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May 11, 2017

PowerStream Battery Chemistry FAQ

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Batteries come in a lot of different varieties, and many years of work have been put into investigating different chemistries. The most common are carbon-zinc, alkaline, lead acid, nickle metal hydride, nickle cadmium and lithium ion. But there are many other battery chemistries, each with their own advantages and disadvantages. Below we discuss different of the battery designs currently used, some of the chemistry involved, and advantages and disadvantages of each design. We have also included some useful definitions and a list of parameters to guide you in matching your battery requirements to a specific battery design.


Table of Contents
  1. Definitions
  2. Battery Parameters
  3. Primary Batteries (disposable)
    1. Leclanché Cells
    2. Alkaline Cells
    3. Mercury Oxide Cells
    4. Zinc/Air Cells
    5. Aluminum/Air Cells
    6. Lithium Cells
    7. Lithium Iron Primary
    8. Magnesium-Copper Chloride Reserve
  4. Secondary Batteries (rechargable)
    1. Lead–Acid Cells
    2. Nickel/Hydrogen Cells
    3. Nickel/Cadmium Cells
    4. Nickel/Metal Hydride Cells
    5. Sodium/Sulfur Cells
    6. Nickel/Sodium Cells
    7. Lithium Ion Cells
    8. Manganese-Titanium (Lithium) Cells
    9. Rechargeable Alkaline Manganese Cells
    10. Nickel Zinc Cells
    11. Iron Nickel Cells
    12. Iron Air Cells
    13. Iron Silver Cells
    14. Redox (Liquid Electrode) Cells
  5. Supercapacitors
  6. References and Other Helpful Sources

Definitions

  1. Anode: The electrode where oxidation (loss of electrons) takes place. While discharging, it is the negative electrode; while charging it becomes the positive electrode.
  2. Amps: Also known as Amperes. This is the rate at which electrons flow in a wire. The units are coulombs per second, or since an electron has a charge of 1.602 x 10-19 coulombs, an amp is 6.24 x 10+18 electrons per second. Think of marbles rolling through a tube. If 6.24 x 10+18 pass by in 1 second you wuld have an amp of marbles.
  3. Amp hours: Also known as ampere hours. This a measure of the amount of charge stored or used. For example if you had an amp of marbles flowing out of your tube into a bucket for an hour, you would have one amp-hour of marbles in the bucket ( 6.24 x 10+18 times 3600 seconds = 2.2 x 10+22 marbles. A 1 amp hour battery contains enough charge to supply 1 amp for 1 hour, if you discharge at the same rate. Usually if you discharge faster than the rate at which the the amp hours were specified you will get fewer amp-hours out. You may notice that amp-hours and coulombs measure the same quantity-charge. One amp-hour is 3600 coulombs, but amp hours are easier to use in battery design. So remember, amps are flow ( "this motor requires 2 amps to run at 1800 rpm.") Amp hours measure capacity, quantity, or amount of charge ("this 100 amp-hour battery will supply 2 amps for 50 hours before recharge." Amp-hours are amps times hours, not amps divided by hours.

So Amp-Hours, (AH), or milliamp-Hours (mAH) is a measure of the size of the battery a 10 mAH battery has half the capacity of a 20 mAH battery, even though they may be in the same physical package.

  • Batteries: Two or more electrochemical cells, electrically interconnected, each of which contains two electrodes and an electrolyte. The redox (oxidation-reduction) reactions that occur at these electrodes convert electrochemical energy into electrical energy. In everyday usage, 'battery' is also used to refer to a single cell.
  • C:C represents the capacity of a battery divided by 1 hour, its units are amps. It represents a 1 hour discharge rate using the nominal capacity of the battery. So a discharge rate of 10C for a 5AH battery would be 50 amps. The concept of "C" is also used for charge currents, since both charge and discharge properties are proportional to the capacity of the battery, so a 5C charge rate for a 5 AH battery would be 25 amps.
  • Capacity: The total quantity of electricity or total ampere-hours available from a fully charged cell or battery.
  • Cathode: The electrode where reduction (gain of electrons) takes place. When discharging, it is the positive electrode, when charging, it becomes the negative electrode.
  • Charge: The conversion of electrical energy, provided in the form of current from an external source, into chemical energy stored at the electrodes of a cell or battery.
  • Discharge: The conversion of the chemical energy of a cell into electrical energy, which can then be used to supply power to a system.
  • Discharge curve: A plot of cell voltage over time into the discharge, at a constant temperature and constant current discharge rate.

    Discharge Curve Example

    Each curve in this graph represents cell performance at a different discharge rate. The farther right the curve ends, the lower the discharge rate (Crompton 31.4).

  • Dry cell: A Leclanché cell, so called because of its non-fluid electrolyte (to prevent spillage). This is achieved by adding an inert metal oxide so that the electrolyte forms a gel or paste.
  • Efficiency: For a secondary cell, the ratio of the output on discharge to the input required to restore it to its initial state of charge under specified conditions. Can be measured in ampere-hour, voltage, and watt-hour efficiency.
  • Electrolyte: The chemistry of a battery requires a medium that provides the ion transport mechanism between the positive and negative electrodes of a cell.
  • Energy density (specific energy): These two terms are often used interchangeably. Energy density refers mainly to the ratio of a battery's available energy to its volume (watt hour/liter). Specific energy refers to the ratio of energy to mass (watt hour/kg). The energy is determined by the charge that can be stored and the cell voltage (E=qV).
  • Fuel cell: A cell in which one or both of the reactants are not permanently contained in the cell, but are continuously supplied from a source external to the cell and the reaction products continuously removed. Unlike the metal anodes typically used in batteries, the fuels in a fuel cell are usually gas or liquid, with oxygen as the oxidant. The hydrogen/oxygen fuel cell is the most common. In this fuel cell, hydrogen is oxidized at the anode:
    half-reaction V vs SHE
    2H2 > 4H+ + 4e- 0
    4H+ + O2 + 4e- > 2H2O 1.2
    Hydrogen/oxygen fuel cell systems work well in space travel applications because of their high efficiency, high power-to-weight and volume ratios, and usable reaction product (water). They can function for many months as long as fuel is supplied and therefore the energy density cannot be measured.
  • Half-reaction: Refers to the chemical processes occurring at each electrode. The potential of the two half-reactions add to give us the overall cell potential. We can see this in the zinc mercury cell, for example:
    Location Reaction Potential
    Anode Zn + 2OH- > Zn(OH)2 + 2e- 1.25 V
    Cathode HgO +H2O + 2e- > Hg + 2OH- 0.098 V
    Overall Zn + HgO + H2O > Zn(OH)2 + Hg 1.35 V
  • Polarization: The voltage drop in a cell during discharge due to the flow of an electrical current. The cell's internal resistance increases with the buildup of a product of oxidation or a reduction of an electrode, preventing further reaction.
  • Power: Defined by voltage (V) and current (I), P=VI.

    Since V=IR, P=I2R and P=V2/R

    Power also can be described by energy emitted per unit of time: P=E/t.

    Thus E=VIt=qV.

  • Power density (specific power): Power density is the ratio of the power available from a battery to its volume (watts/liter). Specific power generally refers to the ratio of power to mass (watts/kg). Comparison of power to cell mass is more common.
  • Primary cells: A cell that is not designed for recharging and is discarded once it has produced all its electrical energy.
  • Prismatic: Just a word to say that the cells are not cylindrical, as nature intended battery cells to be, but fit nicely into a parallelepiped, rectangular or any other such flattened shape.
  • Peukart Effect: When a battery is discharged extremely quickly it will have less capacity than expected. This is the Peukart effect, which is very strong for lead acid batteries and many primary cells, but much less so for nickel cadmium, NiMH and lithium batteries. Peukert's equation is In · t = C, where I is the discharge rate, t is the discharge time and C is the capacity. The exponent "n" depends on the battery chemistry and the temperature. A log-log plot of discharge time versus discharge load will have a slope of "n"
  • Reserve cell: A primary cell that may be kept inactive and which is activated by adding an electrolyte or electrode, or melting an electrolyte which is normally in a solid state.
  • Secondary cells: A cell capable of repeated use. Its charge may be fully restored by passing an electric current through the cell in the opposite direction to that of discharge, thus reversing the redox reactions.


Parameters for Battery Performance

No one battery design is perfect for every application. Choosing one requires compromise. That's why it's important to prioritize your list of requirements. Decide which ones you absolutely must have and which you can compromise on. Here are some of the parameters to consider:

  1. Voltage: Normal voltage during discharge, maximum and minimum permissible voltages, discharge curve profile. Note that secondary cells are characterized by their average voltage during a full discharge, whereas primary cells ar characterized by their peak voltage. Thus an alkaline cells is rated at 1.5V, though its average voltage during discharge is 1.2V. A NiMH cell is rated at 1.2 volts, but its discharge range is 1.4 to 1 volt.
  2. Duty cycle: Conditions the battery experiences during use. Type of discharge and current drain, e.g., continuous, intermittent, continuous with pulses, etc. A 50% duty cycle means that the cell is off for half the cycle, and on for the other half of the cycle.
  3. Temperature: In storage and in use. Temperatures that are too high or too low can greatly reduce battery capacity.
  4. Shelf life: How rapidly the cell loses potential while unused.
  5. Service life: Defined either in calendar time or, for secondary cells, possible number of discharge/charge cycles, depending on the battery application. Service life depends on battery design and operational conditions, i.e., the stress put on a battery. For stationary and motive power application, the end of service life is defined as the point at which a battery's capacity drops to 80% of its original capacity. Exceptions would include car batteries where the service life ends when the capacity falls below 60%.
  6. Physical restrictions: These include dimensions, weight, terminals, etc.
  7. Maintenance and resupply: Ease of battery acquisition, replacement, charging facilities, disposal.
  8. Safety and reliability: Failure rates, freedom from outgassing or leakage; use of toxic components; operation under hazardous conditions; environmentally safe
  9. Cost: Initial cost, operating cost, use of expensive materials
  10. Internal resistance: Batteries capable of a high-rate discharge must have a low internal resistance.
  11. Specific energy: As discussed in the definition section, this is a measurement of possible stored energy per kilogram of mass. This number is purely theoretical as it does not take into account the mass of inactive materials, nor the variation in chemical reactions.
  12. Specific power: Also defined in the definitions section, a P=E/t, so the specific power is discussed at a specific discharge rate. It is possible for batteries with a high specific energy to have a low power density if they experience large voltage drops at high discharge rates. Specific power and specific energy can be compared in a Ragone plot . ragone plot example
  1. Unusual requirements: Very long-term or extreme-temperature storage; very low failure rate; no voltage delay, etc.

Of course the ideal battery would perform well in all these areas with a long shelf and service life, high specific energy and specific power, low initial and maintenance costs, low environmental impact, and good performance in a variety of conditions (temperatures, duty cycles, etc.). When you find one that meets all these requirements, let us know! In the meantime, we have to make do with batteries that work very well in specific applications.

References and Other Helpful Sources

  1. Berndt, D. Maintenance-Free Batteries. New York:: John Wiley & Sons, 1997.
  2. Crompton, T. R. Battery Reference Book. London: Butterworth–Heinemann, 1990.
  3. Linden, D. (Ed), Handbook of Batteries. Maidenhead: McGraw–Hill, 1995.
  4. Linford, R. G. (Ed), Electrochemical Science and Technology of Polymers. New York: Elsevier, 1990.
  5. Ovshinsky, S. R., Fetcenko, M. A., and Ross, J. A. "A Nickel Metal Hydride Battery for Electric Vehicles", Science 260: 1993, 176–81.
  6. Rechargeable Batteries Applications Handbook. Stoneham: Butterworth–Heinemann, 1992.
  7. Wells, A. F. Structural Inorganic Chemistry. Oxford: Clarendon Press, 1975.

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