Microsoft word - understanding battery charging revg.docx
When using batteries for energy storage, whether an off-grid or grid-tie with back-up power, therenewable energy system used to charge the batteries and the metering systems to monitorperformance must be properly configured for optimal performance. While there is a relativelylarge amount of information for programming the correct values in the battery charging systems,whether a charge controller or inverter/charge, with the advent of more sophisticated meteringsystems, e.g., the Pentametric, additional information is required.
Most batteries used in renewable energy systems use lead-acid chemistry, whether flooded orsealed. Therefore, the following discussions are for this basic technology, i.e., not lithium-ion ormetal-hydride, given one must get the specific values for each battery type and manufacturer.
There is a lack of common terminology for concepts and operations regarding batteries, i.e., thebattery manufacturer uses a different terminology than the renewable energy battery chargermanufacturer or the battery meter manufacturer. The following are some terms and definitionsthat are used with in this document and can be used to interpret descriptions in variousmanufacturer documents.
The second phase during the battery recharging cycle where the voltage is fixed
at the bulk/absorb limit and the charging current decreases to an arbitrarily low limit.
ampere-hour (amp-hour; AH):
A measure of current over time, used to measure battery
capacity and state of charge.
The positive electrode within a battery cell that during charging undergoes the chemical
process of oxidation.
a device that stores energy.
NOTE 1 Electrical batteries consist of a liquid, paste, or solid electrolyte, a positive electrode and anegative electrode to convert chemical energy into electrical energy, rechargeable batteries also convertelectrical energy into chemical energy.
NOTE 2 The electrolyte is an ionic conductor; one of the electrodes will react, producing electrons, whilethe other will accept electrons. When the electrodes are connected to a device to be powered, called aload, an electrical current flows.
NOTE 3 Batteries in which the chemicals can be reconstituted by passing an electric current through themin the direction opposite that of normal cell operation are called secondary cells, rechargeable cells,storage cells, or accumulators.
NOTE 4 The electrolyte is a dilute solution of sulfuric acid, the negative electrode consists of lead, andthe positive electrode of lead dioxide. In operation, the negative lead electrode dissociates into freeelectrons and positive lead ions. The electrons travel through the external electric circuit, and the positivelead ions combine with the sulfate ions in the electrolyte to form lead sulfate. When the electrons reenterthe cell at the positive lead-dioxide electrode, another chemical reaction occurs. The lead dioxidecombines with the hydrogen ions in the electrolyte and with the returning electrons to form water,releasing lead ions in the electrolyte to form additional lead sulfate. A lead-acid storage cell runs down asthe sulfuric acid gradually is converted into water and the electrodes are converted into lead sulfate.
When the cell is being recharged, the chemical reactions described above are reversed until the chemicalshave been restored to their original condition.
The total maximum charge, expressed in ampere-hours that can be withdrawn
from battery under a specific set of operating conditions including discharge rate temperature,
state of charge, age, and cutoff voltage.
The time during which a battery is capable of operating above a specified capacity,
typically end-of-life occurs when a fully charged cell can deliver only 80% of the rated capacity
The first phase during the battery recharging cycle when charging current is only
constrained by the limits of the charging system and the voltage rises from the discharged battery
voltage to the bulk/absorb voltage limit.
The negative electrode within a battery cell that during charging undergoes the
chemical process of reduction.
A component of renewable energy systems that controls the charging of the
battery to protect the batteries from overcharge and over-discharge.
The current applied to a battery to restore its available capacity, specified in
relation to total battery capacity.
NOTE A C/20 charge rate is 1/20th of the total battery capacity measured in amp-hours, e.g., if thecapacity were 100 amp-hours, a C/20 would be 5 amps taking at least 20 hours of bulk charging torecharge.
A battery designed to regularly discharge 50 to 80 percent of the battery
capacity before requiring recharging, with minimal impact on battery life.
The ampere-hours removed from a fully charged battery, expressed
as a percentage of rated capacity.
The current removed from a battery measured in amps.
When required, the process of restoring all cells in a lead-acid battery to an equal
state-of-charge, typically for a duration longer than normal recharging.
A trickle charge to keep a battery fully charged at a safe voltage level with
NOTE The float voltage is slightly higher than the intrinsic open-circuit voltage of a fully chargedbattery.
When a battery is overcharged, the production of oxygen gas at the cathode and when
severely overcharged of hydrogen gas at the anode from electrolysis of water in the electrolyte.
intrinsic battery voltage:
The open circuit voltage of a fully charged battery after the gassing
within the electrolyte from the charging operation has stopped and the resulting polarization of
the battery plates has dissipated.
NOTE Sometimes called the battery rest voltage.
open circuit voltage:
The voltage across the battery terminals with no load or charger attached.
The tendency of all batteries to lose energy to internal chemical reactions within
The ampere-hours remaining in a battery, expressed as a percentage of
The formation of lead-sulfate crystals on the plates of a lead-acid battery, which
decreases battery capacity by impeding the opportunity for chemical reaction within a cell,
typically caused by leaving the battery in a discharged state for long periods of time.
NOTE An equalization is often performed to mitigate sulfation.
temperature compensation coefficient:
The value that the charging voltage must be changed as
a function of the difference in temperature between the standard test condition and the battery.
NOTE 1 The temperature compensation coefficient is usually stated as V/ºC-cell (volts per ºC for eachcell).
NOTE 2 When calculating the compensation voltage, the ΔT is positive for colder battery temperatures and negative for hotter battery temperatures, e.g., one adds the ΔV when cold and subtracts when hot.
usable battery capacity:
The number of amp-hours that are available for use on an ongoing
NOTE The usable battery capacity at a given discharge rate is typically 50% of the maximum batterycapacity at that discharge rate. The usable battery capacity is measured from the intrinsic battery voltagelevel to the minimum recommended battery voltage level, while the maximum capacity is measured fromthe intrinsic battery voltage level to the minimum allowed battery voltage.
All renewable energy systems with batteries, will regularly charge and discharge the batteries.
Given the cost of batteries and the limited lifetime of the batteries (relative to the other systemcomponents), one will want to maximize the performance of the batteries and extend the lifetimeas long as possible. The two major components of extending the lifetime of batteries is to limitthe amount of discharge and properly recharge.
The two ways of controlling the amount of discharge are to:
1. Properly size the battery bank, such that the amount of discharge between recharge
periods is no more than the recommended maximum discharge of the battery, typically50%. Thus, one needs to consider both the loads and capacity of the charging source.
For example, if one is using a photovoltaic source and one has 6 hours/day of sunlight,then the ideal usable battery capacity would be the number of bulk charging hoursavailable, (e.g., 6 hours) minus the minimum absorb time. However, since one must stillconsider loads and another charging source, e.g., a generator, may be available, thebattery bank size is generally more in keeping with the load demand for the expected useperiod.
2. Install a metering system so that a warning signal is available when the batteries approach
the maximum recommended safe discharge level. Many inverter/chargers and chargecontrollers have the capability to automatically start a generator or initiate an alarm.
Meters, such as the Pentametric, have even more sophisticated sensors, allowingpreventive actions, prior to reaching the discharged state.
Batteries are usually characterized using essentially unconstrained recharging sources, i.e., nolimitations on current availability. In general, renewable energy systems have current charginglimitations, whether because of component capability or changing resource availability, e.g., acloudy day for a photovoltaic (PV) system. Thus, monitoring the state-of-charge (SOC) of thebatteries becomes very important, in order to maximize the lifetime of the batteries.
In order to properly recharge the batteries (see figure 1), one must use the battery recommendedsettings. Sometimes this is not always possible, e.g., the bulk/absorb timer on many chargecontrollers does not allow the batteries to fully meet the desired recharge time during a shortwinter day. If batteries cannot be regularly charged for the full absorb cycle, then a quarterlyequalization charge may be appropriate, assuming the battery manufacturer allows equalizationcharging. The following uses a Concorde SunXtender battery for illustration, noting differentbatteries and manufacturers will have different values. The examples use a nominal 2-voltbattery, for higher nominal systems, just use the appropriate multiplier, e.g., for a nominal 48 Vsystem multiply by 24.
The battery manufacturer often specifies a recharge voltage and a time for a 50% dischargedbattery. The charge controller (or inverter/charger) manufacturer specifies a bulk and absorbvoltage. Once the recharge cycle is initiated, e.g., when the charge controller turns on sometimeafter sunrise, the system provides the maximum available current at the battery voltage until thebattery voltage reaches the bulk/absorb voltage setting. For example, if the battery is 30%discharged and the bulk current is limited to C/10, then the bulk time will be 3 hours. The
absorb timer then initiates the absorb cycle for the duration of the timer, which may be fixed orprogrammable. During this time, the current will gradually decline to a negligible value.
After the absorb timer times out, the controller supplies current at the battery voltage. Thebattery voltage will decay from the absorb voltage to the float voltage over a period of time,typically 2 to 4 hours with no load; with load the system will settle at the float voltage muchsooner. After this time, the controller will maintain the float voltage (which is slightly higherthan the intrinsic voltage), unless the load is greater than the generating resource. Then thebattery will start to discharge, the amount determined by the loads and the generating resource.
When the battery is discharging, the controller output voltage will track the battery voltage,maximizing the available current at that voltage.
Typical settings for a 2-V battery would be:
A. Intrinsic voltage: 2.167 V.
B. Float voltage: 2.2 V.
C. Recharge/bulk/absorb voltage: 2.4 VD. Absorb time: 4 hoursE. Minimum recommended battery voltage: 2.0 VF. Minimum allowed battery voltage: 1.75 VG. Minimum charge rate: C/20H. Maximum charge rate: 2CI.
Temperature compensation coefficient: 0.00375V/ºC-cell referenced to 25 ºC
J. Temperature compensation range: 0 – 40 ºC (32 – 104 ºF)
Chemical reactions, such as charging a battery, are affected by temperature, i.e., increasing thetemperature increases the reaction rate, decreasing the temperature decreases the reaction rate.
Therefore, for battery temperatures colder than the standard test condition (25 ºC), the chargingvoltage should be increased by the temperature compensation coefficient value (i.e., the numberof cells times the coefficient times the difference in temperature from standard). Similarly, thevoltage should be decreased for higher than standard battery temperatures. All charging voltagesshould be temperature compensated; this is usually accomplished by connecting a batterytemperature sensor mounted on the battery (below the electrolyte level) to the charge controller.
Thus, for a PV charge controller, after sunrise and the minimum on current is reached, the chargecontroller would start the recharge cycle. First, the charge controller would supply maximumavailable current at the battery voltage until the bulk/absorb (2.4 V) voltage is reached, then stayat 2.4 V for 4 hours. Over the next 2 to 4 hours (no load), the battery voltage will decrease tofloat level, which will be maintained until the battery has started to discharge or the nextrecharge cycle begins.
Since there is so much variability in both load and recharging values, a battery monitoringsystem is strongly recommended. The battery monitoring system should measure both batteryvoltage and either SOC or DOD. In order to initiate alarms at the proper levels, the batterymonitoring system needs to have such values as the upper battery voltage limit, e.g., some valueabove the bulk/absorb value, the lower battery voltage limit, e.g., the 50% discharge value of 2V, and usually the float voltage, e.g., 2.2 V.
In order to measure the SOC or DOD, the battery capacity must be entered. Since the batterycapacity is a function of the discharge rate, one must know (at least estimate) the discharge rate.
For most systems that have properly sized the battery bank relative to usage and generationcapability, the discharge rate will be something in the order of 16 to 20 hours, given the batterywill start to discharge immediately after the daily recharge cycle. If one only uses a generator,then the discharge time will be the interval between operating the generator, e.g., could be lessfrequent than once per day.
Given a discharge rate, the usable battery capacity is typically 50% of the stated battery capacity(see Figure 2). The stated battery capacity is usually listed in the manufacturer’s documentationas the 100% discharge value; however, in order to obtain the maximum lifetime of the battery,one should never 100% discharge the battery. The usable battery capacity is what one wouldtypically enter into a metering system that monitors SOC or DOD; that way one would notnormally over-discharge and potentially damage the battery, e.g., reduce the expected lifetime.
Equalization is a specific cycle when re-charging a lead-acid battery. The general parameters forperforming equalization are provided herein. Specific values will vary by battery technology,e.g., flooded or sealed, and by the specific manufacturing technology; therefore, specificationlimits provided by the manufacturer must be carefully reviewed.
In order to properly recharge the batteries (see figure 1), one must use the battery specificrecommended settings (available from the manufacturer). Sometimes this is not always possible,e.g., the bulk/absorb timer on many charge controllers does not allow the batteries to fully meetthe desired recharge time during a short winter day. If batteries cannot be regularly charged forthe full bulk & absorb cycle, then a quarterly (or monthly) equalization charge may beappropriate, assuming the battery manufacturer allows equalization charging, e.g., some sealedbattery manufacturers do not recommend a separate equalization cycle.
The bulk/absorb settings for voltage and current for the normal recharge are the same settingsused for equalization; however, the durations may vary, e.g., when performing an equalizationwith a generator after a normal, but incomplete renewable energy recharge cycle. Essentially,when doing a normal recharge as set by the charge controller or inverter, the cycle is called bulk& absorb; when doing a recharge specifically to address issues such as sulfation or extendedfloat charging, the cycle is called equalization.
Lead-acid batteries require up to 5% more ampere-hours to recharge to the intrinsic batteryvoltage than the ampere-hours used when discharging; this efficiency loss is integrated in therecharge cycle. A good practice is to over-charge flooded batteries by 2% each day and 5% onceper week above the nominal usable battery capacity. This is accomplished by keeping the
battery at the absorb voltage until the number of excess ampere-hours has been transferred to thebattery. For example, if the battery bank is rated at 400 ampere-hours and a 2% overcharge isrequired, maintain the absorb voltage after reaching the normal low trickle charge until anadditional 8 ampere-hours are transferred, e.g., less than 1 hour with a generator or renewableenergy system providing 10 amperes. Regular equalization is a critical component for floodedbatteries in an off-grid system. The renewable energy system and battery bank should be sizedso that a typical daily DOD is < 15% (e.g., at night for a photovoltaic system) and externalrecharging should start is the SOC is < 30% (note a recharge must occur before SOC reaches50% or 100% DOD of usable battery capacity).
Sealed batteries generally do not need equalization in off-grid systems, since the renewableenergy recharging process uses the same settings as would be used for equalization. If thebatteries are not fully charged by the renewable energy system, then the generator based batterycharger will be using the equalization settings to recharge the batteries. In battery backed-up on-grid systems, sealed batteries may require quarterly equalization (if so allowed by themanufacturer), if there are no grid outages, e.g., the batteries are only seeing a float charge (torecover the normal self-discharge losses).
Depth of Discharge
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