Active Material
Chemically active compounds in a cell or battery that convert from one composition to another while producing current (electrical energy) or accepting current from an external circuit. 

Battery Polarity
A battery has two poles or posts. The positive battery post is usually marked POS, P, or + and is larger than the negative post which is usually marked NEG, N, or -. The polarity of the charger and the battery must always match to avoid damage to the battery and charger.

Cell
The basic electrochemical current-producing unit in a battery consisting of a set of positive plates, negative plates, electrolyte, separators and casing. There are six cells in a 12-volt lead-acid battery.

Cold Cranking Amps
Cold Cranking Amps is a rating used in the battery industry to define a battery's ability to start an engine in cold temperatures. The rating is the number of amps a new, fully charged battery can deliver at 0° Farenheit for 30 seconds, while maintaining a voltage of at least 7.2 volts, for a 12 volt battery. The higher the CCA rating, the greater the starting power of the battery.

Container
The polypropylene or hard rubber case which holds the plates, straps and electrolyte.

Cover
The lid for the case/container.

Electrolyte
A solution of sulfuric acid and water which conducts current through the movement of ions (charged particles in the electrolyte solution) between positive and negative plates. It supplies sulfate ions for reaction with the active material of both positive and negative plates.

Grids
A lead alloy framework that supports the active material of a battery plate and conducts current

Ground
The reference potential of a circuit. In automotive use, the result of attaching one battery cable to the body or frame which is used as a path for completing a circuit in lieu of a direct wire from a component. Today, over 99% of autos use the negative terminal of the battery as the ground.

Intercell connections
Connections between the straps of two cells, positive of one cell to the negative of the next.

Open Circuit Voltage (O.C.V.)
The voltage of a battery when it is not delivering or receiving power. It is 2.11 volts for a fully charged battery cell.

Plates
Flat, typically rectangular components that contain the active material and a mechanical support structure called a grid, which also has an electrical function, carrying electrons to and from the active material. Plates are either positive or negative, depending on the active material they hold.

Reserve Capacity (RC)
Reserve Capacity, (RC) is a battery industry rating, defining a battery's ability to power a vehicle with an inoperative alternator or fan belt. The rating is the number of minutes a battery at 80 degrees F can be discharged at 25 amps and maintain a voltage of 10.5 volts for a 12 volt battery. The higher the reserve rating, the longer your vehicle can operate should your alternator or fan belt fail.

Separators
Porous plastic, electrically insulating sheets which allow transfer of ions between plates, but prevent physical contact between plates and resulting electronic conduction

State of Charge
Use this chart to determine the State of Charge for a Deep Cycle Battery
 

Straps
Lead alloy castings that connect a number of same polarity plates together in a cell and carry current

Terminals
The electrical connection from the battery to the external circuit. Each terminal is connected to either the first (positive) or last strap (negative) in the series connection of cells in a battery.

Vents
Components that allow gasses to exit the battery while retaining the electrolyte within the case. Can be permanently fixed to the cover or removable, depending on battery design.

Ampere-hours (A·h)
Ampere-hours (A·h) is the product of the time that a battery can deliver a certain amount of current (in hours) times that current (in amps), for a particular discharge period. This is one indication of the amount of total energy a battery is able to store and deliver at its rated voltage. This rating is rarely stated for automotive batteries.

Cranking amps (CA)
Cranking amps (CA), also sometimes referred to as marine cranking amps (MCA), is the amount of current a battery can provide at 32 °F (0 °C). The rating is defined as the number of amperes a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12 volt battery).

Cold cranking amps (CCA)
Cold cranking amps (CCA) is the amount of current a battery can provide at 0 °F (−18 °C). The rating is defined as the amperage a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery). It is a more demanding test than those at higher temperatures.

Hot cranking amps (HCA)
Hot cranking amps (HCA) is the amount of current a battery can provide at 80 °F (26.7 °C). The rating is defined as the amperage a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery).

Reserve capacity minutes (RCM)
Reserve capacity minutes (RCM), also referred to as reserve capacity (RC), is a battery's ability to sustain a minimum stated electrical load; it is defined as the time (in minutes) that a lead-acid battery at 80 °F (27 °C) will continuously deliver 25 amperes before its voltage drops below 10.5 volts.

Peukert's Law
Peukert's Law expresses the fact that the capacity available from a battery varies according to how rapidly it is discharged. A battery discharged at high rate will give fewer amperehours than one discharged more slowly.

Hydrometer
Hydrometer is an instrument, which is used for measuring the specific gravity of liquids. A hydrometer is a weight with a vertical scale attached. When placed into a liquid, the hydrometer bobs upright, and sinks to a certain level. The specific gravity or solution composition can be read from the liquid level on the vertical scale. Hydrometers have many uses, it is most commonly used by wine, beer, and a mead maker in determining the alcohol content of a homemade beverage. Hydrometers are also used to test batteries by measuring the percentage of sulfuric acid in the battery electrolyte in terms of specific gravity. Thus it has many other uses too. there are different types of hydrometers, which is used for different industrial and commercial purposes.

With batteries, the hydrometer measures the density, and therefore indirectly the amount of sulfuric acid in the electrolyte. A low reading means that sulfate is bound to the battery plates and that the battery is discharged. Upon recharge of the battery, the sulfate returns to the electrolyte.

Open Circuit Voltage
The open circuit voltage, measured when the engine is off. It can be approximately related to the charge of the battery by:

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