A lead-acid deep-cycle battery can withstand repeated deep discharge and is the type of battery you should be using in an off-grid system.
They have thick, solid lead plates to protect them from the stress of prolonged discharge. Over time, the electrochemical reaction that occurs during deep discharge and charge damages a battery. By constructing it with more resilient plates, a deep cycle battery is born.
Batteries are constructed for a variety of purposes. Automotive lead -acid starter batteries provide high currents for a brief amount of time to start an engine. The overall capacity of the battery is hardly used and deep discharges will in fact damage the thinner plates. Deep-cycle lead-acid batteries are designed to do the opposite. Low current and long discharge is their game. They can be used in a variety of applications, one of which is to act as the crucial storage component in an off-grid solar system. Their ability to hold high charge, discharge slowly and deeply, and recharge many thousands of times, makes them suitable for prolonged power consumption.
Lead-acid battery construction and process
A lead-acid cell is made of three basic components: the electrodes (anode + cathode) and electrolyte. The anode is the negative terminal and the cathode is the positive terminal. Connecting these is the electrolyte which drives the electrochemical reaction that provides electricity. In a deep-cycle lead-acid cell, the negative plate is made of solid lead, and the positive plate is made of lead dioxide. Between them is an insulator/separator that prevents them from touching and short circuiting. The electrolyte in a lead-acid battery is water and sulphuric acid. Three reactions take place when discharging, causing the composition of these components to change; in turn reducing the available capacity.
When discharging, current flows from the negative terminal to the positive terminal. Oxidation occurs at the negative, causing it to release electrons and reduction occurs at the positive, causing it to absorb electrons. This is where the plate thickness becomes important. A process called positive grid corrosion occurs, whereby the positive plate gradually gets eroded during the discharge/charge cycle. This is exacerbated by overcharging batteries, especially sealed batteries. Therefore, there is a direct correlation between the positive plate thickness and the lifespan of a battery. When a wire is connected between the positive and negative terminals, these electrons flow in a circuit and the reaction continues until either the lead or sulphuric acid is completely depleted.
The paragraph above describes one cell. One lead-acid cell produces 2.1 volts (V). Therefore, batteries can be constructed with several of these cells alongside each other in a series circuit. This increases the voltage to 6.3V for three cells and 12.6V for six cells.
When a battery discharges, three reactions take place.
At the negative plate, the sulphate ions in the electrolyte react with the lead, to produce lead sulphate. This reaction releases two electrons.
At the positive plate, the sulphate ions react with the lead dioxide, also producing lead sulphate. Meanwhile, the hydrogen in the sulphuric acid reacts with the oxygen from the lead dioxide to produce water. The positive plate requires electrons, and of course it gets them from the negative plate.
These reactions remove sulphuric acid from the electrolyte and eventually they cease when there is no more acid or lead (dioxide) left to react.
The by-product of these reactions - lead sulphate - coats the plates, damaging them, reducing their capacity, and increasing resistance if it left to form hard crystals. Immediate recharging is advised to prevent it from solidifying.
Recharging a battery reverses the discharge process. It reconverts the lead sulphate back to its original form, lead and sulphuric acid, in the process removing the potentially damaging sulphate crystals from the plates.
As electricity passes through the mainly water electrolyte, a process called gassing occurs. Electricity separates the water (H20) into Hydrogen and Oxygen, both flammable and dangerous gases. In un-sealed batteries, this causes water loss, meaning water levels have to be topped up regularly with distilled water. In sealed batteries, the gases are contained within the battery to prevent water loss, however, overcharging may cause excess pressure, causing emergency vents to open and release the gases, in turn reducing the battery life.