Energy Storage

A battery converts chemical energy into electrical energy. The source of this energy is the electrochemical reactions that occur within the battery. The type and amount of the materials participating in these reactions determine the deliverable energy of a battery.

Chemical Reactions

During a spontaneous chemical reaction, substances react to form reaction products. For example, in combustion, a fuel combines with oxygen (O₂) from air to form water (H₂O), carbon dioxide (CO₂), and other species. The fuel is oxidized (loses electrons) during this process. Oxygen from air is reduced (gains electrons). The exact amounts of each that react are determined by the stoichiometry of the reaction. Equations 6-1 to 6-4 are examples of different types of reactions in which oxidation and reduction occur, called “redox” reactions, because electrons are transferred from one reactant to another.

(6-1)

(6-2)

(6-3)

(6-4)

Such chemical reactions occur spontaneously because the products are in a lower energy state than the reactants. The difference in energy appears as heat in the case of combustion, or rusting. A battery or galvanic cell is designed to convert much of this energy difference into electrical energy rather than heat. A battery operates because the electrons transferred during a redox reaction are channeled from one terminal of the battery through the external circuit and back to the battery through its other terminal. The maximum work these electrons can do outside the battery is related to the difference in a thermodynamic quantity called the free energy of the reaction.

Anode and Cathode

The two battery components involved in the electrochemical reaction during discharge are the anode and the cathode. The anode, lithium (Li) in most medical batteries, furnishes electrons to the external circuit, while the cathode, usually an oxide or halogen-rich compound, receives the electrons.

The anode is defined as the electrode at which electrochemical oxidation occurs, and the cathode is the electrode at which electrochemical reduction occurs. Oxidation is a process in which electrons are removed or lost, and reduction is a process in which electrons are gained. This is true in all electrochemical cells. In a spontaneous galvanic cell such as a battery, the anode is electrically negative and the cathode electrically positive. This creates a seeming inconsistency. In nonspontaneous electrochemical reactions, such as those that occur at pacing electrodes, electrochemical reactions are driven by an externally applied voltage. In this case, oxidation (loss of electrons) will occur at the positive electrode and reduction (gain of electrons) at the negative electrode. Thus, for pacing electrodes, the anode is positive and the cathode is negative. The polarities are reversed, but the underlying electrochemical processes defining anode and cathode—oxidation and reduction, respectively—are the same. The terminology is entirely consistent.

Electrolyte

The anode and the cathode must be separated so that they cannot directly react with each other, but they also need to be connected by an ionically conducting medium called the electrolyte. As the battery discharges, it furnishes electrons at one terminal, pushes them through an external circuit, and receives them back at a second terminal. The electrolyte allows electric charge, as ions, to flow within the battery, completing the circuit. The electrolyte must conduct ions but not electrons. If the electrolyte conducted electrons, the battery would be shorted internally just as if the terminals were connected by a metal wire. Batteries with lithium anodes, such as those used to power pacemakers and ICDs, must use nonaqueous electrolytes because water readily reacts with lithium. Most Li-based batteries employ a mixture of organic ethers and esters as solvents for the electrolyte. A dissolved Li salt is used to make the electrolyte conductive. For example, lithium/manganese dioxide batteries, widely used to power automatic cameras, often use electrolytes containing a lithium salt dissolved in dimethoxyethane ether and propylene carbonate ester.

Separator

The separator is a structural member of the battery that keeps the anode and cathode materials physically apart, thus preventing shorting of the battery. In batteries with liquid electrolytes, the separator is usually a porous polymer film that is immersed in and permeated by the electrolyte. In the case of lithium/iodine (Li/I₂) batteries, traditionally used for pacemakers, the separator and electrolyte are the same (i.e., growing layer of solid, ionically conductive LiI discharge product).

Current Collector

The current collector makes the connection between the positive or negative terminal of the battery and its respective active electrode material inside the cell. A current collector is usually a wire connected to a screen, or grid, which is embedded in the anode or cathode material. The current collector may also serve as a structural member of the battery to provide physical integrity and strength to that electrode. Some medical batteries use the case of the cell as the current collector for one of the electrodes. Batteries are termed “case negative” if the anode is in contact with the case and “case positive” if the cathode is in contact with the case.

Primary Batteries

Primary batteries can only be used once. They are not designed to be recharged; in fact, it is dangerous to attempt to recharge them. A familiar example of a primary battery is the alkaline zinc/manganese dioxide flashlight battery. Most batteries used to power modern implantable medical devices are primary batteries that use lithium anodes because such batteries have very high energy densities.

Secondary Batteries

Secondary batteries are designed to be repetitively discharged and recharged. Familiar examples include the lead-acid batteries used in automobiles and lithium-ion batteries, widely used in portable electronic devices, home appliances, power tools, and electric vehicles. Lithium-ion batteries are also the most relevant secondary medical battery, used as supplemental power sources in some left ventricular assist devices and widely used to power implantable neurologic stimulators for control of chronic pain. Although not currently in development for pacemakers and ICDs, features such as more frequent telemetry could lead manufacturers to reconsider lithium-ion battery use in cardiac applications in the future.

Capacity

The fundamental unit of battery capacity is the coulomb (C), or ampere-second (A-sec). This is the amount of charge delivered by one ampere (1 A) of current in 1 second. In the context of implantable devices, it is customary to express battery capacity in terms of ampere-hours (A-hr), which represents the charge carried by a current of 1 A flowing for 1 hour; 1 A-hr = 3600 C. A battery for an implantable medical device usually has a capacity rating of to 2 A-hr. The methods for determining and specifying the capacity of a medical battery produce numbers ranging from upper-limit theoretical values that can never be achieved in the field to realistic values based on detailed models or accelerated testing.^1–3 More sophisticated methods account for limited availability of the components within the battery, kinetic limitations on the chemical reaction, and losses to side-reactions over time.

Estimating the deliverable capacity of implantable medical batteries is especially difficult because of their long service life. The time frame for the operation of most implantable medical devices is so long (5-10 years) that real-time measurements of capacity are not practical. Therefore, accelerated tests and models are typically used to estimate the amount of deliverable capacity in these batteries. Technology in this area is now highly developed, and it is possible to make highly accurate projections of deliverable battery capacity under a range of usage conditions.^4–6

Energy and Energy Density

The fundamental unit of energy is the joule (J). This is the energy given to one coulomb (1 C) of charge that is accelerated by a difference in potential of one volt (1 V). One joule is also the energy transferred by one watt (1 W) of power in 1 second. Just as battery capacity is often measured in ampere-hours, battery energy is often expressed in watt-hours (W-hr) instead of joules; 1 W-hr = 3600 J.

An important battery parameter in the design of an implantable device is energy density, a quantity that can be expressed on either a mass or a volume basis. For medical applications, volume is usually more important than mass, so ratings based on volumetric energy density are most often used. The time integral of the product of voltage and current divided by the total volume of the battery is its deliverable energy density. Modern batteries for implantable devices have energy densities as high as 1 W-hr/cm³, including the case.

Stoichiometry and Cell Balance

The specific amount of anode and cathode materials that will react is determined by the stoichiometry of the battery reaction. A cell that contains exactly the required ratio of anode and cathode materials is said to be a “balanced” cell. However, most medical batteries are not designed with exactly the stoichiometric ratio of the active cathode and anode to provide predictable end-of-service characteristics. In many cases the anode (lithium) is in excess so that the voltage decrease near the end-of-service life is not too abrupt.

Cell Voltage and Current

The open-circuit voltage of a battery can be calculated from the thermodynamic free energy for the discharge reaction. This is the voltage that will be measured when there are no kinetic limitations, a condition that occurs only when an insignificant amount of current is being drawn from the battery. With the onset of current flow, the voltage at the battery terminals will be smaller than the open-circuit value. Both chemistry and battery design determine the relationship between voltage and current drawn from the battery. For example, a lead-acid battery for automotive use is constructed of very conductive materials and is designed with large-surface-area electrodes so that extremely high currents can be drawn from it to run an engine’s starter motor. On the other hand, a transistor radio battery is designed with small electrodes because relatively low currents are typically needed to power small portable electronic devices. Figure 6-2 shows a typical current-voltage relationship in which the load voltage approaches the open-circuit voltage (OCV) as the current approaches zero. At the other extreme, the maximum (short-circuit) current is observed when the load voltage approaches zero.

Figure 6-2 Typical load voltage versus current drain plot for a battery.

The load voltage is always less than the open-circuit voltage (OCV). Current drain increases toward the right. The maximum cell voltage is obtained at zero current, and the maximum current drain occurs at zero load voltage. The exact shape of the plot depends on the chemistry and design of the battery.

Internal Resistance and Impedance

Electrical impedance and resistance are important battery properties that play a significant role in the clinical performance of many implantable devices. The terms impedance and resistance are often used interchangeably but are not the same. Both are terms for the change in voltage per unit change in current in an electric circuit, but they are measured under different conditions. Impedance is the more general term, encompassing effects of resistance, capacitance, inductance, and other circuit elements on the relationship between voltage and current. The resistive component of impedance is measured using direct-current methods. Alternating-current and transient methods are used to measure the additional components of impedance besides resistance.

For simple, resistive electric circuit elements, Ohm’s law, V = IR, accurately describes a linear relationship between voltage drop, V, and the corresponding change in current, I, with resistance, R, as the proportionality constant. However, a battery is a complex electrochemical device with several nonlinear processes operating in series/parallel combinations. Different processes may dominate at different current levels, depths of discharge, and time. Consequently, the relationship between current and voltage for a battery is, in general, nonlinear, even at very low currents. Although this relationship is sometimes characterized by a quantity called R_DC, Ohm’s law should only be applied to batteries with caution.

Non-Ideal Battery Behavior

In addition to the principles and the nomenclature of battery operation, it is also important to understand factors that limit a battery’s ability to power an implantable device.

Polarization

Polarization is any process that causes the voltage at the terminals of a battery to drop below its open-circuit value when it is providing current. The internal resistance of the battery is one important cause. This is well illustrated in Figure 6-3 for the lithium/iodine battery, but the same is true for all batteries to some degree. The differences in the curves for discharge voltage versus capacity at four rates of constant current discharge are mainly caused by the voltage drop associated with internal resistance of the battery. Other contributing elements of voltage loss when a battery provides current are concentration polarization, which is associated with concentration gradients that may develop in the electrolyte or the active electrode materials, and activation polarization, which is associated with the kinetics of the electron-transfer reactions at the electrode/electrolyte interfaces.

Figure 6-3 Load voltage versus capacity plot.

For a lithium/iodine battery discharged at four magnitudes of constant current. Curve A = 40 µA/cm², curve B = 20 µA/cm², curve C = 10 µA/cm², and curve D = 2 µA/cm². The deliverable capacity decreases with increasing current in this current range because of increasing effects of polarization.

When current is drawn from a battery, all these processes occur to some extent. The net effect of these kinetic limitations is always observed as a decrease in the voltage at the terminals of the battery. In general, neither concentration polarization nor electron-transfer polarization conforms to Ohm’s law.

Battery Design Requirements

Implantable medical devices must be viewed as a system. The main requirement in battery selection for implantable devices is high reliability. Other factors include the desired longevity of the device (directly related to battery energy density, circuit design, and overall size) and an appropriate indication of impending battery depletion (end-of-service warning). The basic considerations when designing a battery for an implantable medical device include the current variations that will be imposed by the circuit as the device provides its service for the individual patient. Once the span of these application requirements is formulated, the current, voltage, and capacity requirements of the battery can be determined. An important limitation is the physical space, both volume and shape, within the device that can be allotted to the battery. Once this is determined, the required energy density can be calculated, and various battery systems and designs can be considered and evaluated. To put these decisions in perspective, one needs to realize that the battery occupies about a third of the volume of an ICD. The circuitry occupies another third, and the capacitors fill the remaining volume. In a pacemaker the battery and circuitry each occupy about half the volume.

Power Requirements

An important parameter for a device is the peak power requirement. For example, the rate of energy consumption differs greatly for pacemakers and defibrillators. Pacemakers use very small amounts of energy when they stimulate the heart, on the order of 15 µJ. Defibrillators, on the other hand, deliver as much 40 J for a defibrillation shock. A battery optimized for a pacemaker could never come close to supplying energy at the rate required to power a defibrillator. Likewise, a defibrillator battery is not an optimum choice to power a pacemaker, although it could easily supply the current needed. The high-power design of a defibrillator battery has a significantly lower energy density than that of a pacemaker battery, by a factor of as much as two. Thus, if a defibrillator battery was used primarily for pacing, and everything else were equal, it would need to be twice as large as an optimized pacemaker battery to obtain the same longevity.

Optimizing a battery for longevity and power becomes more complicated when a device performs multiple functions, such as both bradycardia pacing and defibrillation. For example, up to now, the lithium/iodine battery has been the dominant power source for implantable cardiac pacemakers, which typically have peak power demands of 100 to 200 µW. Under these conditions, the lithium/iodine battery, which is still used in many pacemakers, can maintain an adequate voltage, even when its internal resistance reaches several thousand ohms. On the other hand, an implantable cardiac defibrillator may have peak power requirements approximately 10,000 times greater than those of a pacemaker. Under such a high power demand, the voltage of an Li/I₂ battery would drop to almost zero, and the power delivered to the device would be almost nil.

In recent years the distinction between a need for high-rate and low-rate batteries has become more blurred because features such as distance (“wireless”) telemetry and multisite pacing need both more current and more capacity to operate. The result is that battery designers have been challenged to develop medium-rate batteries that can deliver more power than pacemaker batteries of the past while still having a high energy density.

Average vs. Instantaneous Current Drain

Implantable medical devices are often characterized by their average current drain, although events such as therapy delivery and telemetry may require temporary current excursions that can differ greatly from the average value. The effect of the instantaneous current demand can be mitigated by using a capacitor that buffers the battery to allow short bursts of power that may be more than two orders of magnitude higher than it could deliver directly. This allows the use of battery designs with reduced anode and cathode surface areas and improved volumetric efficiency. The same principle involving a battery plus a capacitor is used to deliver the defibrillation therapy to the heart from an ICD, even though the magnitude of the current and voltage is much greater.

Shape, Size, and Mass Constraints

All device requirements (e.g., longevity, end-of-service indication, peak power) must be balanced against device volume, shape, and mass. Volume and thickness are particularly important for safe implantation in children and for esthetic reasons in many adults.

The ratio of volume to surface area in the battery is an important parameter. The performance of a battery of fixed volume will vary substantially depending on its electrode surface area/volume ratios. The operating current and longevity demanded of the battery determine both the minimum areas and the amounts of anode and cathode needed. Increasing the electrode area/volume ratio increases current capability, but this ratio must not be too large, or the battery will be too costly and will have a diminished energy density and most likely greater complexity, raising reliability concerns.

Size, Energy Density, and Current Drain

The relationship between battery size and average current is not one of direct proportionality. For example, decreasing the average current by 50% will not permit a 50% reduction in battery size without compromising longevity because of the inactive materials in a battery (e.g., case, electrolyte, current collectors). Likewise, the usable energy density is also a function of the current demand on the cell. As the current from the cell is increased, the resulting voltage drops significantly (see Fig. 6-3), which reduces the time during which the cell can provide current at or above the minimum voltage necessary to operate the electronic circuits. Thus, usable energy density, which is directly proportional to the area under the discharge (voltage vs. capacity) curve, is also reduced. For very-high-rate batteries such as those used to power ICDs, this is not such an issue because their internal resistance is extremely low.

Effect of Pulse Width on Pacing Current

Increasing the pacing rate, pulse width, or pulse amplitude increases the average pacing current. The average pacing current, excluding static current, is directly proportional to the pacing rate. Recall that the pacing pulse results from the discharge of a capacitor through the electrode-heart interface. This capacitor produces a pulse in which the current decays exponentially with time, as shown for two different pulse widths in Figure 6-4.

Figure 6-4 Comparison of charge delivered to heart as pacing pulse width is shortened.

The two cases (A and B) assume a discharge into the same patient. The pulse width, t_w, is typically between 0.2 and 1.5 msec, and the maximum current is between 2 and 15 mA, depending on the pulse generator output voltage and the lead-heart interface resistance.

Thus, the time-dependent behavior of the current during the pacing pulse is given by the following equation:

(6-7)

where V_A is the amplitude at the beginning of the pacing pulse, R_H is the impedance of the lead plus the heart (discussed later), C is the value of the capacitor that delivers the pacing pulse, and t is the time since the beginning of the pacing pulse. In Figure 6-4, A and B, the pulse width is t_w and t_w/2, respectively. The area under each current-time curve gives the total charge delivered during the pulse. Although the width of the pulse in B is half that of the pulse in A, the charge delivered by this pulse is considerably more than half that of the longer pulse. The exact ratio of the charge delivered in the two cases depends on the values of R_H and C. Nevertheless, reducing the pulse width by a given fraction will always reduce the average pacing current by a substantially smaller fraction because of the exponentially decaying shape of the pacing stimulus current curve.

Effect of Pulse Amplitude on Pacing Current

The definition of pacing pulse amplitude may vary somewhat between manufacturers of implantable pulse generators. For our purposes, pulse amplitude is defined as the voltage delivered to the heart at the beginning of the pacing pulse (“leading edge” voltage). As stated earlier, the area under the current-time curve gives the charge delivered per pulse. Thus, doubling the amplitude doubles the current and the total charge delivered to the heart. Also, because the charge per pulse is doubled, it might seem that the average pacing current drawn from the battery would also be doubled. However, the impact on the pacing current is much larger than that, as seen from the following argument. The energy per pacing pulse is defined as follows:

(6-8)

In Equation 6-8, V_A is the average pacing stimulus output voltage of the pulse generator, I_A is the average pacing current delivered to the heart, and t_w is the pulse width. If we consider the lead-electrode-heart interface to be mainly resistive, Ohm’s law, I = V/R, can be substituted in Equation 6-8, which becomes the following:

(6-9)

In Equation 6-9, R_H