We seem to always want a battery that has the biggest "wallop." But at what cost? For what size? And at what weight? Let's start by considering specifications and terminology.
Impedance: This refers to the AC complex reactance, which is measured in ohms (for the DC resistance, see Internal resistance above). A more complete model of a battery includes inductance, capacitance, and two pure resistances. Typically, a generator that supplies 1,000 Hz is used to stimulate the battery to determine its reactance. For load currents that are continuously and quickly varying, impedance is a better measure than internal resistance (we will discuss this in more detail in a future post).
Specific energy: The energy-to-weight (or energy-to-mass) ratio measured in watt-hours per kilogram (Wh/kg) or megajoules per kilogram (MJ/kg). Also commonly (but perhaps incorrectly) referred to as energy density, this essentially defines how "strong" a battery is for its weight. Some manufacturers use a divided form of this, such as joules per gram (J/g).
Energy density: The energy-to-volume (or energy-to-size) ratio measured in watt-hours per liter (Wh/L) or megajoules per Liter [MJ/L]. This essentially defines how "strong" a battery is for its size. Some manufacturers use a divided form of this, such as joules per liter (J/L).
Energy/consumer-price: The cost of energy measured in Watt-hours per dollar Wh/$US. This allows different energy sources to be compared in terms of cost. Sometimes the ratio and units are converted and reversed, such as $US/kJ, for example.
Specific power: The power-to-weight (or power-to-mass) ratio measured in watts per kilogram (W/kg). This is not an energy specification -- it refers to the battery's ability to deliver power. Higher numbers generally mean a lower internal resistance. Maximum power delivery to a load occurs when the load resistance equals the sum of the internal resistance of the battery along with the resistance of any wires and connectors; however, this condition will not be very efficient in terms of total energy delivered due to the high I2R losses.
Cell: The smallest packaged unit that produces a voltage which is characteristic of the fundamental chemical reaction. When an individual cell won't produce enough voltage, current, power, or run-time by itself, then multiple cells can be connected together to produce modules and packs. For some chemistries, OEM pack or module manufacturers are required to have training and a certification to be able to sell them. This is due to the requirements for safety, devices, circuits, and use of materials (we will discuss this in more detail in a future post).
Equivalent lithium content (ELC): Used by safety, travel, and shipping agents, this is a crude measure (in units of grams) by which lithium batteries are classified specifying how much lithium a battery contains. Eight grams of equivalent lithium content are equal to about 100 watt-hours. To find the gram content of your lithium battery, multiply the watt-hours times (8/100); thus, ELC = Wh x 0.08.
Maximum continuous discharge current: The highest current that can be safely and continuously drawn from the battery that will not substantially degrade the specified overall performance of the battery or cause damage to the battery.
Maximum discharge pulse current: The maximum current at which the battery can be discharged for pulses of up to the manufacturer's rated number of seconds, followed by the rated recovery time. This limit is usually defined by the battery manufacturer in order to prevent excessive discharge rates that would damage the battery or reduce its capacity.
Cycle durability or cycle life: The number of discharge-charge cycles the battery can experience before it fails to meet specific performance criteria. Cycle life is estimated for specific charge and discharge conditions. The actual operating life of the battery is affected by the rate and depth of the cycles and by other conditions such as temperature and humidity -- the higher the depth-of-discharge, the lower the cycle life.
Charge/discharge efficiency: Specified as a percentage, this is a measure of the actual transfer of energy to the rated energy. For example, a calculation for discharges might be: (Actual-Energy/Rated-Energy) x 100.
Self-discharge rate: The percent of total capacity lost each month at no load. Slow chemical reactions that occur, even without any load, reduce the battery capacity. This is the primary reason that batteries have a limited shelf life.
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