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EV Battery Glossary




Absorbed Glass Mat



Absorbed (or absorptive) Glass Mat

A technique for sealed lead-acid batteries. The electrolyte isabsorbed in a matrix of glass fibers, which holds the electrolytenext to the plate, and immobilizes it preventing spills. AGMbatteries tend to have good power characteristics, low internalresistance, and good behavior during charging.

Amp, Ampere

Unit of electrical current. Abbreviated "A"

Amp-hour EV Battery Glossary

Unit of electrical energy, one amp of current flowing for one hour.Abbreviated Ah

Anode EV Battery Glossary

The negative electrode of abattery (or other device).

Automotive post

A battery terminal style found on starting batteries ininternal-combustion vehicles. A round post made of lead. Seeterminal styles forcomparison.


Battery council international. Promoters of battery standards, notably the"Group sizes" which specifythe external dimensions of a battery. See also the group size chart. BCI has a web page.

C20, C6, C1, etc

An expression describing rate of discharge. The number indicatesthe number of hours to completely discharge the battery at aconstant current. So C/20 is the current draw at which the batterywill last for 20 hours, C/1 is the current at which the batterywill last 1 hour. The useful capacity of a battery changesdepending on the discharge rate, so battery capacities are statedwith respect to a particular rate. For instance, a particular modelof Hawker battery is ratedat 42 amp-hours at the C/10rate of 4.2amps, but only 30 Ah at the C/1 rate of30A. Also written as the 20-hourrate, 1-hour rate, etc.


Cold Cranking Amps


The positive electrode of abattery (or other device).

Cold Cranking Amps

A performance rating for automobile starting batteries. It isdefined as the current that the battery can deliver for 30 secondsand maintain a terminal voltage greater than or equal to 1.20 voltsper cell, at 0degrees Fahrenheit (-18Celsius), when the battery isnew and fully charged. Starting batteries may also be rated forCranking Amps, which is the same thing but at a temperatureof 32F (0C).

Cycle Life

How many charge/discharge cycles the battery can endure before itloses its ability to hold a useful charge. Cycle life typicallydepends on the depth ofdischarge. For example, if a hypothetical battery packwill propel your car for a maximum range of 100 miles, and youdrive 50 miles between charges, (50% DOD) then you may get 600 trips beforereplacing the pack; but if you drove 80 miles between charges, youmight only get 400 trips before the pack wears out. (All numberswere made up.)


Depth of Discharge.

Depth of Discharge

The amount of energy that has been removed from a battery (orbattery pack). Usually expressed as a percentage of the totalcapacity of the battery. For example, 50% depth of discharge meansthat half of the energy in the battery has been used. 80% DOD meansthat eighty percent of the energy has been discharged, so thebattery now holds only 20% of its full charge.


Energy Conversion Devices


A conductor by which electrical current enters or leaves anon-metallic medium, such as the electrolyte in a battery (as wellas vacuum tubes and lots of other devices).


An elctrically conductive medium, in which current flow is due tothe movement of ions. In a lead-acid battery, the electrolyte is asolution of sulfuric acid. In other batteries, the electrolyte maybe very different.


Makers of "Horizon"advanced lead-acid batteries. They have a web site.

Energy Conversion Devices

Parent company of Ovonics.

Energy Density

The amount of energy that can be contained in a specific quantityof the fuel source. Typically quoted in watt-hours per pound,wh/lb, or watt-hours per kilogram, wh/kg. For example, floodedlead-acid batteries generally have about 25 wh/kg, the latestadvanced lead-acid designs claim about 50 wh/kg, and newer batterytechnologies such as NiMH and LiON are in the 80-135 wh/kgrange.

Flooded cell

A design for lead-acid batteries. The electrolyte is an ordinary liquid solutionof acid. Flooded cells are prone to making gas while being charged.Flooded cells must be periodically checked for fluid level andwater added as necessary. Flooded cells are also typically lessexpensive than AGM orgel type lead-acidbatteries.

Fuel Cell

A battery where reactants are supplied to the cell from an externalsource. The most commonly cited example is the hydrogen fuel cell,in which hydrogen and oxygen are combined, producing electriccurrent and water.


GNB Industrial Battery company, maker of lead-acidbatteries.

Gel Cell

A technique for sealed lead-acid batteries. The electrolytesolution is in a gel form, usually silica gel, instead of plainliquid.


Brand of advanced lead-acid battery made by Hawker Energy Products.

Group size

A set of standard sizes for the external dimensions of a battery,standardized by BCI. All"group 27", etc, batteries are the same size, though they maydiffer in weight and capacity. We have a chart of the group sizes most likely to befound in a car.




Manufacturer of "Genesis"advanced lead-acid batteries. They have a web site.


Brand of advanced lead-acid batteries made by ElectroSource. (also apparently the nameof a stationary power supply system made by Exide.) The plates in aHorizon battery are arranged horizontally, instead of the morecommon vertical arrangement, hence the name. Horizon batteries havevery high energy density for lead-acid batteries, but are notgeneral available at retail.


Unit of rate of doing work. Defined as 550 foot-pounds per second.One horsepower is about 746 watts. (A horsepower has also been definedas the amount of power needed to drag a dead horse 500 feet in 1second, but this was not accepted by the international standardscommunity.)


A tool for testing the specific-gravity of a fluid, such as theelectrolyte in a flooded battery. Typically a squeeze-bulb is usedto suck up a sample of the fluid, and a float indicates thespecific gravity.


A tool for measuring the humidity of the air. It is not unheard-offor people to say "hygrometer" when they mean "hydrometer".

Immobilized Electrolyte

A technique for lead-acid batteries. The electrolyte (the acid) isheld in place against the plates instead of being a free-flowingliquid. The two most common techniques are gel and glassmat.


One thousand watts.Equivalent to about 1.34 horsepower.


A style of battery terminal, shaped like an L, with a flat verticalpart to which the cable is bolted. See terminal styles for comparison.


A technique for rechargable batteries. Electrodes of lead oxide and metallic leadare separated by an electrolyte of sulfuric acid. 

LiON, LiIon

Lithium ion. A technique for rechargeable batteries. Instead ofusing metalic lithium as the anode, lithium ions are added to acarbon electrode.


A light metal, atomic number 3. Used in advanced rechargeablebatteries.

Lithium Polymer

A technique for rechargeable batteries. The lithium anode isseparated from the cathode by a thin polymer electrolyte.

More Information:



Nickel Metal Hydride


Nickel Cadmium. (historical note, Nicad is/was a registered trademark, but haseffectively passed into the public domain, like aspirin.)

Nickel Cadmium

"Old" rechargeable battery technology. For many years, rechargeabledry cell meant nickel-cadmium. Recently, newer technologies such asnickel-metal hydride have mostly replaced nicad, since they havebetter energy characteristics and don't contain toxic cadmium. Thebattery has a nickel-hydroxide cathode, a cadmium anode, and aqueous potassium hydroxideelectrolyteSaft is a leading manufacturer ofnickel-cadmium batteries for EV applications.

Nickel Metal Hydride

A technique for making rechargeable batteries. NiMH batteries arecommon in laptop computers and cellular phones. The battery issimilar to nickel-cadmium but uses an anode of a metal hydride; a variety ofmetal alloys are used. 


Manufacturer of sealed lead-acid batteries: the Red Top and Yellow Top. They have a web site.


Division of Energy Conversion Devices, makes nickel metal-hydride batteries. They havea web site.

Peukert's equation

A formula that shows how the available capacity of a lead-acidbattery changes according to the rate of discharge. The capacity ofa battery is expressed in Amp-Hours, but it turns out that thesimple formula of current times hours doesn't accurately representthe situation. Peukert found that the equation:

C = I  T

fits the observed behavior of batteries. "C" is the theoreticalcapacity of the battery, "I" is the current, "T" is time, and "n"is the Peukert number , aconstant for the given battery. The equation captures the fact thatat higher currents, there is less available energy in thebattery.

Peukert number

A value that indicates how well a lead-acid battery performs underheavy currents. The Peukert number is the exponent inPeukert's equation. A valueclose to 1 indicates that the battery performs well; the higher thenumber, the more capacity is lost when the battery is discharged athigh currents. The Peukert number of a battery is determinedempirically. For Peukert numbers for typical EV batteries, seeUve W. Rick's BatteryPage.


Maker of lead-acid batteries.

Red Top

A sealed battery made by Optima, so-called because of the color ofits case, it is not the official product name. The red top is astarting battery, sometimes used in racing applications. It is notdesigned as a tractionbattery

Reserve Capacity

A performance rating for automobile starting batteries. It is thenumber of minutes at which the battery can be discharged at 25 Ampsand maintain a terminal voltage higher than 1.75 volts per cell, ona new, fully charged battery at 80degreesFahrenheit(27C).


Specific Gravity. A specific gravity of 1.300 is sometimes written as 1300SG,multiplying by 1000 to avoid the use of fractional parts.

SLI battery

Starting, Lighting, and Ignition battery, a battery designed foruse in a conventional gasoline automobile. An SLI battery isdesigned to give a lot of current during starting, but then to berecharged immediately by the car's alternator. Deeply dischargingan SLI battery will greatly shorten its life. SLI batteries aresometimes used in electric vehicles, especially for racing, but aregenerally not considered suitable because of their shortcycle life.


State of Charge


Manufacturer of Nickel-Cadmium, Nickel-Metal Hydride, and Lithium Ion batteries. Web Site

See Also

Lead Industry Associates
Delco's battery glossary:

Uve W. Rick's battery page

Mike Thompson's battery informationpage


Specific Gravity

The density of a material, expressed as the ratio of the mass of agiven volume of the material and the mass of the same volume ofwater; a specific gravity greater than 1 means heavier than water,less than 1 means lighter than water. The specific gravity of theelectrolyte in a batterycan be used to measure the state of charge of the battery.


A particular design for the electrodes in a lead-acid battery.Instead of having the electrodes as flat plates, the electrodes arerolled up in a spiral, like a cinnamon roll. The Optima is an example of a spiral-wound battery.

State of Charge

The amount of electrical charge in the battery, expressed as apercentage of the difference between the fully-charged andfully-discharged states.

Starved Electrolyte

A technique for "maintenance free" lead-acid batteries. Thesebatteries are less prone to gassing, so they don't require frequentchecking and addition of water. "Starved" because the batterycontains just enough electrolyte to provide the necessary chemicalreaction, as opposed to the older "flooded" arrangement whichcontains considerably more electrolyte than needed to make therated amp-hour capacity.

Terminal styles

Lead-acid batteries are made with several different styles for theposts that connect to the cables. The most common styles are



"Automotive" -- the round post familiar on startingbatteries in gas-powered cars. The cable lug fits around theterminal.


"Universal" -- Like an automotive post, with an extra studin the center of the post. The cable lug fits over the stud and anut holds them together


"L" -- A flat tang with a hole through it. A bolt throughthe hole connects the terminal to the cable lug.


There is considerable difference of opinion as to which style is"best".

Traction Battery

A battery designed to be used to provide the power to move avehicle, e.g. to be used in an electric car.


Manufacturer of flooded cell traction batteries. Popular and(relatively) inexpensive, Trojans seem to be the default choice forconverters.

Universal post

A style of battery terminal, with a round post similar to theautomotive post, but with a threaded stud in the center of thepost. See terminal stylesfor comparison.


Valve-Regulated Lead Acid

Valve Regulated

A technique for making lead-acid batteries. Instead of simple ventcaps on the cells to let gas escape, VRLA have pressure valves thatopen only under extreme conditions. Valve-regulated batteries alsoneed an electrolyte design that reduces gassing, usually involvinga catalyst that causes the hydrogen and oxygen to recombine intowater.






Watt-hours per kilogram, unit of energydensity.


Watt-hours per pound, unit of energydensity.


Unit of electricity, the rate at which work is done. The equationis Watts = Volts x Amps. One watt is equivalent to about 0.00134horsepower.


Unit of electrical energy, or work. 1 watt expended continuouslyfor 1 hour equals 1 watt-hour

Yellow Top

A sealed battery manufactured by Optima, so-called because of the color ofthe case, it is not the official product name. The Yellow Top isdesigned to be a tractionbattery. A relative newcomer to the market, the yellowtop is popular in some circles because it can deliver highcurrents, providing good acceleration.

X-hour rate

The discharge rate of a battery is usually quoted in the number ofhours that the battery will last at that current. So at the 6-hourrate, the battery can produce current for 6 hours. Same asC6.

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