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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Charging sealed batteries with Alton alternators
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<blockquote data-quote="redbloke1956" data-source="post: 43206" data-attributes="member: 2299"><p>Hi Jim, some of this may sound like gobbledeegook but if you filter out that which is over your (our) head then you should get some valuable info from it on SLA (sealed lead acid) batteries. <table style='width: 100%'><tr><td><p style="text-align: center">Figure 1: Typical SLA Battery Construction</p> </td></tr></table><p>When an SLA battery is being discharged; the lead (Pb) on the negative plate and the lead dioxide (PbO2) on the positive plate are converted to lead sulphate (PbSO4). At the same time the sulphuric acid (H2SO4) is converted to water (H2O).</p><p>In a normal charge, the chemical reaction is reversed. The lead sulphate and water are electro-chemically converted to lead, lead dioxide and sulphuric acid. During a full charge cycle any gasses produced need to be re-combined in a so called ‘oxygen cycle’. Oxygen is generated at the positive plates during the latter stages of the charge cycle, this reacts with and partially discharges in the sponge lead of the negative plates. As charging continues, the oxygen produced also re-combines with the hydrogen being produced on the negative plate forming water. With correct and accurate cell voltage control all gasses produced during the charge cycle will be re-combined completely into the negative plates and returned to water in the electrolyte.</p><p>If an SLA battery is over-charged, the excess cell voltage will result in the conversion of electrolyte into large amounts of hydrogen and oxygen gasses which cannot be recombined by the normal processes. A pressure-release valve will open and vent the excess gas, resulting in the loss of electrolyte and a loss of capacity.</p><p><strong>If the battery is undercharged; the low cell voltage will cause the charge current to diminish to zero well before full capacity is reached. This will allow some of the lead sulphate produced during discharge to remain on the plates, where it will crystallise, which also causes a permanent loss of capacity.</strong></p><p>It is also important to remember that SLA batteries have a self discharge rate of approximately 5% per month. This is less than most other forms of rechargeable batteries, but has to be considered.<u> Manufacturers recommend recharging when the battery reaches about 70% of its capacity </u>(approximately 2.1 volts per cell). They use this to calculate the maximum life of the battery, but this is very difficult to implement in a real world application.</p><p style="text-align: left">So let us look at different charging techniques: -<strong><u>Constant Voltage Charging</u></strong>: this method is the most commonly used for SLA batteries as the individual cells tend to share the voltage and equalize the charge between them. It is important to limit the initial charging current to prevent damage to the battery. However, with a single fixed voltage, it is impossible to properly balance the requirements of a fast charge cycle against the danger of overcharge.<u><strong>Constant Current Charging</strong></u>: this method can be used for a single 2V cell but is not recommended for charging a number of series connected cells, a battery, at the same time. This is because some cells will reach full charge before others and it is very difficult to determine when the battery has reached a fully charged state. If the charge is continued at the same rate, for any extended period of time, severe overcharge may occur to some cells, resulting in damage to the battery.Taper Current Charging: this method is not really recommended for charging SLA batteries as it can often shorten battery service life due to poor control of the final fully charged voltage. However, because of the simplicity of the circuit and subsequent low cost, taper current charging is often used to charge a number of series connected batteries that are subject to cyclic use. When using this method it is recommended that the charging time is either limited or that a charging cut-off circuit is incorporated to prevent overcharge.Two Stage Constant Voltage Charging: this method is a recommended for charging SLA batteries in a short period of time and then maintaining them in a fully charged float (or standby) condition.</p><p>Each of the above has its advantages and disadvantages, but using a simple charger design may not be cost effective in the long term. Checking battery condition and replacing batteries with lost capacity is very costly and environmentally unfriendly. So designing a charger to maximise the life of the SLA battery is very important.</p><p>Another important factor that has to be considered when charging an SLA battery is temperature. As the temperature rises, electrochemical activity in a battery increases, so the charging voltage should be reduced to prevent overcharge. Conversely as temperature falls, the change voltage should be increased to avoid undercharge.</p><p>I hope some of this helps mate.</p><p></p><p>Kevin</p></blockquote><p></p>
[QUOTE="redbloke1956, post: 43206, member: 2299"] Hi Jim, some of this may sound like gobbledeegook but if you filter out that which is over your (our) head then you should get some valuable info from it on SLA (sealed lead acid) batteries.[TABLE="align: center"] [TR] [TD][CENTER]Figure 1: Typical SLA Battery Construction[/CENTER] [/TD] [/TR] [/TABLE] When an SLA battery is being discharged; the lead (Pb) on the negative plate and the lead dioxide (PbO2) on the positive plate are converted to lead sulphate (PbSO4). At the same time the sulphuric acid (H2SO4) is converted to water (H2O). In a normal charge, the chemical reaction is reversed. The lead sulphate and water are electro-chemically converted to lead, lead dioxide and sulphuric acid. During a full charge cycle any gasses produced need to be re-combined in a so called ‘oxygen cycle’. Oxygen is generated at the positive plates during the latter stages of the charge cycle, this reacts with and partially discharges in the sponge lead of the negative plates. As charging continues, the oxygen produced also re-combines with the hydrogen being produced on the negative plate forming water. With correct and accurate cell voltage control all gasses produced during the charge cycle will be re-combined completely into the negative plates and returned to water in the electrolyte. If an SLA battery is over-charged, the excess cell voltage will result in the conversion of electrolyte into large amounts of hydrogen and oxygen gasses which cannot be recombined by the normal processes. A pressure-release valve will open and vent the excess gas, resulting in the loss of electrolyte and a loss of capacity. [B]If the battery is undercharged; the low cell voltage will cause the charge current to diminish to zero well before full capacity is reached. This will allow some of the lead sulphate produced during discharge to remain on the plates, where it will crystallise, which also causes a permanent loss of capacity.[/B] It is also important to remember that SLA batteries have a self discharge rate of approximately 5% per month. This is less than most other forms of rechargeable batteries, but has to be considered.[U] Manufacturers recommend recharging when the battery reaches about 70% of its capacity [/U](approximately 2.1 volts per cell). They use this to calculate the maximum life of the battery, but this is very difficult to implement in a real world application. [LEFT]So let us look at different charging techniques: -[B][U]Constant Voltage Charging[/U][/B]: this method is the most commonly used for SLA batteries as the individual cells tend to share the voltage and equalize the charge between them. It is important to limit the initial charging current to prevent damage to the battery. However, with a single fixed voltage, it is impossible to properly balance the requirements of a fast charge cycle against the danger of overcharge.[U][B]Constant Current Charging[/B][/U]: this method can be used for a single 2V cell but is not recommended for charging a number of series connected cells, a battery, at the same time. This is because some cells will reach full charge before others and it is very difficult to determine when the battery has reached a fully charged state. If the charge is continued at the same rate, for any extended period of time, severe overcharge may occur to some cells, resulting in damage to the battery.Taper Current Charging: this method is not really recommended for charging SLA batteries as it can often shorten battery service life due to poor control of the final fully charged voltage. However, because of the simplicity of the circuit and subsequent low cost, taper current charging is often used to charge a number of series connected batteries that are subject to cyclic use. When using this method it is recommended that the charging time is either limited or that a charging cut-off circuit is incorporated to prevent overcharge.Two Stage Constant Voltage Charging: this method is a recommended for charging SLA batteries in a short period of time and then maintaining them in a fully charged float (or standby) condition.[/LEFT]Each of the above has its advantages and disadvantages, but using a simple charger design may not be cost effective in the long term. Checking battery condition and replacing batteries with lost capacity is very costly and environmentally unfriendly. So designing a charger to maximise the life of the SLA battery is very important. Another important factor that has to be considered when charging an SLA battery is temperature. As the temperature rises, electrochemical activity in a battery increases, so the charging voltage should be reduced to prevent overcharge. Conversely as temperature falls, the change voltage should be increased to avoid undercharge. I hope some of this helps mate. Kevin [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Charging sealed batteries with Alton alternators
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