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When learning about car batteries you may come across terms you have never seen before. That’s we have supplied a list of common battery terms to help you understand your battery better. These terms can also help when talking with one of the experts at your local NAPA AUTO PARTS store.

ACTIVE MATERIAL – The active material in the positive plates is lead dioxide and that in the negative is metallic sponge lead. When an electrical circuit is created, these materials react with sulfuric acid during charging and discharging according to the following chemical reaction:

PbO2 + Pb + 2H2SO4 = 2PbSO4 + 2H20.

AGM (ABSORBED GLASS MAT) – A spill proof battery that utilizes a unique highly absorbent separator to hold the acid like a sponge. These batteries utilized a non-woven separator made from spun-glass microfibers that absorb and hold the acid. AGM batteries also feature a pressurized venting system that nearly eliminates water loss through oxygen recombination.

AMPERE (Amp. A.) – The unit of measure of the electron flow rate, or current, through a circuit.

AMPERE-HOUR (Amp.-Hr, AH) – A unit of measure for a battery’s electrical storage capacity, obtained by multiplying the current in amperes by the time in hours of discharge. (Example: A battery which delivers 5 amperes for 20 hours delivers 5 amperes X 20 hours = 100 Amp-Hr of capacity.)

CAPACITY – The ability of a fully charged battery to deliver a specified quantity of electricity (Amp-Hr, AH) at a given rate (Amp, A), over a definite period of time (Hr). The capacity of a battery depends upon a number of factors such as: active material weight, density of the active material, adhesion of the active material to the grid, number, design and dimensions of plates, plate spacing, design of separators, specific gravity and quantity of available electrolyte, grid alloys, final limiting voltage, discharge rate, temperature, internal and external resistance, age, and life history of the battery.

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.

CIRCUIT – An electric circuit is the path followed by a flow of electrons. A closed circuit is a complete path. An open circuit has a broken, or disconnected, path.

CIRCUIT (Series) – A circuit which has only one path for the flow of current. Batteries arranged in series are connected with negative of the first to positive of the second, negative of the second to positive of the third, etc. If two 12-volt batteries of 50 ampere-hours capacity each are connected in series, the circuit voltage is equal to the sum of the two battery voltages, or 24 volts, and the ampere-hour capacity of the combination is 50 ampere-hours.

CIRCUIT (Parallel) – A circuit which provides more than one path for the flow of current. A parallel arrangement of batteries (usually of like voltages and capacities) has all positive terminals connected to a conductor and all negative terminals connected to another conductor. If two 12 volt batteries of 50 ampere-hour capacity each are connected in parallel, the circuit voltage is 12 volts, and the ampere-hour (Amp-Hr) capacity of the combination is 100 ampere-hours.

COLD CRANKING RATING – The number of amperes a lead acid battery at 0ºF (-17.8ºC) can deliver for 30 seconds and maintain at least 1.2 volts per cell.

CORROSION – The destructive chemical reaction of a liquid electrolyte with a reactive material – e.g., dilute sulfuric acid on iron, producing corrosion products such as rust. Battery terminals are subject to corrosion if they are not properly maintained.

CURRENT – The rate of flow of electricity, or the movement of electrons along a conductor. It is comparable to the flow of a stream of water. The unit of measure for current is the ampere.

CURRENT (ALTERNATING) (AC) – A current that varies periodically in magnitude and direction. A battery does not deliver alternating current (AC).

CURRENT (DIRECT) (DC) – An electrical current flowing in an electrical circuit in one direction only. A battery delivers direct current (DC) and must be recharged with direct current in the opposite direction of the discharge.

CYCLE – In a battery, one discharge plus one recharge equals one cycle.

DISCHARGING – When a battery is delivering current, it is said to be discharging.

ELECTROLYTE – In a lead-acid battery, the electrolyte is sulfuric acid diluted with water. It is a conductor that supplies water and sulfate for the electrochemical reaction:

PbO2 + Pb + 2H2SO4 = 2PbSO4 + 2H2O.

ELEMENT – In a battery, a set of positive and negative plates assembled with separators.

FORMING – In battery manufacturing, formation is the process of charging the battery for the first time. Electrochemically, formation changes the lead oxide paste on the positive grids into lead dioxide and the lead oxide paste on the negative grids to metallic sponge lead.

GEL CELLS – A spill proof battery that utilizes a very unique gelled electrolyte instead of conventional liquid sulfuric acid. The acid electrolyte had the consistency of “Petroleum Jelly” and is made by mixing the sulfuric acid with silica powder. Gel Cell batteries also feature a pressurized venting system that nearly eliminates water loss through oxygen recombination.

GRID- 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 of a vehicle which is used as a path for completing a circuit in lieu of a direct wire from a component. Today, over 99% of automotive and LTV applications, use the negative terminal of the battery as the ground.

HYDROMETER – A float type device used to determine the state of charge of a battery by measuring the specific gravity of the electrolyte (i.e., the concentration of sulfuric acid in the electrolyte).

LOAD TESTER – An instrument which draws current (discharges) from a battery using an electrical load while measuring voltage. It determines the battery’s ability to perform under actual discharge conditions.

LOW WATER LOSS BATTERY – A battery which does not require periodic water addition under normal driving conditions; also referred to as a maintenance-free battery.

NEGATIVE – Designating, or pertaining to, electrical potential. The negative battery terminal is the point from which electrons flow during discharge.

OHM – A unit for measuring electrical resistance or impedance within an electrical circuit.

OHM’S LAW – Expresses the relationship between volts (V) and amperes (A) in an electrical circuit with resistance (R). It can be expressed as follows:

V = IR Volts (V) = Amperes (I) x Ohms (R).

If any two of the three values are known, the third value can be calculated using the above equation.

OPEN CIRCUIT VOLTAGE – The voltage of a battery when it is not delivering or receiving power. It is 2.11 volts for a fully charged battery cell, or 12.66 for a fully charged 12 volt battery. (6.33 for a 6 volt battery.)

POSITIVE – Designating, or pertaining to, a kind of electrical potential; opposite of negative. A point or terminal on a battery having lower relative electrical potential.

PRIMARY BATTERY – This type of battery can store and deliver electrical energy but cannot be recharged.

RESERVE CAPACITY RATING – The time in minutes that a new, fully charged battery will deliver 25 amperes at 80ºF and maintain a terminal voltage equal to, or higher than, 1.75 volts per cell. This rating represents the time the battery will continue to operate essential accessories if the alternator or generator of a vehicle fails.

RESISTANCE (Electrical) – The opposition to the free flow of current in a circuit. It is commonly measured in Ohms.

SECONDARY BATTERY – A battery which can store and deliver electrical energy and can be recharged by passing direct current through it in a direction opposite to that of discharge.

SEPARATOR – A divider between the positive and negative plates of an element which allows the flow of current to pass through it. Separators are made from numerous materials such as: polyethylene, polyvinyl chloride, rubber, glass fiber, cellulose, etc.

SHORT CIRCUIT – An unintended current-bypass in an electric device or wiring, generally very low in resistance and thus causing a large current to flow. In a battery a cell short circuit may be permanent enough to discharge the cell and render the battery useless.

SPECIFIC GRAVITY (Sp. Gr.) – The density of a liquid compared to the density of water. The specific gravity of the electrolyte is the weight of the electrolyte compared to the weight of an equal volume of pure water.

STATE OF CHARGE – The amount of electrical energy stored in a battery at a given time expressed as a percentage of the energy when fully charged.

VOLT – The unit of measure for electrical potential.

VOLTAGE DROP – The net difference in the electrical potential (voltage) when measured across a resistance or impedance (ohms). Its relationship to current is described in Ohm’s law.

WATT – The unit for measuring electrical power, i.e. the rate of doing work, in moving electrons by, or against, an electrical potential. Formula: Watts = Amperes x Volts.

WATT-HOUR (Watt-Hr, WH) – The unit of measure for electrical energy expressed as Watts x Hours.

Check out all the electrical system products available on NAPA Online or trust one of our 16,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on common battery terms, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

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