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Definitions

Ampere (A)
The amount of electrical current flowing in an electrical circuit.
Ampere-Hour Capacity (Ah)
The total amount of current (Amps) that can be discharged by a battery over a given period of time (hours) under specified conditions. Ah capacity will be dependent upon factors such as temperature, rate of discharge and depth of discharge. Battery Ah ratings are typically specified for a 20 hour rate for lead acid batteries and a 5 hour rate for Nickel Cadmium, Nickel Metal Hydride and Lithium Iron Phosphate batteries at a 25C ambient temperature. For example, a sealed lead acid battery with a 10 Ah rating can sustain a 0.5A current draw for 20 hours at 25C before its terminal voltage falls to a discharged level. In general, a battery’s capacity decreases with increasing discharge rates.
Battery
Two or more electrochemical cells electrically connected (in series or parallel) so that they provide the required operating voltage and current. The word battery is also commonly used to refer to a single cell.
C-Rate
The charge or discharge current in Amps, expressed in multiples of the rated Ah capacity. For example, a 2C discharge current for a battery this is rated 5 Ah would be 5 x 2 = 10A. Likewise, a C/20 discharge current for a battery that is rated 5Ah would be 5 x 1/20 = 0.25A.
Capacity
The specific energy in Ampere-hours (or watt-hours) that can be discharged from a fully charged battery under specified operating conditions.
Capacity Retention
The percentage of the full capacity remaining in a battery after it has been stored for a period of time.
Cell
The basic electrochemical unit of the battery, including electrode, electrolyte and case, that is used to store electrical energy. Cells are available in various shapes and sizes. Common sizes of Ni-Cd and Ni-MH rechargeable batteries (as well as non-rechargeable batteries) are AA, AAA, C and D.
Charge
The process of supplying direct current (electrical energy) to a battery, in the direction opposite of discharge, to restore the chemical energy used during discharge.
Chemistry
The most common battery chemistries are lead, nickel and lithium and each requires a specific charging algorithm. This is why different battery chemistries cannot be interchanged in the same device (e.g. emergency lighting unit). Each chemistry also has different regulatory requirements regarding recycling. Please recycle properly.
Constant Charging Voltage
The voltage applied to a rechargeable battery during the constant voltage charging process. The individual cells tend to share this voltage and equalize the charge between them. The initial charging current should be limited to prevent damage to the battery.
Cut-off Voltage
The battery voltage at the end of a full discharge. The cut-off voltage will vary by manufacturer and can be a function of the discharge rate.
Cycle
One consecutive discharge and charge sequence.
Cycle Life
The number of cycles under specified conditions that a rechargeable battery can provide before it fails to meet the device’s specified performance criteria.
Depth of Discharge
The percentage of the total rated capacity removed from the battery due to discharge operation.
Direct Current (DC)
Electrical current that flows in one direction only. Batteries only produce direct current (as opposed to alternating current) as the current flows from a negatively charged electrode to a positively charged electrode.
Discharge
The process of converting the chemical energy of a battery into electrical energy, resulting in the flow of direct current from a battery into a load or device; i.e. in the direction opposite of charge.
Drain
The current withdrawn from a battery during discharge.
Energy
The output capability of a cell or battery, typically expressed in watt-hours (Wh).
Energy Density
The ratio of the energy capability of a battery to is volume or weight, expressed in Wh/L or Wh/kg. Lithium ion batteries have a higher energy density than most other types of rechargeable batteries.
Float (Trickle) Charge
A low rate charge that compensates for the self discharge of a sealed lead acid battery and maintains it in a fully charged condition.
Internal Resistance
The opposition to the flow of direct current within a cell or battery, expressed in Ohms or milliOhms. It is composed of both electronic and ionic components. The greater the internal resistance, the larger the voltage drop (IR drop) of a battery and the less efficient it is.
Lead Acid
The oldest and still the most popular rechargeable battery in use today, main applications include motor vehicles and back-up power systems such as emergency lighting units. Otherwise known as Sealed Lead Acid (SLA), Absorbed Glass Mat (AGM), Valve Regulated Lead Acid (VRLA) or Gel Cell, its advantages include a high voltage per cell (approx. 2 volts), a good capacity life, maintenance-free designs and relatively low cost.
Lithium-Ion
One of the more recent rechargeable battery technologies, Lithium-Ion batteries have a lower self discharge rate and a higher energy density than most other types of rechargeables, providing up to 40% more capacity than similarly sized Nickel-Cadmium (Ni-Cd) batteries. As one of the lightest rechargeable batteries available, they are widely used in portable and low-profile devices.
Nickel-Cadmium
One of the most established and widely used rechargeable battery chemistries, Nickel-Cadmium (Ni-Cd) batteries are considered very reliable and robust with a high discharge rate capability, depth of discharge tolerance and excellent cycling performance. These characteristics make them very popular in high drain application such as portable power tools, cordless phones and emergency lighting. Due to the toxicity of Cadmium, Ni-Cd batteries are slowly being supplanted by other chemistries such as Lithium-Ion and Nickel-Metal Hydride.
Nickel-Metal Hydride
Available in the same standard sizes (AAA, AA, C and D), Nickel-Metal Hydride (Ni-MH) batteries are generally used in the same type of applications as Ni-Cd batteries. They provide up to 25% more capacity than Ni-Cds with an energy density similar to that of Lithium Ion batteries. Ni-MH batteries are more environmentally friendly since they do not contain cadmium, however, as with all rechargeable batteries, they should be properly recycled.
Nominal Voltage
The rated DC voltage of a battery or the voltage at which a device is designed to operate.
Polarity
In electrical terms, the condition of being either positively charged or negatively charged. On batteries or in DC circuits, the positive pole or terminal is typically marked red or “+” and the negative pole or terminal is typically marked black or “-“. Always maintain the correct polarity (“+” to “+” and “-“ to “-“) when terminating batteries.
Pure Lead
A type of sealed lead acid battery that utilizes a high purity lead-tin grid (99.99% pure lead), resulting in a rechargeable battery with a long life on a float charge. Depending on operating conditions, a float life of 10 years is possible.
Rated Capacity
The number of Ampere-hours a battery can deliver under specific operating conditions, such as ambient temperature, discharge rate and end voltage.
Rechargeable (Secondary) Battery
Comprised of one or more electrochemical cells, it is an energy source which, after being discharged, can be restored to its fully charged state by the application of an electrical current through the cells in the direction opposite to that of discharge.
Safety Vent (Valve)
A venting mechanism designed into some sealed batteries that prevents excessive gas accumulation and internal pressure by activating during specific conditions of voltage or current overcharging.
Self Discharge
The loss of a rechargeable battery’s useful capacity due to internal chemical reactions. Self discharge will occur in varying rates in all battery chemistries, whether the battery is connected to a load or not, and is highly dependent on ambient temperature. In general, the higher the temperature, the higher the self discharge rate.
Service Life
Expected period of useful life of a battery before a specified end voltage is reached. This period is highly dependent on operating conditions such as charging voltage and ambient temperature. In general, the higher the temperature the shorter the service life.
Shelf Life
The amount of time a battery can be stored under specified conditions and still retain its ability to meet specified performance requirements; generally a percentage of its stated capacity.
Standard (Constant) Charging Current
The constant direct current used to fully charge a rechargeable battery within the specified time period.
Standby (Float)
The use of batteries in an application that charges them to be ready for use if the primary (AC) power to the application fails; in emergency lighting units for example.
State of Charge
The remaining percentage of capacity remaining in a battery.
Terminal
A component at the end of a cell, battery or wire for making an electrical connection to an adjoining cell, battery or wire. Terminals typically used include Faston quick disconnect tabs (1/4” F2 or 3/16” F1), L terminals and threaded posts and inserts on sealed lead batteries, and 2-pole crimp connectors on Ni-Cd and Ni-MH wire leads.
Valve Regulated Lead Acid
A type of sealed lead acid battery that includes a venting mechanism designed to prevent excessive gas accumulation and internal pressure by activating during specific conditions of voltage or current overcharging.
Voltage (V)
The amount of electric potential difference between two points in a circuit, or of an energy source, such as a battery. All batteries are rated in Volts DC (VDC).
Watts (W)
A measure of electrical power, it is calculated by multiplying the electrical current flowing in a circuit by the voltage across which that current flows. One Ampere of DC current flowing across one Volt DC equals one Watt.
Watt-Hours (Wh)
A measure of electrical power produced or expended in a given amount of time. It is calculated by multiplying the ampere-hours by the voltage that the current flows across. For example, 5 amperes flowing for 2 hours across 6 volts would yield 60 Watt hours.