![]() ![]() However, witha single fixed voltage, it is impossible to properly balance the requirements of a fast chargecycle against the danger of overcharge. It isimportant to limit the initial charging current to prevent damage to the battery. This method is the most commonly used for SLA batteries asthe individual cells tend to share the voltage and equalize the charge between them. So let us look at different charging techniques: – Constant Voltage Charging: They use this to calculate the maximumlife of the battery, but this is very difficult to implement in a real world application. Manufacturers recommend recharging when the battery reaches about70% of its capacity (approximately 2.1 volts per cell). This is less than most other forms of rechargeable batteries, buthas to be considered. It is also important to remember that SLA batteries have a self discharge rate ofapproximately 5% per month. This will allow some of the lead sulphate produced during discharge to remain on the plates, where it will crystallise, which also causes apermanent loss of capacity. If the battery is undercharged the low cell voltage will cause the charge current to diminish to zero well before full capacity is reached. ![]() A pressure-release valve will open and vent the excess gas, resulting inthe loss of electrolyte and a loss of capacity. If an SLA battery is over-charged, the excess cell voltage will result in the conversion ofelectrolyte into large amounts of hydrogen and oxygen gasses which cannot be recombined bythe normal processes. With correct and accurate cell voltage control all gasses produced during the charge Guide to charging Sealed Lead Acid batteriescycle will be re-combined completely into the negative plates and returned to water in the electrolyte. As charging continues, the oxygen produced also re-combines with the hydrogen being produced on the negative plate formingwater. Oxygen is generated atthe positive plates during the latter stages of the charge cycle, this reacts with and partiallydischarges in the sponge lead of the negative plates. During a full charge cycle any gasses produced need to be re-combined in a so called ‘oxygen cycle’. The lead sulphate and water are electrochemically converted to lead, lead dioxide and sulphuric acid. In a normal charge, the chemical reaction is reversed. At the same timethe sulphuric acid (H2SO4) is converted to water (H2O). When an SLA battery is being discharged the lead (Pb) on the negative plate and the leaddioxide (PbO2) on the positive plate are converted to lead sulphate (PbSO4). The electrolyte is usually sulphuric acid mixed with a gelling agentand is largely absorbed and held by insulating separators between the plates, see Figure 1. Simple constantcurrent / constant voltage chargers will do the job for a while, but the battery life expectancyquoted by the manufacturer will be greatly reduced by using non-intelligent chargers like this.Maximising the life of your SLA battery by using an intelligent charger is not only costeffective, it is also better for the environment.Before looking at the different charging techniques it is important to understand the batterychemistry and what happens during normal charge and discharge cycles.Typically the positive plates in an SLA battery are made from lead dioxide and the negativeplates from a sponge lead. Sealed lead acid batteries are widely used, but charging them can be a complex processas Tony Morgan explains:Charging Sealed Lead Acid (SLA) batteries does not seem a particularly difficult process, butthe hard part in charging an SLA battery is maximising the battery life. ![]()
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