new materials to reduce battery costs.

Lithium-ion batteries have the potential to be used in medical electronics manufacturers because of their small size, light weight, high energy, long cycle life, good durability, high voltage and good heat resistance, which can be used to expand the market for their products and ultimately bring therapeutic benefits to consumers, medical professionals and patients.