Battery Terms and Definitions

ABS: The outer case material used for most SLA batteries.  There are different types of flame retardant ABS material as rated by Underwriter Laboratories (UL).  UL94-HB and UL94-VO are both used and UL94-VO has the highest rating for flame retardant requirements, but is much more expensive than regular ABS material.  Another case material used in battery manufacturing is Polypropylene. 

Active Material: The active electro-chemical materials commonly called paste used in the manufacture of positive and negative plates. 

AGM: Stands for Absorbent Glass Mat, which is the separator used between the positive and negative plates inside some SLA batteries. 

Ah: Stands for Ampere Hour which is a relational term describing the amount of amps the battery can continuously deliver for a set number of hours to an end point voltage at a set temperature. (Ah = Amps x Hours @ 25°C) 

Ambient Temperature: The average external temperature experienced by the battery. 

Ampere: Unit of electrical current.  (Sometimes called Amp or A) 

Battery: In a SLA battery; single or parallel groups of positive and negative plates determining the Ah in a cell.  Cells are then connected together in a series connection thereby increasing the voltage.  

C-Rate or C: A current rate expressed in amperes or milliamperes, which is relational to a battery's ampere hour rating. For example: in a 6 Ah rating, 1C = 6 amps; 3C = 18 Amps; 0.05C = 300 milliamps. 

Cell: The minimum unit of which a storage battery is composed. Note: The nominal voltage of a single lead acid cell is 2.0 volts. Cells are comprised of positive and negative plates connected together in a parallel connection.  A cell must have at least 1 positive and 1 negative plate. 

Constant Current Charge: A method of charging batteries by applying a fixed current and allowing the voltage to move freely.  This can be used on SLA batteries as long as the charge time period is monitored and controlled and the recommended voltage level is not exceeded.  All SLA batteries have maximum charging current that must be followed. If these conditions are not met the battery will throw off the excess charge forced into the battery in the form of heat leading to battery failure and out-gassing drying out the electrolyte. 

Constant Voltage Charge: A method of charging batteries by applying a fixed voltage and allowing the current to vary to the degree that the batteries counter electromotive force (EMF) reduces the current to a final charge current level at the end of the charge cycle. Recommended for sealed lead acid batteries.  Charging voltage limits must be followed. 

COS: “Cast-On Strap” that connects the plate lugs together inside each cell.  There is a negative bar or strap and positive bar or strap in each cell.  These straps are welded by hand with a mold or with an automatic machine to increase uniformity of construction. 

Cutoff Voltage: The final voltage of a cell or battery at the end of charge or discharge 

Counter EMF: (Counter Electromotive Force) The batteries ability to suppress the amount of current it will accept during the charge process.  If this is exceeded the battery will throw off the excess charge in the form of heat. 

Cycle: A single charge and discharge of a battery. 

Cycle Life: The number of cycles a battery can endure before battery fails to produce required capacity. 

Discharge Rate: The amount of current taken from a cell or battery and expressed as a fraction of C (Rated Ampere-hour (Ah) capacity of the cell or battery). 

End-of-Charge Voltage: The voltage reached by battery at the end-of-charge, while the charger is still attached. 

Electrolyte: Solution added to the cell that conducts ions in the cell between the positive and negative plates. Lead acid batteries use a diluted sulfuric acid solution.  Specific gravities of the electrolyte vary by size and design. 

Energy Density: Ratio of cell or battery energy to unit weight (pound or kilogram) or unit volume (cubic inch or cubic meter) 

Fast-on: Common terminal on most small SLA batteries.  Fast-on is a brand that manufacturers some of these tabs and describes these terminal types in the SLA battery industry.  There are .250 (1/4” wide) and a .187 (1/8” wide) tab versions commonly used on SLA types. 

Final Charge Current: The amount of current the battery will continue to accept even though it is fully charged.  Usually the final charge current is equal to the current needed to offset the battery internal resistance.  Once this value to achieved charging must be stopped or switched to a float charge. 

Float Use or Charge: To charge or maintain a battery at recommended voltage and current that will only compensate for internal resistance and self-discharge values while still being sufficient to recharge the battery after a discharge event.  A float charge system constantly monitors battery status and adjusts charging levels to ensure full state of charge is maintained at all times. 

Formation: The process of placing a charge into the positive and negative plates which activates the active materials.  In-battery formation takes place after the battery assembly.  In-battery formation must be done slowly or with some type of heat control like water baths as high heat generated during this process is detrimental to battery life. Tank formation forms the plates outside the battery in large tanks.  Dry charged batteries require tank formation since the batteries are assembled and sold before electrolyte addition. 

Gas Absorption: The ability of the negative plate to absorb oxygen gas generated within the battery. 

Gel or Gelled: Fumed Silica is added to battery electrolyte to thicken and immobilize the electrolyte.  This gelled electrolyte is thixotropic meaning it can break-down into liquid and reset back into a thick gel over time.  Commonly used to increase cycle life since it creates a reserve amount of electrolyte needed to recover as a result of deep-discharge events.  However, it does out-gas slightly more than AGM designs especially at the beginning of its life and can be higher in internal resistance in some designs.  By construction is also a SLA, VRLA, and with some suppliers AGM/Gel hybrid. 

Grid: The backbone or structure of the plate.  It has interior holes like a large screen mesh material where the active material or paste is applied.  Grids can be cast with a mold or punched in a continuous machined process.  The cast grids can be rectilinear (square hole design), radial (wires angle to the lug), or branch radial (wires merge into larger wires in route to the lug) in design.  Some grids are punched and then expanded into expanded metal grids. The positive plate grid carries the current to the top lug and into the straps.  The negative plate active material is conductive so grid provides plate structure. 

Recombination process: The process where internal gas is recombined into liquid form. Charging voltage/current should not exceed the batteries ability to internally recombine the gasses, otherwise out gassing and dry-out occurs. 

High-Rate Discharge: A very rapid discharge of the battery. Normally in multiples of the battery C-rate or can be in terms of watts or amperage rates at differing temperatures (like Cold Cranking Amps – CCA). 

Immobilized Electrolyte: In order to enable the SLA batteries to be used in different positons the electrolyte must be immobilized and held against the plate surfaces.  Generally there are two ways to hold the electrolyte in place, one is absorbed in AGM material and another is to add fumed silica to the electrolyte creating a thick immobilized Gel. 

Internal Impedance or Resistance: The resistive value of the battery expressed in ohms or milliohms. Normally measured at 1 khz at full charge.  Can also be expressed in terms of conductance. 

Low Voltage Cutoff: A sensing device designed to end discharge at a predetermined voltage level.  Used to keep batteries from over-discharge. 

Lug: The top tab on each grid that is used to connect all the grids together in a parallel connection in each cell in SLA batteries. 

Nominal Capacity: The nominal or named value of the rated capacity. In sealed lead acid (SLA) batteries, nominal capacity is usually measured at the 20 hour rate, which means the amount of current the battery can deliver for 20 hours before reaching the end point voltage of 1.75 volts per cell at 25°C. 

Nominal Voltage: The nominal or named value of battery voltage. In lead acid batteries, nominal voltage is 2 volts per cell. The actual full charge voltage for SLA batteries is 2.15 – 2.20 volts per cell depending on the specific gravity of the electrolyte.  Electrolyte density directly affects the actual voltage. 

Open Circuit Voltage: The measured voltage of a battery without a load attached. 

Overcharge: The continuous charging of a battery after it achieves 100% of rated capacity or too fast for the battery to absorb the charge. Battery life is reduced by prolonged over-charging from excess heat generation and severe overcharging will also quickly dry out the electrolyte. 

Parallel Connection: A group of batteries with all terminals of the same polarity connected, thereby increasing the Ah capacity of the connected battery group while the voltage remains the same (+ → +, - → -). Batteries connected together should be similar in age, size, and construction.  Batteries that differ in a series or parallel connection will not charge or discharge equally reducing the life of the overall string.   

Plates: The grids and active material or paste together in a formed or uniformed state.  There are both positive and negative active material plates. 

Primary Cell: A cell which can be discharged only once. For example: household alkaline batteries. 

Rated Capacity: The capacity of a battery expressed in ampere hours (Ah or Amps X Hours). Expression of a constant current discharge for a designated number of hours to a specified depth of discharge at 25°C. 

Resealable Safety Vent: The safety device built into the top of the cell of a battery to relieve the excess pressure of gassing and prevent case rupture. This usually happens when charged faster than the batteries ability to recombine internal gassing rates.  Under proper charging conditions small venting can take place at end of charge cycles to equalize internal pressure. Valves usually open at about 5-6 PSI (lbs. per square inch) internal pressure. 

Secondary Battery: A battery which can be charged and discharged repeatedly like SLA, NiCd, NiMh, and Li Ion chemistry batteries. 

Self-Discharge: The loss of capacity of a battery while stored in an unused condition without external drain.  Heat accelerates this condition, while cold slows the self-discharge. 

Separator: The materials which separate the battery plates. In a sealed lead acid battery, they are usually constructed of micro-porous glass fiber (Absorbent Glass Mat – AGM) and additionally serve to immobilize the electrolyte so the battery will function in different positions. Separator types used in some other battery types are SMPE (sub-microporous polyethylene) envelope type and ribbed leaf separators. 

Series Connection: A group of batteries with terminals connected to the opposite polarity (+ → - → + → -), thereby increasing the voltage of the battery group while the Ah capacity remains the same. Batteries connected together should be similar in age, size, and construction.  Batteries that differ in a series or parallel connection will not charge or discharge equally reducing the life of the overall battery string.   

Service Life: Expected life of a battery expressed in the number of total cycles or years of standby service to a designated remaining percentage of original capacity. Life is affected by number of cycles, depth of cycles, charging regime, maintenance, and temperature. 

Shelf Life: The maximum period of time a battery can be stored under specific conditions, without supplementary charging.  The self-discharge to a recoverable state of charge is used to calculate this value. 

Specific Gravity: A term to describe the weight of the electrolyte relative to the weight of water.  So 1.000 weight is the weight of water and 1.300 is the average relative weight of SLA battery finished electrolyte.  Full strength Sulfuric acid specific gravity is 1.840. 

SLA: Stands for Sealed Lead Acid because the batteries are sealed from the outside in, although they are valve regulated from the inside out to relieve internal pressure as needed. Also called VRLA, AGM, and Gel batteries. 

Standby Service: (See also Float use) A state in which the battery is continuously maintained in a fully charged condition by trickle or float charge so it is always ready for use. 

Sulfation: Chemical condition occurring in the active material when the battery is left in a discharged state for a period a time causing the sulfates that accumulated in the active material during normal discharge to crystalize.  The crystallization of the sulfates can cause permanent damage to the battery. 

Terminal: The point of connection for a battery to external load.  SLA Batteries have a positive and negative connection point terminal.  Usually several different terminal options are available to meet the needs of the application. 

Thermal Runaway: Condition where the battery is overcharged in a high heat environment where the charging level is allowed to continue and increase as the battery gets hotter creating a runaway charging environment.  Since heat increases capacity and the need for more charging, the batteries begins to call for more charge than necessary creating this failure mode. 

Trickle Charge: Continuous charging by means of a small current designed to only compensate for self-discharge in an unloaded battery.  Some of type of microprocessor controlled charger is necessary to monitor the battery state and only deliver the charge necessary to maintain the battery capacity. 

Under-Charge: The state of a battery where insufficient charging has occurred.  Small levels of continuous under-charge can be harmful to a battery causing sulfation to occur on a portion of the plates.  This effectively reduces the available capacity every time the battery is undercharged until insufficient capacity remains for usage. 

VRLA: Stands for Valve Regulated Lead Acid batteries which is another name for SLA and Gel batteries.  VRLA is a more accurate term as the batteries are only sealed from the outside in, so internal pressures can be released from the inside of the battery through the pressure relief valves. 

Volt: A volt is the electrical unit of voltage or potential difference (symbol: V). One volt is equal to a current of 1 amp times resistance of 1 ohm (1V = 1A x 1Ω) 

Voltage Cutoff: A sensing device used to terminate a charge or discharge when the battery reaches a predetermined voltage level. 

Watts: A power value relational to Amps and volts.  Amps x Volts = Watts to an end point voltage and specific temperature.  Can be expressed in watts per battery or watts per cell.  

Watts per cell (WPC): Amount of Watts that each cell in a battery can produce at a given end point volts for the stated time and temperature. To calculate the watts per battery (WPB) multiply the watts per cell times the number of cells. (WPB = WPC X #cells) 

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