Lead-acid battery

Lead-acid battery is the first rechargeable battery ever created. It is commonly used as an automotive battery for starting the engine but also as the main energy source for electric vehicles such as motorised wheelchairs, golf carts and electric scooters. In addition, it is also used for storage in backup power supplies.

How do lead-acid batteries work?

A lead-acid battery consists of negative electrodes (anodes), positive electrodes (cathodes), separators between them, and electrolyte. When fully charged, the anode consists mainly of lead (Pb) and the cathode of lead dioxide (PbO2). The electrolyte is a concentrated sulfuric acid solution (H2SO4). When using the battery, the lead reacts with the sulfuric acid releasing electrons and hydrogen ions (H+) that move from anode to cathode. On the cathode, the PbO2 reacts with sulfuric acid. As a result of the total reaction, both electrodes are covered in lead sulfate (PbSO4) and the electrolyte becomes primarily water.

Total reaction:
Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O

When charged, the charger creates an excess of negative electrons on the anode and hydrogen ions are attracted to it. This forces the reverse reaction to take place.

Two main types of lead-acid batteries

By design, there are two types of lead-acid batteries: starting batteries and deep-cycle batteries.

Starting batteries have a large number of thin electrodes to maximise surface area. These batteries give out a high burst of current at once. They are designed to be used for starting an engine. When the engine is running, the alternator charges the battery and holds the charge at the optimal level. Starting batteries are not designed for repeated deep discharge and charge cycles.

Deep-cycle batteries have thick electrodes to improve cycling performance. These batteries are designed to have a good capacity and high cycle count. Deep-cycle batteries are used to give continuous power to electric vehicles such as wheelchairs and scooters, and also for power storage.

Advantages and disadvantages

Lead-acid batteries have a large power-to-weight ratio which makes them good for supplying high surge currents. They are also relatively inexpensive. Because of this, lead-acid batteries are widely used even in places where newer technologies would suit better. For example, lithium-ion batteries have a much higher energy density and therefore are suited for use in places where the size of the battery is important such as electric scooters. But lead-acid batteries are still used in cheaper models to bring down the cost.

The main downsides of lead-acid batteries are that they charge slowly, their cycle life is limited and they need regular maintenance. It is important to charge them with optimal voltage. Excessive charging with high voltages can lead to gases building up and the battery could explode.

From an environmental standpoint, lead-acid batteries are a problem. Both lead and sulfuric acid can contaminate soil and groundwater. Some lead compounds are extremely toxic and could cause lasting health problems. Because of this, it is important to properly recycle lead-acid batteries. The recycling process is often regulated by governments and people are paid to properly dispose of their batteries.

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