Medical Equipment Batteries & Power Management -1

Battery Types -Application- Power Management

Batteries are the critical components of a Medical Device. Almost all the medical devices Pacemakers, Glucometers, Ventilators, OT Tables, Surgical Saw, etc; works on Battery. The Type of battery used also defines device Reliability and Performance.

At the time of Equipment design battery selections are made based on the Power requirements of Hardware. Electrical and physical Characteristics of Batteries are crucial, some of them are..

• Voltage, Backup capacity (mAh)

• Cost

• Charging/Discharging Time

• Life Cycle

• Dimensions, Weight

• Discharge Current

Battery-powered medical devices are regulated by the Federal Drug Administration (FDA). The products should demonstrate their compliance's with the regulation standards, such as IEC 62133, for Secondary Cells and Batteries and IEC 60086-4 for Primary Batteries like Lithium ion. The Electrical Safety of Medical devices is covered with IEC 60601-1.

What is a Battery?

A battery is a collection of one or more cells that undergo chemical reactions to create flow of electrons within a circuit. Battery cells are made up of Three main components: i) Anode; Negative Electrode which produces electrons, ii) Cathode; Positive Electrode and iii) Electrolyte; provides path for electron movement when an electrical circuit is connected.

Batteries achieve the desired operating voltage by connecting several cells in series; each cell adds its voltage potential to derive at the total terminal voltage. Parallel connection can attains higher capacity by adding up the total ampere-hour (Ah).

Types of Batteries

1. Primary Battery : Non-rechargeable (Alkaline, Coin Cells)

2. Secondary Battery : Rechargeable Batteries

1.1 Primary Battery : Non-rechargeable (Alkaline, Coin Cells)

Alkaline Battery types include AAA (1.5V), AA (1.5V), and 9V Battery.

AAA is commonly used in Fingertip Pulse Oximeters since they are low energy devices. Wide display of Pulse Oximeter uses rechargeable NiMH Batteries or even Lithium ion. 9V Alkaline battery is most commonly found in Multimeters. Coin cell batteries are widely used as Real Time Clock Battery (RTC) in Motherboards of Medical Equipments. These batteries are non rechargeable.

2.1 Lithium Ion

Li Ion Cell consists of a four layer structure: Li Metal Oxide Cathode with thin Aluminium foil as current collector, Carbon (Graphite) Anode with thin Copper foil as current collector, Separator fine porous Polymer film and Electrolyte made with Lithium as conductor.

Lithium batteries deliver high current pulses and are used in AED (Automated External Defibrillators), Monitors, Ventilators; and other Applications including implantable Pacemakers, Surgical Drills, etc.

Merits :

• The nominal Voltage is 3.6 or 3.85 V/cell.

• High Energy Density (250: 693 W-h/L) and Low Safety risks.

• Battery Capacity 150Wh of electricity in 1 Kg battery.

• Fast Charge/discharge rates (80-90%).

• It can operate in Extreme temperatures (–80 °C to 125 °C).

• 3-5% of self discharge per month.

• Recyclable : Composition Materials of Li are recyclable

Demerits:

• Rigid Physical shape.

• Expensive.

• Degrades at a faster rate with life of 400: 1200 cycles.

• Needs protection circuit to prevent Thermal runaway if stressed.

Lithium batteries are Low Maintenance battery, with no memory and no scheduled cycling requirement to prolong the battery's life. These are usually monitored for temperature variations using additional Electronics included inside battery. The charging, discharging cycles are controlled by a charger circuit; to prevent Overheating and run the risk of Explosion.

2.2 Lithium Polymer

Li Polymer Cell consists of four components: Li Metal Oxide Cathode, Carbon (Graphite) Anode, solid polymer Electrolyte such as Poly Ethylene Oxide, and a micro porous Polyethylene Separator with some moisture. These cells are protected by an electronic circuit, which prevents overcharging and over discharging of batteries when in use.

Merits:

▪ High Specific Energy.

▪ It can be made thinner than conventional Li-ion Batteries.

▪ Low cost compared to Lithium Ion.

▪ Weight is reduced more than 20% compared to hard shell Lithium ion

▪ Operating temperature ranges from 5 °C to 60 °C.

Demerits:

▪ Less Durable compared to Li Ion Battery.

▪ Protection circuits are needed.

▪ Gas may build up during charging in Prismatic & Pouch cells resulting in Swell.

Flexibility, Compact shape and Low Weight of these batteries make them suitable for Wearable Medical Device Applications. Lithium Polymer can be alternative for Lithium Ion.

2.3 Lead Acid

A Lead Acid battery consists of a spongy or Porous Lead Anode, Lead Oxide (PbO2) Cathode and Sulphuric Acid and Water as Electrolyte. A Typical Lead Acid battery can store 25Wh of electricity in 1 Kg battery.

A Sealed Lead Acid (SLA) and Valve-Regulated Lead Acid (VRLA) prevents the loss of water from electrolyte by preventing/ minimising the escape of hydrogen gas from battery; which prevents drying out during cycling. These are also available with a pressure vent to prevent build-up of excess pressure.

These batteries find application in electrically powered wheel chairs, Ventilators, Anaesthesia machine, etc. High discharge current of these batteries makes them suitable for Automobile applications like Car Engine starter.

Merits :

▪ Inexpensive, Low cost per Watt-Hour.

▪ The nominal Voltage is 2 V/cell.

▪ Low Self Discharge.

▪ Operating temperature ranges from –20 °C to 50 °C.

Demerits:

▪ Heavy in weight & Large size.

▪ High Maintenance and limited Life Cycle.

▪ Lead is Toxic and can result in Lead Contamination.

2.4 Nickel Metal Hydride (Ni-MH)

NiMH batteries consists of a Cathode Nickel-hydroxide; Hydrogen absorbing alloys (Hydride) Anode; and Potassium-Hydroxide (KOH) Electrolyte.

These are generally used in Low voltage, low backup requirement, and for Non transporting Medical Devices applications like Syringe or Infusion Pumps, Defibrillator etc.

Note : The above devices also have transport applications where battery specifications varies.

Merits:

▪ Single cell Voltage is 1.2 V.

▪ It can operate in temperatures ranging from –30 °C to 75 °C.

▪ Higher Life of 500-2000 cycles.

▪ Cheaper in cost compared to Li Ion.

▪ Less prone to memory.

▪ 40 % higher capacity over a standard Ni-Cd.

▪ Environmentally friendly.

▪ Profitable for recycling.

Demerits:

▪ Low Energy Density (140-300 W-h/L).

▪ Slow charging.

2.5 Nickel Cadmium (Ni-Cd)

It consists of Nickel Hydroxide Ni(OH)2 Cathode, Cadmium Anode and alkaline Potassium Hydroxide KOH Electrolyte. The Batteries are sealed and consists of a Recombinant System, which prevents loss of electrolyte by recombining gases released in chemical reaction rather than venting it to atmosphere. Operating temperature ranges from –20 °C to 45 °C.

Nickel Cadmium batteries are low cost batteries and they are preferred in short term battery backup applications like 1-3 Hours; but it requires longer Charging Time to prevent overheating.

Merits:

▪ The nominal Voltage is 1.2 V/cell.

▪ Low Cost.

▪ Compact size.

▪ Simple charging/Discharge circuit.

Demerits:

▪ Low Operating Temperature Range (Upto 45°C).

▪ Slow charging & Fast Discharging.

▪ Relatively Low Specific Energy 40-60W-h/kg.

▪ Cadmium is Toxic and should not be disposed of in landfills.

▪ Short Life Cycle : 500 and Above.

▪ Susceptible to Memory Effect.

Next Article of this series will be on other Power Management Devices.