Aug. 19, 2024
Invented by the French physician Gaston Planté in , lead acid was the first rechargeable battery for commercial use. Despite its advanced age, the lead chemistry continues to be in wide use today. There are good reasons for its popularity; lead acid is dependable and inexpensive on a cost-per-watt base. There are few other batteries that deliver bulk power as cheaply as lead acid, and this makes the battery cost-effective for automobiles, golf cars, forklifts, marine and uninterruptible power supplies (UPS).
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The grid structure of the lead acid battery is made from a lead alloy. Pure lead is too soft and would not support itself, so small quantities of other metals are added to get the mechanical strength and improve electrical properties. The most common additives are antimony, calcium, tin and selenium. These batteries are often known as &#;lead-antimony&#; and &#;leadcalcium.&#;
Adding antimony and tin improves deep cycling but this increases water consumption and escalates the need to equalize. Calcium reduces self-discharge, but the positive lead-calcium plate has the side effect of growing due to grid oxidation when being over-charged. Modern lead acid batteries also make use of doping agents such as selenium, cadmium, tin and arsenic to lower the antimony and calcium content.
Lead acid is heavy and is less durable than nickel- and lithium-based systems when deep cycled. A full discharge causes strain and each discharge/charge cycle permanently robs the battery of a small amount of capacity. This loss is small while the battery is in good operating condition, but the fading increases once the performance drops to half the nominal capacity. This wear-down characteristic applies to all batteries in various degrees.
Depending on the depth of discharge, lead acid for deep-cycle applications provides 200 to 300 discharge/charge cycles. The primary reasons for its relatively short cycle life are grid corrosion on the positive electrode, depletion of the active material and expansion of the positive plates. This aging phenomenon is accelerated at elevated operating temperatures and when drawing high discharge currents. (See BU-804:How to Prolong Lead Acid Batteries)
Charging a lead acid battery is simple, but the correct voltage limits must be observed. Choosing a low voltage limit shelters the battery, but this produces poor performance and causes a buildup of sulfation on the negative plate. A high voltage limit improves performance but forms grid corrosion on the positive plate. While sulfation can be reversed if serviced in time, corrosion is permanent. (See BU-403: Charging Lead Acid)
Lead acid does not lend itself to fast charging and with most types, a full charge takes 14&#;16 hours. The battery must always be stored at full state-of-charge. Low charge causes sulfation, a condition that robs the battery of performance. Adding carbon on the negative electrode reduces this problem but this lowers the specific energy. (See BU-202: New Lead Acid Systems)
Lead acid has a moderate life span, but it is not subject to memory as nickel-based systems are, and the charge retention is best among rechargeable batteries. While NiCd loses approximately 40 percent of their stored energy in three months, lead acid self-discharges the same amount in one year. The lead acid battery works well at cold temperatures and is superior to lithium-ion when operating in subzero conditions. According to RWTH, Aachen, Germany (), the cost of the flooded lead acid is about $150 per kWh, one of the lowest in batteries.
The first sealed, or maintenance-free, lead acid emerged in the mid-s. Engineers argued that the term &#;sealed lead acid&#; was a misnomer because no lead acid battery can be totally sealed. To control venting during stressful charge and rapid discharge, valves have been added that release gases if pressure builds up. Rather than submerging the plates in a liquid, the electrolyte is impregnated into a moistened separator, a design that resembles nickel- and lithium-based systems. This enables operating the battery in any physical orientation without leakage.
The sealed battery contains less electrolyte than the flooded type, hence the term &#;acid-starved.&#; Perhaps the most significant advantage of sealed lead acid is the ability to combine oxygen and hydrogen to create water and prevent dry out during cycling. The recombination occurs at a moderate pressure of 0.14 bar (2psi). The valve serves as a safety vent if the gas buildup rises. Repeated venting should be avoided as this will lead to an eventual dry-out. According to RWTH, Aachen, Germany (), the cost of VRLA is about $260 per kWh.
Several types of sealed lead acid have emerged and the most common are gel, also known as valve-regulated lead acid (VRLA), and absorbent glass mat (AGM). The gel cell contains a silica type gel that suspends the electrolyte in a paste. Smaller packs with capacities of up to 30Ah are often called SLA (sealed lead acid). Packaged in a plastic container, these batteries are used for small UPS, emergency lighting and wheelchairs. Because of low price, dependable service and low maintenance, the SLA remains the preferred choice for healthcare in hospitals and retirement homes. The larger VRLA is used as power backup for cellular repeater towers, Internet hubs, banks, hospitals, airports and more.
The AGM suspends the electrolyte in a specially designed glass mat. This offers several advantages to lead acid systems, including faster charging and instant high load currents on demand. AGM works best as a mid-range battery with capacities of 30 to 100Ah and is less suited for large systems, such as UPS. Typical uses are starter batteries for motorcycles, start-stop function for micro-hybrid cars, as well as marine and RV that need some cycling.
With cycling and age, the capacity of AGM fades gradually; gel, on the other hand, has a dome shaped performance curve and stays in the high performance range longer but then drops suddenly towards the end of life. AGM is more expensive than flooded, but is cheaper than gel. (Gel would be too expensive for start/stop use in cars.)
Unlike the flooded, the sealed lead acid battery is designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging causes gassing, venting and subsequent water depletion and dry-out. Consequently, gel, and in part also AGM, cannot be charged to their full potential and the charge voltage limit must be set lower than that of a flooded. This also applies to the float charge on full charge. In respect to charging, the gel and AGM are no direct replacements for the flooded type. If no designated charger is available for AGM with lower voltage settings, disconnect the charger after 24 hours of charge. This prevents gassing due to a float voltage that is set too high. (See BU-403: Charging Lead Acid)
The optimum operating temperature for a VRLA battery is 25°C (77°F); every 8°C (15°F) rise above this temperature threshold cuts battery life in half. (See BU-806a: How Heat and Loading affect Battery Life) Lead acid batteries are rated at a 5-hour (0.2C) and 20-hour (0.05C) discharge rate. The battery performs best when discharged slowly; the capacity readings are substantially higher at a slower discharge than at the 1C-rate. Lead acid can, however, deliver high pulse currents of several C if done for only a few seconds. This makes the lead acid well suited as a starter battery, also known as starter-light-ignition (SLI). The high lead content and the sulfuric acid make lead acid environmentally unfriendly.
Lead acid batteries are commonly classified into three usages: Automotive (starter or SLI), motive power (traction or deep cycle) and stationary (UPS).
The starter battery is designed to crank an engine with a momentary high-power load lasting a second or so. For its size, the battery is able to deliver high current but it cannot be deep-cycled. Starter batteries are rated with Ah or RS (reserve capacity) to indicate energy storage capability, as well as CCA (cold cranking amps) to signify the current a battery can deliver at cold temperature. SAE J537 specifies 30 seconds of discharge at &#;18°C (0°F) at the rated CCA ampere without the battery voltage dropping below 7.2 volts. RC reflects the runtime in minutes at a steady discharge of 25. (SAE stands for Society of Automotive Engineers.) See also BU-902a: How to Measure CCA.
Starter batteries have a very low internal resistance that is achieved by adding extra plates for maximum surface area (Figure 1). The plates are thin and the lead is applied in a sponge-like form that has the appearance of fine foam, expanding the surface area further. Plate thickness, which is important for a deep-cycle battery is less important because the discharge is short and the battery is recharged while driving; the emphasis is on power rather than capacity.
The deep-cycle battery is built to provide continuous power for wheelchairs, golf cars, forklifts and more. This battery is built for maximum capacity and a reasonably high cycle count. This is achieved by making the lead plates thick (Figure 2). Although the battery is designed for cycling, full discharges still induce stress and the cycle count relates to the depth-of-discharge (DoD). Deep-cycle batteries are marked in Ah or minutes of runtime. The capacity is typically rated as a 5-hour and 20-hour discharge.
A starter battery cannot be swapped with a deep-cycle battery or vice versa. While an inventive senior may be tempted to install a starter battery instead of the more expensive deep-cycle on his wheelchair to save money, the starter battery would not last because the thin sponge-like plates would quickly dissolve with repeated deep cycling.
There are combination starter/deep-cycle batteries available for trucks, buses, public safety and military vehicles, but these units are big and heavy. As a simple guideline, the heavier the battery is, the more lead it contains, and the longer it will last. Table 3 compares the typical life of starter and deep-cycle batteries when deep cycled.
Depth of Discharge
Starter Battery
Deep-Cycle Battery
100%
12&#;15 cycles
150&#;200 cycles
50%
100&#;120 cycles
400&#;500 cycles
30%
130&#;150 cycles
1,000 and more cycles
Table 3: Cycle performance of starter and deep-cycle batteries.Ever since Cadillac introduced the starter motor in , lead acid batteries served well as battery of choice. Thomas Edison tried to replace lead acid with nickel-iron (NiFe), but lead acid prevailed because of its rugged and forgiving nature, as well as low cost. Now the lead acid serving as starter battery in vehicles is being challenged by Li-ion.
Figure 4 illustrates the characteristics of lead acid and Li-ion. Both chemistries perform similarly in cold cranking. Lead acid is slightly better in W/kg, but Li-ion delivers large improvements in cycle life, better specific energy in Wh/kg and good dynamic charge acceptance. Where Li-ion falls short is high cost per kWh, complex recycling and less stellar safety record than lead acid.
Lead is toxic and environmentalists would like to replace the lead acid battery with an alternative chemistry. Europe succeeded in keeping NiCd out of consumer products, and similar efforts are being made with the starter battery. The choices are NiMH and Li-ion, but the price is too high and low temperature performance is poor. With a 99 percent recycling rate, the lead acid battery poses little environmental hazard and will likely continue to be the battery of choice.
Table 5 lists advantages and limitations of common lead acid batteries in use today. The table does not include the new lead acid chemistries. (See also BU-202: New Lead Acid Systems)
[1] Source: Johnson Control
Dependable performance and long service life of your sealed lead acid battery will depend upon correct battery charging. Following incorrect charging procedures or using inadequate charging equipment can result in decreased battery life and/or poor battery performance. The selection of a suitable SLA battery charger and the methods used to charge it is just as important as choosing the right battery for the application.
Power Sonic recommends you select a charger designed for the chemistry of your battery. This means we recommend using a sealed lead acid battery charger, like the the A-C series of SLA chargers from Power Sonic, when charging a sealed lead acid battery.
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Sealed lead acid batteries may be charged by using any of the following charging techniques:
To obtain maximum battery service life and capacity, along with acceptable recharge time and economy, constant voltage-current limited charging is best.
To charge a sealed lead acid battery, a DC voltage between 2.30 volts per cell (float) and 2.45 volts per cell (fast) is applied to the terminals of the battery. Depending on the state of charge (SoC), the cell may temporarily be lower after discharge than the applied voltage. After some time, however, it should level off.
During charge, the lead sulfate of the positive plate becomes lead dioxide. As the battery reaches full charge, the positive plate begins generating dioxide causing a sudden rise in voltage due to decreasing internal resistance. A constant voltage charge, therefore, allows detection of this voltage increase and thus control of the current charge amount.
During constant voltage or taper charging, the battery&#;s current acceptance decreases as voltage and state of charge increase. The battery is fully charged once the current stabilizes at a low level for a few hours. There are two criteria for determining when a battery is fully charged: (1) the final current level and (2) the peak charging voltage while this current flows.
Typical sealed lead acid battery charge characteristics for cycle service where charging is non-continuous and peak voltage can be higher.Typical characteristics for standby service type battery charge. Here, charging is continuous and the peak charge voltage must be lower.Selecting the appropriate charging method for your sealed lead acid battery depends on the intended use (cyclic or float service), economic considerations, recharge time, anticipated frequency and depth of discharge (DoD), and expected service life. The goal of any charging method is to control the charge current at the end of the charge.
Constant voltage charging is the best method to charge sealed lead acid batteries. Depending on the application, batteries may be charged either on a continuous or non-continuous basis. In applications where standby power is required to operate, for example a security system or uninterruptible power supply (UPS), when the AC power has been interrupted, continuous float charging is recommended. Non-continuous cyclic charging is used primarily with portable equipment where charging on an intermittent basis is appropriate, such as electric wheelchairs and mobile medical carts.
The constant voltage charge method applies a constant voltage to the battery and limits the initial charge current. It is necessary to set the charge voltage according to specified charge and temperature characteristics. Inaccurate voltage settings can cause either over-charge or under-charge. This charging method can be used for both cyclic and standby applications.
Constant voltage charging circuitConstant voltage charging characteristicsConstant current charging is suited for applications where discharged ampere-hours of the preceding discharge cycle are known. Charge time and charge quantity can easily be calculated, however an expensive circuit is necessary to obtain a highly accurate constant current. Monitoring of charge voltage or limiting of charge time is necessary to avoid excessive overcharge of the battery.
While this charging method is very effective for recovering the capacity of a SLA battery that has been stored for an extended period of time, or for occasional overcharging to equalize cell capacities, it lacks specific properties required in today&#;s electronic environment.
The taper current charging method is not recommended as it is somewhat abusive of sealed lead acid batteries and can shorten service life. However, because of the simplicity of the circuit and low cost, taper current charging is extensively used to charge multiple numbers and/or for cyclic charging.
When using a taper current battery charger the charger time should be limited or a charging cut-off circuit needs to be incorporated to prevent over-charge.
In a taper current charging circuit, the current decreases in proportion to the voltage rise. When designing a taper charger always consider power voltage fluctuations. In this event the internal resistance drop will convert to heat. Heat generated by the circuit should be measured and if required a heat sink should be incorporated in the design.
Taper current charging circuitTaper current charging characteristics for this type of basically unregulated chargerAs a result of too high a charge voltage excessive current will flow into the battery, after reaching full charge, causing decomposition of water in the electrolyte and premature aging.
At high rates of overcharge a battery will progressively heat up. As it gets hotter, it will accept more current, heating up even further. This is called thermal runaway and it can destroy a battery in as little as a few hours.
If too low a charge voltage is applied, the current flow will essentially stop before the battery is fully charged. This allows some of the lead sulfate to remain on the electrodes, which will eventually reduce battery capacity.
Batteries which are stored in a discharged state, or left on the shelf for too long, may initially appear to be &#;open circuited&#; or will accept far less current than normal. This is caused by a phenomenon called &#;sulfation&#;. When this occurs, leave the charger connected to the battery. Usually, the battery will start to accept increasing amounts of current until a normal current level is reached. If there is no response, even to charge voltages above recommended levels, the battery may have been in a discharged state for too long to recover, and in which case a replacement SLA battery will be needed.
Cyclic (or cycling) applications generally require recharging be done in a relatively short time. The initial charge current, however, must not exceed 0.30 x C amps. Just as battery voltage drops during discharge, it slowly rises during charge. Full charge is determined by voltage and inflowing current. When, at a charge voltage of 2.45 ± 0.05 volts/cell, the current accepted by the battery drops to less than 0.01 x C amps (1% of rated capacity), the battery is fully charged and the charger should be disconnected or switched to a float voltage of 2.25 to 2.30 volts/cell. The voltage should not be allowed to rise above 2.45 ± 0.05 volts/cell.
Standby applications generally do not require that the battery be charged as fast or as frequently as in cycle operation. However, the battery must be kept constantly charged to replace the energy that is expended due to internal loss and deterioration of the battery itself. Although these losses are very low in Power Sonic lead acid batteries, they must be replaced at the rate the battery self discharges; at the same time the battery must not be given more than these losses or it will be overcharged. To accomplish this, a constant voltage method of charging called standby or float charging is used.
The recommended constant float voltage is 2.25 &#; 2.30 volts per cell. Maintaining this float voltage will allow the battery to define its own current level and remain fully charged without having to disconnect the charger from the battery. The trickle current for a fully charged battery floating at the recommended charge voltage will typically hover around the 0.001C rate (7mA for a 7AH battery, for example.)
The float charger is basically a constant voltage power supply. As with cycle chargers, care must be taken not to exceed the initial charge current of 0.30 x C amperes.
This method uses two constant voltage devices. In the initial charge phase the high voltage setting is used. When charging is nearly complete and the charge voltage has risen to a specified value (with the charge current decreased), the charger switches the voltage to the lower setting. This method allows rapid charging in cycle or float service without the possibility of overcharging, even after extended charging periods.
Dual stage current limited SLA battery chargerTwo-step constant voltage charging characteristics.Lead acid batteries are strings of 2 volt cells connected in series, commonly 2, 3, 4 or 6 cells per battery. Strings of lead acid batteries, up to 48 volts and higher, may be charged in series safely and efficiently. However, as the number of batteries in series increases, so does the possibility of slight differences in capacity. These differences can result from age, storage history, temperature variations or abuse.
Fully charged batteries should never be mixed with discharged batteries when charging batteries in series. The discharged batteries should be charged before connection.
When a single constant voltage charger is connected across an entire high voltage string, the same current flows through all cells in the string. Depending on the characteristics of the individual batteries, some may overcharge while others remain in a slightly undercharged condition.
To minimize the effects of individual battery differences, use batteries of the same age, manufacturer, amp hour, and history and, if possible, charge in strings of no greater than 24 or 48 volts.
Lead acid batteries may be used in parallel with one or more batteries of equal voltage. When connecting batteries in parallel, the current from a charger will tend to divide almost equally between the batteries. No special matching of batteries is required. If the batteries of unequal capacity are connected in parallel, the current will tend to divide between the batteries in the ratio of capacities (actually, internal resistances).
When charging batteries in parallel, where different ratios of charge are to be expected, it is best to make provisions to assure that the currents will not vary too much between batteries.
Power Sonic sealed lead acid batteries perform well both at low and high temperatures. At low temperatures, however, charge efficiency is reduced; at temperatures above 45°C (113°F), charge efficiency increases so rapidly that there is a danger of thermal runaway if temperature compensation is not precise.
The effect of temperature on charge voltage is less critical in float applications than in cyclic use, where relatively high charge currents are applied for the purpose of short recharge times.
Temperature effects should definitely be considered when designing or selecting a charging system. Temperature compensation is desirable in the charging circuit, especially when operating outside the range of 5°C to 35°C
(41°F to 95°F). The temperature coefficient is -2mV/cell/ºC below 20°C (68°F) in float use and -6mV/cell/ ºC below 20°C in cyclic use. For higher temperatures the charge voltage should be correspondingly decreased.
The battery temperature compensation chart below shows the recommended charge voltages for different temperatures based on ambient charge voltage per cell.
TemperatureCyclic Use (V)Float Use (V)-40°C (-40°F)2.85 &#; 2.952.38 &#; 2.43-20°C (-4°F)2.67 &#; 2.772.34 &#; 2.39-10°C (14°F)2.61 &#; 2.712.32 &#; 2.370°C (32°F)2.55 &#; 2.652.30 &#; 2.°C (50°F)2.49 &#; 2.592.28 &#; 2.°C (68°F)2.43 &#; 2.532.26 &#; 2.°C (77°F)2.40 &#; 2.502.25 &#; 2.°C (86°F)2.37 &#; 2.472.24 &#; 2.°C (104°F)2.31 &#; 2.412.22 &#; 2.°C (122°F)2.25 &#; 2.352.20 &#; 2.25All batteries lose capacity through self-discharge, it is recommended that a &#;top up charge&#; be applied to any battery that has been stored for a long period of time, prior to putting the battery into service.
To successfully top up charge a battery stored for more than 12 months, the open circuit voltage must be higher than 2.0 volts per cell, in this case, always confirm open circuit voltage prior to attempting top up charging.
The charging efficiency (η) of a battery is expressed by the following formula:
The charging efficiency varies depending upon the state of charge of the battery, temperatures, and charging rates. The below graph illustrates the concept of the state of charge and charging efficiency.
The below graph shows Power Sonic sealed lead acid batteries exhibit very high charging efficiency, even when charged at low charging rates.
It is always important to match your charger to deliver the correct current and voltage for the battery you are charging. For example, you wouldn&#;t use a 24V charger to charge a 12V battery.
If you have any questions about an existing charger&#;s capability with one of our products, please give us a call or send us an . We would be happy to assist you with your charging needs.
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