6. Batteries and ControllersHerb WadeConsultant Solar PV Design Implementation O& M March 31- April 11, 2008 Marshall Islands
6. Batteries and Controllers • Contents 6-1. Batteries for Solar Systems 6-2. Controllers
Types of Batteries • Lead-Acid • Cheapest, mature technology, readily available in a wide range of types • Easily damaged by improper discharge control, some types require periodic maintenance • Nickel-Cadmium • Expensive, mature technology, not readily available • Not sensitive to overcharging and high discharge levels • Long life, minimal maintenance • Other • Under development for electric and hybrid cars. Main advantage light weight and high energy density, no maintenance, not touchy regarding charging and discharging. Expensive.
Lead Acid Battery Construction • Basic requirement: To allow the reversible chemical reaction that absorbs then releases electricity to function efficiently:Pb + 2H2SO4 + PbO2 PbSO4 + 2H2O + PbSO4Lead Sulfuric Acid Lead Oxide Lead Sulfate Water Lead Sulfate- + - + Lead is a metal. Lead oxide is a hard, black gritty solid.Lead Sulfate is a softer whitish solid. Sulfuric Acid isa liquid as is water.
Cycle life • A basic measure of the life of a battery for any application is its rated cycle life, a characteristic provided by the manufacturer based on actual tests by cycling batteries • The number of full charge/discharge cycles that a battery can provide before losing 20% of its rated capacity • A battery slowly loses capacity as it ages but once about 20% of capacity is lost the ageing process accelerates and the battery quickly fails completely. So a battery is considered to be at the end of its useful life when its capacity has decreased to 80% of its rated capacity
Cycle life (2) • Partial cycles add up to make full cycles. So five 20% discharge cycles = 1 full cycle. Ten 10% cycles = 1 full cycle, etc. So in theory a battery that is always discharged 20% each day and has a cycle life of 1000 cycles will in theory last 1000/0.20 = 5000 days (over 13.5 years). But cycle life usually represents the maximum life not average life so actual life usually is less than indicated by cycle life. Still cycle life is a good indicator of the comparative life of batteries. A 100 cycle battery probably will only last half as long as a 200 cycle battery. • Cycle life changes with rate of discharge. Cycle life at C10 is much less than at C100 for example.
Cycle life relative to DoD • For a high quality solar battery, if the average DoD is 80% the cycle life is rated at 600 cycles (about 2 years) • For an average DoD of 40%, the cycle life is rated at 1450 cycles (about 10 years) • For an average DOD of 20%, the cycle life is rated at 2000 cycles or more than 25 years. Unless maintenance is excellent and the system very well designed, it is possible for a very good battery to last longer than 15 years in solar service and more than 20 years in stationary backup service. So cycle life should be specified to allow it to survive at least that long at the average depth of discharge for the solar system.
Internal construction • Determined by the type of use and cost • Type of use mainly the speed that the battery must deliver and/or accept power and depth of discharge • 1. Starting battery: Very high current (speed of energy delivery) for a short time. Never discharged more than 1% or 2% • 2. Traction battery (like for electric car or boat motor): Medium current delivery for medium time. Often discharged 50%-80% • 3. Solar battery: Slow current delivery for long time. Sometimes discharged 80%, mostly 20%-30% • 4. Backup battery: Most of the time kept at full charge then must reliably deliver energy to operate equipment (telecom, UPS, etc.) to deep discharge
Postive plate construction • Flat plate: Large surface area in contact with acid allows fast chemical reaction and high current delivery • Need lots of surface in contact with electrolyte to allow chemical reaction to work. Tiny grains of lead oxide (like sand) packed (pasted) into a lead grid. Huge surface area but grains deeper into the pack are harder for the electrolyte to get to quickly. • The more external surface area the more current so for starting batteries that need high current, many thin plates are used. But fragile and high surface area also means high probability of loss of grains of lead oxide from the plate surface
Positive Plate Construction (2) • Tubular plate • Lead oxide grains packed in a tubular shape around a central electrode inside a porous tube. Minimal surface area to lose grains of lead oxide and held in place by tube so the battery has a long life but because electrolyte is slower to penetrate the thick layer of active material high current cannot be maintained. Best for lower current but long times of discharge. Excellent for deep discharge applications
Internal changes with charging • Lead sulfate is a larger molecule than lead oxide so when discharging and lead oxide is converted to lead sulfate the material swells. The difference in bulk between charged and discharged plates is around 10%. • Deep discharge of flat pasted plates causes swelling that can push grains of active material off the surface of the plates and they then fall to the bottom of the battery and are no longer available to be charged or discharged. Causes loss of Ah capacity • Can be reduced by putting porous sheets over the plate surface but that slows down rate of chemical reaction and reduces the maximum current the battery can produce • Starting batteries are therefore quickly damaged by deep discharge
Starting battery • Starting battery: Large number of thin plates to maximize the rate of the chemical process and therefore to instantly produce high current for engine starting. Only produces current for a few seconds so the total depth of discharge (DoD) is normally 1% or less. So swelling of the plates is minimal and few problems with loss of surface grains. • If used in solar with DoD of 20%-30% there is substantial swelling and the large surface area of the many thin plates allows rapid loss of grains from the surfaces and a short life (6 mo to 2 years according to the quality of construction of the battery) • Type of use effectively C0.2 I.e. very high rate of discharge though for a very short time usually • Cycle life only 5-50 according to quality of construction
Traction battery for solar use • Traction batteries are used for electric vehicles like golf carts, small boats (trolling), industrial fork lifts, etc. Fairly high current but much less than starting motors. Need to deliver energy as long as possible between charges so DoD of 80% is common. • Flat plates to provide fairly high surface area and high enough current but better batteries can use porous sheet to reduce surface grain losses • Much thicker plates than starting battery since lower current needed. The longer time it takes for electrolyte to get to internal grains in the plate (and for the water that is produced to get out) the lower the current production. But thicker plates have less surface area for loss of active material grains so longer life • Typical type of use is C5 to C10 so the motor can run all day before the battery is discharged. • Typical cycle life 200-500 according to quality of construction and rate of discharge
Tubular positive plate batteries • Relatively low maximum current capability because getting electrolyte to interior grains in the tube is fairly slow • Least possible surface area for losing grains of material so very long life • Physically bigger than flat plate batteries because tubular construction takes more internal space for the same amount of active material • Best for C20-C100 applications. Modest current delivered over a long period - e.g. most solar systems such as SHS and remote telecom power • Cycle life 500-3000+ according to quality of construction and rate of discharge
Back up batteries • Sometimes called “stationary” batteries. Used as an emergency power source if the main source fails. Typically kept at full charge for long periods but then may be deeply discharged • Telecom back up batteries, UPS batteries for computer power backup, electronic control equipment backup, etc. • Service life based on frequency of power outages. Typically very long service life relative to other applications. Cycle life not very relevant since batteries are not normally cycled between charge and discharge. Life largely determined by resistance to sulfation, water loss rate and resistance to internal corrosion.
General Battery Characteristics • Nominal voltage (number of 2V cells) • Capacity in Ampere hours • Open cell or sealed • Liquid or Gel electrolyte • Cycle life • Acceptable repeated depth of discharge • “Starting Amps” “Number of Plates” or “Starting Minutes” not useful for solar specifications
The effect of discharge rates • The more hours taken to discharge a battery, the more energy can be transferred because with a slow discharge the chemical process that produces electricity is more efficient. So a battery delivers more Ah at C100 than at C10 by a quite significant amount. Note that C100 means that the battery takes 100 hours to discharge fully while C10 means it only takes 10 hours to discharge fully. A 100Ah battery at C100 may become a 65 Ah battery at C10 discharge rate.
Battery Ah ratings • A 100 Ah battery at C100 may be a 65 Ah battery at C10. So to compare batteries, the battery Ah rating must include the Cx rate for the stated capacity and you must compare at the same Cx rate. • Manufacturers of solar batteries, particularly those of questionable quality, often give Ah ratings using a C100 discharge rate. That gives a substantially inflated Ah value but that capacity is never reached in practice. • Always compare battery capacities at the same discharge rate, preferably C10 or C20 (C20 represents the typical solar discharge rate for SHS and is best though C10 comparisons are commonly done and are ok. Just be sure all comparisons are at the same Cx rate.)
Physical strength of construction • For longest service life, pure lead is the best material for making negative plates and for other components internal to the battery. Pure lead batteries have the lowest water loss rate of any lead-acid battery and the fewest problems with internal corrosion. • Unfortunately pure lead is very soft and not physically strong. So for batteries that will be in vehicles or that will have to survive rough transport, pure lead batteries are too weak internally and vibration and physical shocks cause internal damage. • Most batteries have either antimony or calcium added to lead to make it stronger • Antimony added to the lead provides long cycle life but tends to cause a battery to lose water faster than may be desirable • Calcium added to the lead causes a shorter cycle life but very low water loss
Battery uses vs lead additive • For stationary (backup) batteries that have to have very low water loss and very long service life, pure lead is usually used but they are sensitive to shock and vibration so have to be handled carefully, They have the longest cycle life. • For sealed batteries that have to have very low water loss and need to be able to withstand shocks and vibration, calcium is often added to the lead structure. The disadvantage is that the cycle life is reduced. • For open cell batteries where water can be added to cells when electrolyte levels fall, antimony is often added to the lead structure. The disadvantage is that the water loss is higher than either pure lead or calcium added to lead. The cycle life is about as long as pure lead however.
Measuring the level of charge • Battery voltage. About 10.5V represents full discharge. Full charge voltage for a battery with no current flow in or out will be around 12.6V. When charging, full charge is reached at about 14.2V • Electrolyte specific gravity. This is an indication of acid concentration and is measured using a hydrometer. The higher the value the greater the charge (1.26 to 1.28 is full charge, 1.0 to 1.1 is about fully discharged). Neither battery voltage nor specific gravity is an accurate measure of charge, especially in old batteries.
Accuracy of charge estimation • The use of voltage to determine level of charge is not very accurate because the rate of charge or discharge affects the voltage too. As the battery gets older the accuracy of voltage readings as an indicator of state of charge during charging or discharging gets less and less because the battery’s internal resistance goes up. • The use of specific gravity is accurate when a battery is new but as the battery ages, the hydrometer tends to show a lower charge than is actually present due to increasing sulfation
Procedure for Hydrometer Measurements • Clean the top of the battery with a rag and water • Flush out the hydrometer with clean battery water • Open one cell at a time placing the cap upside down on the top of the battery. Never open all caps at once. • Draw the cell electrolyte into the hydrometer so the float does not touch the bottom of the hydrometer tube and read the value • Return the electrolyte to the cell. Add water if needed. • Replace the cell cap and open the next cell. • Draw the liquid into the hydrometer and read. Repeat for all cells. • After completing, flush out the hydrometer with battery water and replace in shock proof carrier.
Interpreting Hydrometer Readings • The reading indicates the relative level of charge for the cell. All cells should be about the same (within about 0.03 of each other) • If any cells are significantly different from the others, it is a bad sign. An equalizing charge should be made if possible.
Equalizing Charge in the Field • Connect the battery directly to the panels (bypass the controller) • Disconnect all loads and ask the user not to reconnect at night during the charging process • Bring the battery to full charge • Continue charging the battery for at least one bright sun day after full charge is reached. • Keep a close watch on water level and keep the cells filled but do not over fill.
Causes of Battery Failures • Sulfation – Most common problem. Sulfation is where part of the cell becomes resistant to charge. Caused by the battery remaining at partial charge for long periods. May be offset by equalizing charges when cells are seen to have unequal specific gravity. • Internal corrosion – results in high internal resistance and open circuits. Caused by cheap design, adding acid instead of water and stratification of the acid in some types of batteries • Internal shorts – results in one or more cells not producing voltage. May be due to cheap construction, overheating or mechanical damage • Loss of active material from plates – caused by excessive depth of discharge and age. Mostly a problem with cheaper batteries.
What is Sulfation? • When a battery discharges, Lead Sulfate is created. When the battery is recharged, the Lead Sulfate is supposed to dissolve. But if the Lead Sulfate is not dissolved after a week or so because the battery is not fully charged, it tends to form a mass that is very difficult to dissolve when charging does take place. Over time the amount of Lead Sulfate increases and the battery loses its ability to be charged fully. The effect is a loss of capacity. A 100Ah battery may become a 50Ah battery after serious sulfation has occurred.
What are the signs of sulfation? • When battery voltage indicates a full charge but the hydrometer reading indicates a partial charge, that is a strong indicator that serious sulfation has occurred in the battery. • Lead Sulfate is white in color. When looking at the plates in a battery, if the battery has been fully charged and the plates look light in color or have white sections, that is an indication of sulfation
Failure modes of batteries • Total loss of power. Zero volts, cannot charge. Caused by an internal open circuit. This may be because of corrosion eating through a cell connector or mechanical damage • Gradually decreasing capacity. The time to charge and discharge gets shorter and shorter. Caused by sulfation or loss of active material from the plates or both. Accelerated by deep discharge conditions and operation at partial charge levels for weeks at a time. • Reduced voltage at full charge. Cannot get the battery to charge to more than about 10V. Caused by a short in a cell making one cell inoperative. Excessive discharge and mechanical damage are typically the reasons for this mode of failure.
Main Causes of Early Battery Failure (Open Cell Batteries) • Use of wrong kind of battery • Panel capacity inadequate so the battery does not come to full charge regularly • System design not based on lowest solar month causing batteries to stay at partial charge condition during low months • Controlers not working properly • Over use of electricity keeping the battery in a constant state of partial discharge • Inadequate or incorrect maintenance • Addition of acid to cells instead of water • Addition of impure water to cells • High temperature of operation (35º or more) • Too deep a discharge for the type of battery being used • Mechanical damage caused by hammering on the posts, lifting the battery by the terminals, jolting the battery too much in transport.
Main Causes of Early Battery Failure (Sealed Batteries) • Repeated overcharging (wrong controller setting) • Failure to maintain a high average charge level thereby encouraging sulfation • High operating temperature (over 30°C) • Repeated deep discharge • Mechanical damage caused by hammering on the terminals, lifting the battery by the terminals, jolting the battery too much during transport
How are batteries damaged by excess discharge? • When a battery discharges the plates swell. The deeper the discharge the more the plates swell. Batteries that are not designed for deep discharge have flat plates that have the active material pressed into pockets in the plate. When the plates swell greatly due to deep discharge, some of the active material is pushed out of the pockets and falls to the bottom of the battery causing loss of capacity and possible shorting of cells. Batteries that are designed for deep discharge have plates that have the active material wrapped in porous membranes to prevent the swelling from causing the active material to fall off. This adds considerably to the cost but increased battery life.
Time between charging for idle batteries If a battery is being stored fully charged, a hightemperature of storage means the battery mustbe recharged more frequently than if a coolerstorage temperature can be maintained. Delayingcharging beyond these limits allows sulfation tooccur and a permanent loss of some capacity.
Self-discharge • The effect of storage temperature on self-discharge percentage of high quality tubular cell batteries The number in the table is the percent of charge lost at the given time period and temperature. For example, a charged battery stored at 40C for 4 months will lose 36% of its charge
Storing dry charged batteries • High quality dry charged batteries can usually be safely stored for up to 2 years without significant damage provided: • Storage is at 25°C or cooler • Cell seals remain in place (usually a special cell cap or a tape seal over the cap vent hole) • Storage is in original packing and batteries remain upright in the normal operating position • Shorter storage life will result if these conditions are not observed. In particular breaking the cell seal or storage at 35°C or higher will result in rapid deterioration of batteries. NEVER store wet or dry charged batteries in unshaded, unventilated containers.
Filling dry charged batteries • Fill slowly. It is advisable to take 30 seconds or more per liter of electrolyte put in a battery. Small batteries with more than one cell should have the liter split about equally among all the cells. • If the cells do not get hot to the touch, let the battery sit for about two hours and then charge to full charge, effectively an equalizing charge. It is important that there be NO LOAD applied during the time of the first charge. The service life of the battery may be seriously reduced if a load it turned on before the first charge is fully completed.
Damaged dry charged batteries, commissioning in the field • If the cell gets noticeably hot to the touch, that is an indication that the cell has been damaged by poor storage conditions or excess time in storage. The battery will have a drastically shortened life if it is not properly charged after filling. Do NOT commence charging sooner than 12 hours after filling • For proper charging, connect directly to the solar panels without a controller in the circuit and allow to charge WITH NO LOAD APPLIED for at least two weeks checking the battery water daily and replacing any that has been lost.
Damaged dry charged batteries, commissioning with a charger • If a mains or generator powered charger is available, a shorter time can be taken for charging. Method one is to charge at constant voltage of 2.40V per cell for 96 hours continuously or until the SG of all cells is the full charge rated value. Or method 2: • Charge continuously at a constant current of between 0.02 C10 and 0.05 C10 for 10-12 hours or until the SG of all cells is the full charge rated value. Constant current charging can restore seriously degraded dry charged batteries. NO LOAD SHOULD BE APPLIED DURING THE CHARGING PROCESS.
Choices for Solar Use • Open cell batteries, either flat plate or tubular cell types, provide the best value and longest life but electrolyte levels have to be checked and water added when needed • Tubular cell, deep discharge batteries provide the longest life and should be used where access for replacement is expensive or very difficult • Valve regulated, sealed batteries are only recommended where there is no one to properly maintain an open cell battery • Worst choice is an automotive type “maintenance free” battery.
Increasing battery voltage • Add cells or batteries in series. Increments may be 2V (single cells for large batteries), 6V (three cells in one case – medium sized batteries), 12V (six cells in one case, smaller batteries). No problems usually develop because of series connections though a battery with a shorted cell can create overcharging conditions for the rest of the cells because the lower battery voltage that results from a shorted cell makes the controller think that the battery still needs charging so the charging current is not shut off when the good cells do come to full charge. The result is excessive water loss from the battery, a definite symptom of a shorted cell in an SHS battery along with voltage that is lower than expected.
Increasing Ah capacity • Put in a larger battery. It is always preferred to use a single large battery than to connect batteries in parallel • It is possible to parallel identical batteries just as it is possible to parallel panels. However never should more than two batteries be placed in parallel and even then do not expect as long a life as a single larger battery. If one cell of either battery loses capacity, both batteries may rapidly develop sulfation problems. Note that many battery manufacturers void battery warranties if more than two are paralleled.
Large battery Bank 48V battery bank (Cook Islands)
Battery Safety • Most injuries relating to batteries are the result of dropping them or being hurt somehow by their weight. Do not carry batteries by the connections, always support the battery from the bottom or sides of the case. Preferably use a special carry strap made for the purpose. For large batteries share the load with another person. Many smaller batteries have built in handles. Use them • For open cell batteries, note that the electrolyte is dilute sulfuric acid and can cause mild chemical burns on the skin and is toxic if swallowed. If the acid gets into your eyes, immediate flushing with water is vital to avoid eye damage. For that reason, keep a full bucket of water nearby when working with batteries and battery acid. • Be sure cell caps have clear ventilation holes. A plugged ventilation hole will cause pressure build up in the cell and will cause the battery case to swell and may cause damage to the battery. • Never lay tools on top of the battery. A short circuit could occur and may damage the battery, cause an explosion or cause burns. • Do not smoke around batteries that are charging. Explosive gas is present in the cells. • Do not take the caps off battery cells when charging.
When should you add more acid? • Unless electrolyte is actually spilled out of the battery, you should never add acid, only pure water. It is not the sulfuric acid that evaporates, it is water only. Adding more acid gradually increases the strength of the acid and increases the rate of internal corrosion but in no way increases the charge in the battery or makes it easier to charge.