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Absorbed Glass Mat - AGM - "Absorbed Glass Mats", or AGM is packed between the plates. The Boron-Silicate glass mat consists of very fine fibers . These type of batteries have all the advantages of gelled batteries, but can take more abuse.

Nearly all AGM batteries are recombinant,  the Oxygen and Hydrogen recombine inside the battery. These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost.

During cell overcharging, hydrogen gas (H2) and oxygen gas (O2) are contrived through the dissociation of water (H2O) in the cell’s positive plate electrolyte.  AGM cells incorporate an oxygen recombination process during overcharge, a process where oxygen gas is recombined to the negative cell plates.  While oxygen is produced and fully recombined, hydrogen is not; excess hydrogen can accumulate within the cell jar and cause the internal cell pressure to increase.  With increased voltage level, gases are produced at a faster rate.  To mitigate cell explosion, a vent plug alleviates internal cell pressures by expelling excessive gas build-up.

Amp (A) - is a measure of electric current; one A of current represents one coulomb of electrical charge moving past a specific point in one second (1 C/s = 1 A).

Amp Hour Rating — The amp hour (AHr) rating describes the size of this storage capacity of a battery over a period of time. A battery with a 125 AHr rating over 20 hours means it can supply 6.25 Amps of current for a period of twenty hours before it is 100% discharged.  More Ampere hours can be drawn from a battery if the current drawn from the battery is low.  Some battery companies name their battery models and publishes their 100 Ahr ratings, however most provide a 20hr rating.  This rating as it implies, is the amount of ampre hours that can be drawn from a battery.

Antimony - is  added to lead to increase hardness. The high antimony content also reduces long discharge capability and increases the gases produced by the cells during charging.

Autonomy -is the length of time which a battery bank can support a specific load without overcharging. A value used to measure battery reserve capacity and system reliability.

Battery — Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.

Battery bank - is a group of batteries wired in series or parallel (or both), to store the electrical energy produced for later usage.

Battery Available Capacity — The total maximum charge, expressed in ampere-hours, that can be withdrawn from a cell or battery under a specific set of operating conditions including discharge rate, temperature, initial state of charge, age, and cut-off voltage (also see amp hour rating).

Battery Cell — are the most basic individual component of a storage battery. They consist of one or more positive electrodes or plates, an electrolyte that permits ionic conduction, one or more negative electrodes or plates, separators between plates of opposite polarity, and a container for the above.

Battery Cycle Life — The number of cycles, to a specified depth of discharge, that a cell or battery can undergo before failing to meet its specified capacity or efficiency performance criteria.

Battery Energy Capacity — The total energy available, expressed in watt-hours (kilowatt-hours), which can be withdrawn from a fully charged cell or battery. The energy capacity of a given cell varies with temperature, rate, age, and cut-off voltage. This term is more common to system designers than it is to the battery industry where capacity refers to ampere-hours (see amp hour rating).

Battery Energy Storage — Energy storage using electrochemical batteries. The three main applications for battery energy storage systems include spinning reserve at generating stations, load leveling at substations, and peak shaving on the customer side of the meter.

Battery Life — The period during which a cell or battery is capable of operating above a specified capacity or efficiency performance level. Life may be measured in cycles and/or years, depending on the type of service for which the cell or battery is intended.

Capacity -Deep cycle batteries are rated in amp-hours. An amp-hour is one amp for one hour, or 10 amps for 1/10 of an hour. Stated mathematically (amps x hours).  If you have something that consumes 20 amps, and you use it for 30 minutes, then the amp-hours used would be 20 (amps) x .5 (hours), or 10 AH.  The the most common accepted AH rating time period for batteries used in solar electric and backup power systems is the "20 hour rate".  This means that a battery is discharged down to 10.5 volts (100% capacity) over a 20 hour period while the total actual amp-hours it supplies is measured. The 100 hour rate is sometimes given just to make the battery look better than it really is. See Peukert Effect below.

The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

  • Two 6V, 225Ah batteries are wired in Series, the voltage is doubled but the amp-hour capacity remains 225Ah (Total Power = 2700 Watt-hours [225A * 12V]).
  • Two 6V, 225Ah batteries wired in Parallel will have a total storage capacity of 450Ah at 6V (or 2700 Watt hours [225A * 2 = 450Ah] and [450Ah * 6V = 2700] ).
  • Series-Parallel looks and sounds more complicated, however the principal is the same. Consider, four 6V cells are wired in two "strings" of 12VDC that were then wired in parallel. Using 6V, 225Ah batteries, this system will have a storage capacity of 450Ah at 12V or 5400Wh.

Capacity (False)- A battery can meet all the tests for being at full charge, yet be much lower than it's original capacity. If plates are damaged, sulfated, or partially eroded from long use, the battery may give the appearance of being fully charged, but act like a battery of smaller size.  This can also occur in gelled cells if they are overcharged and gaps (bubbles) have formed in the gel.

Captive Electrolyte Battery — A battery having an immobilized electrolyte (gelled or absorbed in a material).

Cell (battery) — A single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected together to produce higher voltages. (Sometimes the terms cell and battery are used interchangeably). Also see photovoltaic (PV) cell.

Charge Factor — A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours. Related to charge rate.

Charge Rate — The current applied to a cell or battery to restore its available capacity. This rate is commonly normalized by a charge control device with respect to the rated capacity of the cell or battery.

Battery charging takes place in 3 basic stages: Bulk, Absorption, and Float.

Bulk Charge - The first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate  until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts. There is no "correct" voltage for bulk charging, but there are limits on the maximum current that the battery and/or wiring can take.

Absorption Charge: The second stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2 to 15.5 volts.

Float Charge: The third stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it's main purpose is to keep an already charged battery from discharging. PWM, or "pulse width modulation" accomplishes the same thing. In PWM, the controller or charger senses tiny voltage drops in the battery and sends very short charging cycles (pulses) to the battery. This may occur several hundred times per minute. It is called "pulse width" because the width of the pulses may vary from a few microseconds to several seconds. Note that for long term float service, such as backup power systems that are seldom discharged, the float voltage should be around 13.02 to 13.20 volts.

Concrete — We frequently hear that storing batteries on concrete is bad as it will drain the batteries.  ...well it used to be bad back in the days when batteries were constructed out of wooden crates soaked in tar.  A long time ago batteries used to ooze electrolyte onto the concrete eventually creating a ground allowing current to leak out as well.  This hearsay has been perpetuated as batteries will lose their charge over time.  Batteries stored at less than full charge for periods of time will form large, hard sulphate crystals further degrading the battery.  Go ahead and store your battery on concrete, just make sure it is charged when you put it away and give it a charge from time to time to prevent sulphation.

Conductance - is the ability of a battery to conduct current. It is a measurement of the plate surface available in a battery for chemical reaction, which determines how much power the battery can supply.  Conductance can be used to detect cell defects, shorts, and open circuits which can cause the battery to fail.

Conversion Efficiency - is how well a battery converts an electrical charge into chemical energy and back again. The higher this factor, the less energy is converted into heat and the faster a battery can be charged without overheating.

Current -is the rate at which electricity flows through a conductor; measured in amps (A).  Current flow over time is defined as ampere-hours (a.k.a. amp-hours or Ah), a product of the average current and the amount of time it flowed.

Cycles (Battery)- A period of discharge and recharge is called one cycle. A battery cycle is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20% DOD, and then back to 100%. Battery performance may be measured by the expected number of cycles it may deliver at varying depths of discharge.

Cycle Life - is the measure of how many charge and discharge cycles a battery can take before its lead-plate grids/plates are expected to collapse and short out. The greater the average depth-of-discharge, the shorter the cycle life. Be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged. A battery that is rated for a 20 year life expectancy if discharged by only 5% may have a 5 year life expectancy if discharged to 50%.  A battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. At shallow discharge cycles Lead Dioxide builds up on the the positive plates rather in an even film.  Calculate based on an average DOD of around 50% for the best storage vs. cost factor.

Deep-Cycle Battery — A battery with large plates that can withstand many discharges to a low state-of-charge.  Deep Cycle batteries have thicker lead plates that make them tolerate deep discharges better. They cannot dispense charge as quickly as a starter battery but can also be used to start combustion engines. Thicker lead plates lead to a  longer the life span.  Battery weight is a simple indicator for the thickness of the lead plates used in a battery. The heavier a battery for a given group size, the thicker the plates, and the better the battery will tolerate deep discharges.

Deep Cycling — This is a battery restoration technique employing the repeated to deep discharge and recharge of most of the battery's total storage capacity.

Deep Discharge — Discharging a battery to 20% or less of its full charge capacity.

Deficit Cycling  — When a battery is not being fully charged on a regular basis

Depth of Discharge (DOD) — is a measure of how deeply a battery is discharged. When a battery is 100% full, then the DOD is 0%. The ampere-hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity. For example 25 Ah are removed from a 100 Ah battery, thus it's depth of discharge is 25% and the battery is at a 75% state of charge.

Direct current ( DC )- electric current which flows in only one direction in a wire. Solar voltaic panels and batteries are DC.

Discharge — The withdrawal of electrical energy from a battery.

Discharge Cycle — When connected to a load, the chemical reaction within the battery between sulfuric acid and the lead plates releases releases electrons from the electrolyte. The chemical reaction coats both positive and negative plates with a substance called lead sulfate also known as sulfation during during discharge. If immediately recharged the lead sulfate a soft material is easily back into lead and sulfuric acid during the recharge cycle.

Discharge Factor — A number equivalent to the time in hours during which a battery is discharged at constant current usually expressed as a percentage of the total battery capacity, i.e., C/5 indicates a discharge factor of 5 hours.( see discharge rate and amp hour rating).

Discharge Rate — The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.

Electrolyte — A nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell.

End of Life  — When a battery's capacity reaches 80% of it's rating the battery is considered to have reached end of life.  This is the industry standard definition, it does not mean the battery cannot be used nor does it mean it should be disposed.

Equalization Charge — The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short time. This is a continuation of normal battery charging, at a voltage level slightly higher than the normal end-of-charge voltage, in order to provide cell equalization within a battery.  This is a process of balancing the state of charge in all the cells that form a battery bank to remove sulphates from battery plates and restore capacity.

Float Charge / Float Voltage — The lowest voltage that will maintain the battery at full charge. The voltage required to counteract the self-discharge of the battery at a certain temperature.

Freezing Point of all Lead Acid batteries — The temperature at which the electrolyte will freeze based on the battery's state of Chage (SoG)

Approx. (SoC)

Approx.  Electrolyte Freeze Point











Gassing -  is when batteries start to gas when you attempt to charge them faster than they can absorb the energy. The excess energy is turned into heat, which then causes the electrolyte to boil and evaporate.

Gassing Current — The portion of charge current that goes into electrolytic production of hydrogen and oxygen from the electrolytic liquid. This current increases both voltage and temperature.

Gel-Type Battery — Lead-acid battery in which the electrolyte is composed of a silica gel matrix.

Hydrometer  — A direct-reading instrument for indicating the density, specific gravity, or some similar characteristic of liquids.  Specific gravity hydrometers to indicate specific gravity of a liquid, with reference to water, at a particular temperature. The Hydrometer used to measure the specific gravity of battery electrolyte is called a bulb hydrometer and   consists of a small commercial hydrometer contained in a larger glass tube into which the solution to be tested is drawn by the action of a rubber bulb. It is used to measure the specific gravity of the sulfuric acid solution in lead acid batteries.

Lead-acid battery - is a electrical storage device that uses a reversible chemical reaction to store energy. It uses a combination of lead plates or grids and an electrolyte consisting of a diluted sulphuric acid to convert electrical energy into potential chemical energy and back again.

Liquid Electrolyte Battery — A battery containing a liquid solution of acid and water. Distilled water may be added to these batteries to replenish the electrolyte as necessary. Also called a flooded battery because the plates are covered with the electrolyte.

Load Voltage - As a battery is charged the plates polarize and develop a resistance to the charge (surface charge). This resistance adds to the battery voltage.  The voltage reading will not reflect the true state of charge.

Maintenance-Free Battery — A sealed battery to which water cannot be added to maintain electrolyte level.

Nickel Cadmium Battery — A battery containing nickel and cadmium plates and an alkaline electrolyte.

Nominal Voltage — A reference voltage used to describe batteries, modules, or systems (i.e., a 12-volt or 24-volt battery, module, or system).

Open Cell Voltage- (OCV) Cell voltage taken when the battery is at rest.  Accurate battery voltage may only be measured after all loads have been taken off the battery for a period of at least 4 hours.

Overcharge — Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.

Peukert Effect- The Peukert value is directly related to the internal resistance of the battery. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. The faster a battery is used (discharged), the LOWER the AH capacity. Conversely, if it is drained slower, the AH capacity is higher

Pilot Cell - One cell in in one battery is usually selected as a pilot cell for specific gravity readings. Since all cells in all of the batteries receive the same amount of charge or discharge current, their specific gravities will fall or rise proportionately to that of the pilot cell.  It is advisable to change pilot cells after about 10 readings, because a slight amount of electrolyte is lost each time a hydrometer reading is taken.

Plates — A metal plate, usually lead or lead compound, immersed in the electrolyte in a battery.

Power - is the product of voltage and current and is measured in Watts. Power over time is usually defined in Watt-hours (Wh), the product of the average number of watts and time. Utility usually bills are usually in  per kilowatt hour (kWh), which is 1,000 watt-hours.

Rated Battery Capacity — The term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery under specified discharge rate, end voltage and temperature. (see battery capacity)

Recharge Cycle — During the recharge cycle electrons are forced back into the storage battery and lead sulfate (sulfation) reconverts to lead and sulfuric acid. During the recharge cycle some water is separated into its component elements of Oxygen and Hydrogen.

Self Discharge - is a measure of how much batteries discharge on their own. The Self-Discharge rate is governed by the construction of the battery and the metallurgy of the lead used inside. Batteries that are stored for long periods will eventually lose all their charge. Self discharge varies considerably with battery type, age, & temperature. It ranges from about 1% to 15% per month  Batteries self-discharge faster at higher temperatures. In general new AGM batteries have the lowest, and old industrial (Lead-Antimony plates) are the highest self discharge rates. A major cause of batteries failure is caused by being stored in a partly discharged state for a few months.  A "float" charge should be maintained on the batteries especially if they are not used.

Series - a connection pattern placing module or batteries in a string so that current flows from positive to negative through each component. The current in series wiring remains the same as that of a single module, while the voltage is added.

Specific Gravity — The ratio of the weight of the solution to the weight of an equal volume of water at a specified temperature. Used as an indicator of battery state-of-charge.

Starter Battery - have many thin lead plates which allow them to discharge a lot of energy very quickly for a short amount of time. They do not tolerate being discharged deeply, as the thin lead plates needed for starter currents will degrade quickly under deep discharge and re-charging cycles. Most starter batteries will only tolerate being completely discharged a few times before being irreversibly damaged.

Starved Electrolyte Cell — A battery containing little or no free fluid electrolyte.

State-of-Charge (SOC) — The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

Storage Battery — A device capable of transforming energy from electric to chemical form and vice versa. The reactions are almost completely reversible. During discharge, chemical energy is converted to electric energy and is consumed in an external circuit or apparatus.

Stratification — A condition that occurs when the acid concentration varies from top to bottom in the battery electrolyte. Periodic, controlled charging at voltages that produce gassing will mix the electrolyte. See equalization.

Sulfation — A condition that afflicts unused and discharged batteries; large crystals of lead sulfate grow on the plate, instead of the usual tiny crystals, making the battery extremely difficult to recharge.

Surface Charge — As a battery is charged the plates will polarize and develop a resistance to the charge; surface charge.   Surface charge  builds as a battery reaches full charge reducing the rate of charge acceptance. Surface charge is removed as a battery is discharged.

Temperature affect on Batteries - COLD Battery capacity is reduced as temperature goes down, and increased as temperature goes up. This is why your car battery dies on a cold winter morning, even though it worked fine the previous afternoon.  If your batteries spend part of the year in a cold area, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at room temperature - 25 degrees C (about 77 F). At approximately -22 degrees F (-27 C), battery AH capacity drops to 50%. At freezing battery, capacity is reduced by 20%. Capacity is increased at higher temperatures - at 122 degrees F, battery capacity would be about 12% higher.

Temperature affect on Batteries - HEAT While battery capacity at high temperatures is higher, battery life is shortened. Battery capacity is reduced by 50% at -22 degrees F - but battery LIFE increases by about 60%. Battery life is reduced at higher temperatures - for every 15 degrees F over 77, battery life is cut in half. This holds true for ALL types of Lead-Acid battery, sealed, gelled, AGM, or industrial.

Thermal run-away - is a very dangerous condition that can occur if batteries are charged too fast. One of the byproducts of Gassing are Oxygen and Hydrogen. As the battery heats up, the gassing rate increases as well and it becomes increasingly likely that the Hydrogen around it will explode. The danger posed by high Hydrogen concentrations is one of the reasons that  batteries be installed in separate, well-ventilated areas.

Thermohydrometer - A Hydrometer containing a thermometer for measuring the temperature of liquids . 

Vented Cell — A battery designed with a vent mechanism to expel gases generated during charging.

Voltage - is a measure of the electric potential difference between two points; usually expressed as volts (V). Lead-Acid batteries supply about 2.14 volts per cell (12.6 to 12.8 for a 12 volt battery) when fully charged.

Wet Shelf Life — The period of time that a charged battery, when filled with electrolyte, can remain unused before dropping below a specified level of performance.

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