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 is maximising the battery life. 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. 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.
Figure 1: Typical SLA Battery Construction
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). At the same timethe sulphuric acid (H2SO4) is converted to water (H2O).In a normal charge, the chemical reaction is reversed. The lead sulphate and water are electrochemically converted to lead, lead dioxide and sulphuric acid. During a full charge cycle anygasses produced need to be re-combined in a so called ‘oxygen cycle’. 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. As charging continues, the oxygenproduced also re-combines with the hydrogen being produced on the negative plate formingwater. 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 theelectrolyte.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. A pressure-release valve will open and vent the excess gas, resulting inthe loss of electrolyte and a loss of capacity.If the battery is undercharged; the low cell voltage will cause the charge current to diminish tozero well before full capacity is reached. This will allow some of the lead sulphate producedduring discharge to remain on the plates, where it will crystallise, which also causes apermanent loss of capacity.It is also important to remember that SLA batteries have a self discharge rate ofapproximately 5% per month. This is less than most other forms of rechargeable batteries, buthas to be considered. Manufacturers recommend recharging when the battery reaches about70% of its capacity (approximately 2.1 volts per cell). They use this to calculate the maximumlife of the battery, but this is very difficult to implement in a real world application.So let us look at different charging techniques: -Constant Voltage Charging: this method is the most commonly used for SLA batteries asthe individual cells tend to share the voltage and equalize the charge between them. It isimportant to limit the initial charging current to prevent damage to the battery. However, witha single fixed voltage, it is impossible to properly balance the requirements of a fast chargecycle against the danger of overcharge.Constant Current Charging: this method can be used for a single 2V cell but is notrecommended 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 todetermine when the battery has reached a fully charged state. If the charge is continued at thesame 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 batteriesas it can often shorten battery service life due to poor control of the final fully chargedvoltage. However, because of the simplicity of the circuit and subsequent low cost, tapercurrent charging is often used to charge a number of series connected batteries that are subjectto cyclic use. When using this method it is recommended that the charging time is eitherlimited or that a charging cut-off circuit is incorporated to prevent overcharge.Two Stage Constant Voltage Charging: this method is a recommended for charging SLAbatteries in a short period of time and then maintaining them in a fully charged float (orstandby) condition.Each of the above has its advantages and disadvantages, but using a simple charger designmay not be cost effective in the long term. Checking battery condition and replacing batterieswith lost capacity is very costly and environmentally unfriendly. So designing a charger tomaximise the life of the SLA battery is very important. Guide to charging Sealed Lead Acid batteriesAnother important factor that has to be considered when charging an SLA battery istemperature. As the temperature rises, electrochemical activity in a battery increases, so thecharging voltage should be reduced to prevent overcharge. Conversely as temperature falls,the change voltage should be increased to avoid undercharge.Using a combination of the constant current charging and two stage constant voltage chargingtechniques and also by monitoring the battery terminal voltage and temperature a multi-stagecharge profile can be implemented to reduce stress on the battery while giving the shortestpossible charge time.Figure 2 shows the multi-stage charge profile used by the Silvertel Ag102 module, for a 6-cellbattery that will achieve this. The upper trace shows the charge voltage and the lower traceshows the charge current.Charge CurrentCharge VoltageBattery Voltage / Charge CurrentCharging TimeConstant Current(Bulk Charge)Two Stage Constant Voltage(Absorption Charge)Pulsed(Float Charge)BatteryCapacityCheckResume Pulsed(Float Charge)Good BatteryPoor BatteryBatteryCapacityThreshold~1hr ~2mins
Figure 2: Ag102 Charge Profile
he first part of the multi-stage charge cycle is constant current mode “Bulk Charge”; Ag102nce the terminal voltage reaches 14.4V; the Ag102 charge cycle automatically moves on tohe final stage of the charge cycle is the “Float Charge”. This can be done by accuratelyTlimits this to a maximum of 0.25C Amps (a quarter of the battery capacity), as required bySLA batteries. For example, if the capacity of the SLA battery being charged is 4Ah, then theconstant current should be limited to 1 Amp. During this stage, the Ag102 monitors thebattery terminal voltage until the terminal voltage reaches 14.4V (2.40V/cell).Othe second stage “High Absorption Charge”. The Ag102 output changes from constantcurrent to constant voltage and now monitors the charge current. When the charge currentdrops to 0.05C Amps, which is 0.2 Amps for a 4Ah battery, the battery will have recoveredapproximately 70-80% of its charge. At this point the Ag102 output voltage reduces to13.65V (2.275V/cell) - this is the “Low Absorption Charge”. The remaining 20-30% of thecharge is carried out at this lower voltage in order to prevent over-charge. Ag102 will stay inthis mode until the battery is fully charged.Tmaintaining the low absorption voltage level, or as with the Ag102, by providing anintermittent float charge as shown in Figure 2. These methods ensure that the battery is notbeing over-charged, as over-charging will result in battery stress, reducing the battery life. Guide to charging Sealed Lead Acid batteriesll the above charge voltages are based on an ambient temperature of between 20˚C to 25˚C.here are limits to the battery operating temperature and SLA battery life is greatly reduced atony Morgantions Engineer.comDuring the intermittent float charge, Ag102 will periodically check battery capacity, and flagthis information.AFor the best performance these voltages will need to be temperature compensated byapproximately 4mV/˚C/Cell (reduced at higher temperatures and increased at lowertemperatures). Ag102 has the ability to provide temperature compensation via PTC thermistorinput – the PTC thermistor connects to the battery, and the Ag102 automatically adjusts allcharging voltages to compensate for temperature variances, thus always maintaining optimumcharge performance for the SLA battery – maximising battery life, and minimising chargetimes.Thigher ambient temperatures. For more information on this you will need to refer to thebattery manufacturer’s datasheet.