Charging Lead-Acid Batteries by CellPack Solutions

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The charge time of a sealed lead-acid battery is 12-16 hours (up to 36 hours for larger capacity batteries). Lead-acid cannot be fully charged as quickly as nickel or lithium-based systems.

The way in which you charge your battery will very much depend on its application. As with most batteries, it is important to seek advice from your battery manufacturer with regards to both the correct battery and charging techniques for your application.

Sealed Lead-acid batteries (generally used for stand-by-power applications) need to be charged with a constant voltage charger with a current limiter. Generally when first purchased, the battery should be put on charge from anything from 24hrs to several days. From this point onwards, a constant charge current is applied which is slightly more than the self-discharge rate of the battery (float charge).

Charging with a power supply

Lead-acid batteries such as car and bike batteries (DO NOT connect Sealed Lead Acid batteries directly to the mains) can be charged manually with a commercial power supply featuring voltage regulation and current limiting. Calculate the charge voltage according to the number of cells and desired voltage limit. Charging a 12-volt battery (6 cells) at a cell voltage limit of 2.40V, for example, would require a voltage setting of 14.40V.

The charge current for small lead-acid batteries should be set between 10% and 30% of the rated capacity (30% of a 2Ah battery would be 600mA). Larger batteries, such as those used in the automotive industry, are generally charged at lower current ratings. Cells constructed of a non-antimonial lead grid material allow higher charge currents but have a lower capacity. The cylindrical Cyclone is sealed and can sustain a pressure of up to 3.5 Bar (50 psi). A pressurized cell assists in the recombination of gases.

Observe the battery temperature, voltage and current during charge. Charge only at ambient temperatures and in a ventilated room. Once the battery is fully charged and the current has dropped to 3% of the rated current, the charge is completed. A good car battery will drop to about 40mA when fully charged; a bad battery may not fall below 100mA. After full charge, remove the battery from the charger.

General Charging tips

    * Batteries should be charged often, frequent full discharge wear the battery down.
    * Partial recharge-discharge cycles do not affect the batteries performance.
    * Lead-acid batteries must always be stored in a charged state.

How to restore and prolong lead-acid batteries

The sealed lead-acid battery is designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating state during charge. This prevents water depletion of the sealed system. Consequently, these batteries will never get fully charged and some sulfation will develop over time.

SLA batteries with mild sulfation can be restored but the work is time consuming and the results are mixed. Reasonably good results are achieved by applying a charge on top of a charge. This is done by fully charging an SLA battery, then removing it for a 24 to 48 hour rest period and applying a charge again. The process is repeated several times and the capacity is checked with a final full discharge and recharge.

Another method of improving performance is by applying an equalizing charge, in which the charge voltage threshold is increased by about 100mV, typically from 2.40V to 2.50V. This procedure should last no longer than one to two hours and must be carried out at moderate room temperature. A careless equalize charge could cause the cells to heat up and induce venting due to excessive pressure. Observe the battery during the service.

Sealed lead-acid batteries are commonly rated at a 20-hour discharge. Even at such a slow rate, a capacity of 100% is difficult to achieve. For practical reasons, most battery analyzers use a 5-hour discharge when servicing these batteries. This produces 80% to 90% of the rated capacity. SLA batteries are normally overrated and manufacturers are aware of this practice.

Cycling an SLA on a battery analyzer may provide capacity readings that decrease with each additional cycle. A battery may start off at a marginal 88%, then go to 86%, 84% and 83%. This phenomenon can be corrected by increasing the charge voltage threshold from 2.40V to 2.45V and perhaps even 2.50V. Always consider the manufacturer's recommended settings. Avoid setting the charge voltage threshold too high. In an extreme case, the limiting voltage may never be reached, especially when charging at elevated temperatures. The battery continues charging at full current and the pack gets hot. Heat lowers the battery voltage and works against a further voltage raise. If no temperature sensing is available to terminate the charge, a thermal runaway can be the result.

The recovery rate of SLA batteries is a low 15%. Other than reverse sulfation, there is little one can do to improve SLA. Because the SLA has a relatively short cycle life, many fail due to wear-out.

Self Discharge is the phenomenon which occurs in all batteries when they are stored. All batteries lose charge when stored even though they are not connected to a device. The rate at which a battery self-discharges depends on the technology. or a full comparison see our Battery Technology Comparison Table. A standard Sealed Lead Acid battery stored at room temperature will lose approximately 40% of its capacity per year.

Sulfation is a natural occurrence in the life of a battery. As the battery ages, lead sulphate accumulates on the plates of the battery and sediment will deposit at the bottom of the battery. This creates increased resistance and reduces the flow of electrons between the positive and negative electrodes. This results in a tired, slow battery.
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