W4 Batteries & Transcutaneous Energy

Question 1

Define “Battery”

Answer 1

• Device consisting of one or more electrically connected electrochemical cells which is designed to receive, store, and deliver electric energy. • (An electrochemical cell is a self-contained system consisting of an anode, cathode, and an electrolyte, plus such connections (electrical & mechanical) as may be needed to allow the cell to deliver or receive electrical energy)

Question 2

Difference between Anode and Cathode

Answer 2

• Anode - electrode whose active material is oxidised during discharge • Cathode - electrode whose active material is reduced during discharge (remember ‘Roman Catholic’ - RC for Cathode = Reduced)

Question 3

Implantable devices must take into consideration stimulus vs power requirements. Define the approximate thresholds for low and medium power requirements and list some examples

Answer 3

• Low power (average <1mW): pacemakers, spinal cord stimulators • Medium power (average ~1-100mW): defibrillators, cochlear implants

Question 4

Label the following components of the Pacemaker battery. What type of battery is it?

Answer 4

Li/I2-PVP

Question 5

Describe the specifications of the Li/I₂-PVP battery

Answer 5

Non-rechargeable

Life ~5-10 years

LiI is a good Li⁺-conducting solid while not conducting electrons or I^- ions, thus acting as electrolyte/separator

Structure greatly modified by coating the anode with polyvinylpyridine (PVP) - helps the battery to work more effectively

I₂-PVP cathode as an electronic conductor

Used in most pacemakers to-date

Question 6

Describe the specifications of the Li-ion battery.

What implantable is it commonly used for?

Answer 6

Neurostimulator battery

• Wound electrode assembly, typically case-negative, glass-to-metal seal for positive terminal
• C/LiCoO₂
• Rechargeable/zero volt discharge capability
• Longevity ~10 years (spinal cord stimulators)
• Typical de-ratings compared to non-implantable Li-ion batteries:
  
  • reduced energy density
  • reduced charge voltage
  • increased impedance
    
    • reduced maximum load capability (1-hour rate vs 0.1-hour rate)
    • safer, longer cycle life
• Declining voltage characteristic may serve as state-of-charge indicator
• Constructive precautions to avoid Li metal plating across electrode ridges or glass-to-metal seal of feedthrough
• Used in most rechargeable neurostimulators to-date

Question 7

Describe ideal charging characteristics for implantable Li-ion batteries

Answer 7

Faster the better, but for an implant with a receiver coil that transmits energy, any loss of energy turns into heat - cannot enter a zone that becomes unsafe for the patient (heat dissipation)

~80% charge in constant current phase

Manage discharge process; recharge a little, more often

Simple design (load regulation in implant) will have to appreciate corresponding internal heat losses

Complex design (external charge current regulation) is better, more challenging, takes longer

Question 8

What regulatory series governs the requirements for power sources?

Answer 8

ISO 14708 series (global)

1. General requirements for safety, marking & information
2. Cardiac pacemakers
3. Implantable neurostimulators
4. n/a
5. n/a
6. Implantable defibrillators
7. Cochlear implant systems

EN 45502 series (Europe) predated this ISO which was modelled off of it

US AAMI CI 86 - more detailed & stringent

EN / IEC 60601 series, Medical Electrical Equipment

Question 9

Describe the current exception limits for cells & batteries when it comes to Transportation (as of 1 Jan 2017)

Answer 9

Mediated by ICAO and IATA

< 2.7Wh Li ion

• Package cannot exceed 2.5 kg

Li metal < 1 g/cell

No more than 2 batteries or 8 cells per package, otherwise ship as fully regulated Class 9 dangerous goods

Li0ion < 30% SOC

No Li metal cells/batteries on passenger aircraft

Pacemaker batteries are lithium metal batteries, and have to be shipped separately. These constrains apply to implantable batteries!

Question 10

If you are to build a neurostimulator with the following parameters:

Amplitude 5mA

Pulse width 200us

Repetition rate 100Hz

Implant base load 0.5mA

Minimum supply voltage 2.0V, working off battery voltage

Desired minimum battery longevity 5 years

Calculate the total current, and decide whether it would more suited to a Li/I2-PVP or Li-ion battery type, and explain why?

Answer 10

Av current: 5x200810^-6 x 100mA = 0.1mA

Total current = 0.1mA + 0.5mA = 0.6mA

No further losses from min supply voltage - can use Ah capacity

Li/I2-PVP has 1.6Ah; 0.6mA current creates 2667 hours, = 111d < 5 years

Li-ion has 200mAh; 0.6mA current creates 333h = 14d per charge = 130 full cycles for 5y (full recharge every fortnight or partial recharges every day/week/as you like)

Question 11

What is the resonant frequency of a parallel circuit?

Answer 11

Where the inductance & capacitance combine to produce infinite impedance or an open circuit

w (omega, lowercase; radians/second) = 2*pi*f (Hz)

Question 12

Voltage in an implantable device peaks at ~14 VDC, however, our electronics can only handle a max of 12 VDC. How can this be rectified?

Answer 12

Use a Zener Diode - has a reverse breakdown voltage; e.g. if you cap output at 10 V at some node, when this node reaches the 10V threshold, then the diode will accept and level off the voltage to take the excess to group, to keep it 10V.

Question 13

What is an alternative to batteries for a circuit that maintains its power?

Answer 13

Full power supply circuit contains the components: tuning capacitor for resonance, rectifying capacitor, power supply capacitor to store charge, Zener diode to keep voltage clipped at e.g. 10V.

If this power supply cap can maintain enough power to get you through what you need to do - do not need battery