w5 Flashcards
(26 cards)
What is a battery?
a device consisting of one or more electrically connected electrochemical cells which is designed to receive, store, and deliver electric energy.
what is an electrochemical cell
a self contained system consisting of an anode, cathode, and an electrolyte,
plus some connections (electrical and mechanical)
as may be needed to allow the cell to deliver or
receive electrical energy.
what is an anode and cathode
Anode: Electrode whose active material is oxidised (gains e-) during discharge. Negative electrode as seen by the user (delivering electrons to the load).
Cathode: Electrode whose active material is reduced (loses e-) during discharge. Positive electrode as seen by the user (receiving electrons from the load)
what is an electrolyte
Ionically conductive salt by itself or dissolved
what does a separator do
prevents the electrodes from shorting
what are the two metrics for battery life
Ah/ampere hour capacity, equivalent to charge (current x time)
Wh/watt-hour enegy, equivalent to voltage x charge.
what are the two metrics for energy density
Gravimetric: Energy per mass unit. mWh/g = Wh/kg
Volumetric: Energy contained in a volume element. mWh/cm^3 = Wh/l
what is the average for “low” and “medium” power requirement
Low power: average <1mW
Medium power: average 1-100mW
given the complex impedance of implantable batteries, it is more complex to obtain a constant current output.
what is the maximum phase width to ensure that the battery of a cochlear implant will preceive phase durations as an average load (flat current, with some voltage ripple) i.e. the maximum phase width before pulsed load considerations are needed.
usually 10-50us.
what type of solid state battery does a pacemaker have?
Li/I2-PVP (Lithium/Iodine-Polyvinylpyridine)
are pacemaker batteries rechargable
no, must replace pacemaker at knee point
what is the equilibrium potential difference of pacemaker batteries
2.8V
what is the battery life of pacemakers
5-10 years depending on the number of pulses required
what is the base and pulse load of pacemakers
base: load required to keep device running
pulse: load required for pacemaking function
what are the advantages of Lithium iodine batteries
good conducting solid while not conducting electrons or I ions
LiI is both electrolyte and separator; self forming, self healing. allows for energy dense battery
Structure greatly modified by coating anode with PVP - helps the battery to work more effectively
the cathode is conducting
what type of battery is used in neurostimulators
Li-ion
what is the capacity, energy and energy density of neurostimulor batteries
capacity: 200mAh
energy: 200mAh x 3.6V = 720mWh
energy density: 720mWh / 3.3cm^3 = 220mWh/cm^3
how do li-ion batteries typically work? (charge and discharge)
anode is Li-graphite, cathode is Li-oxides
Charge: powering the battery, Li+ to anode, e- to anode.
Discharge: powering the device, Li+ to cathode, e- to load (towards cathode?).
what are some advantages of li-ion batteries
They are rechargable.
The cycle life / energy content / charge voltage of the battery can be modified by modifying the chemical composition ( # of Li atoms)
when is the best time to discharge a li-ion battery
around knee point for best battery life per charge
how do you design a li-ion battery to obtain zero volt discharge capacity
zero volt discharge capacity means the battery is fully charged - desired if battery is hard to charge or if it is inconvenient to charge often
Requires additives or certain current collector materials.
how do you make a li-ion battery safer and more long lasting (cycle life)
Reduce energy density.
Increase impedance.
Reduce charge voltage,
Constructive precautions to prevent Li metal from plating across electrode ridges or glass to metal seal (feedthrough).
what is a cycle life
The cycle life is the number of complete charge/discharge cycles that the battery is able to support before that its capacity falls under 80% of it’s original capacity.
what effect does reducing the charge voltage of a battery have, and what is the rule of thumb for this effect
reducing charge voltage reduces the % capacity of the battery, therefore increasing the cycle life.
Rule of thumb: Vcharge -0.1V => Cycle life x2 & capacity -10%
