Lead acid battery how does it work

The remaining sulfate combines with the active material on the plates to form lead sulfate. This reduces the strength of the electrolyte, and the sulfate on the plates acts as an electrical insulator. The excess electrons flow out the negative side of the battery, through the electrical device, and back to the positive side of the battery.

At the positive battery terminal, the electrons rush back in and are accepted by the positive plates. The oxygen in the active material lead dioxide reacts with the hydrogen ions to form water, and the lead reacts with the sulfuric acid to form lead sulfate. The ions moving around in the electrolyte are what create the current flow, but as the cell becomes discharged, the number of ions in the electrolyte decreases and the area of active material available to accept them also decreases because it's becoming coated with sulfate.

Remember, the chemical reaction takes place in the pores on the active material that's bonded to the plates. Many of you may have noticed that a battery used to crank a bike that just won't start will quickly reach the point that it won't even turn the engine over. However, if that battery is left to rest for a while, it seems to come back to life. On the other hand, if you leave the switch in the "park" position overnight only a couple of small lamps are lit , the battery will be totally useless in the morning, and no amount of rest will cause it to recover.

Why is this? Since the current is produced by the chemical reaction at the surface of the plates, a heavy current flow will quickly reduce the electrolyte on the surface of the plates to water. The voltage and current will be reduced to a level insufficient to operate the starter.

It takes time for more acid to diffuse through the electrolyte and get to the plates' surface. A short rest period accomplishes this. The acid isn't depleted as quickly when the current flow is small like to power a tail light bulb , and the diffusion rate is sufficient to maintain the voltage and current. That's good, but when the voltage does eventually drop off, there's no more acid hiding in the outer reaches of the cell to migrate over to the plates.

The electrolyte is mostly water, and the plates are covered with an insulating layer of lead sulfate. Charging is now required. One not-so-nice feature of lead acid batteries is that they discharge all by themselves even if not used. A general rule of thumb is a one percent per day rate of self-discharge. This rate increases at high temperatures and decreases at cold temperatures. Don't forget that your Gold Wing, with a clock, stereo, and CB radio, is never completely turned off.

Each of those devices has a "keep alive memory" to preserve your radio pre-sets and time, and those memories draw about 20 milliamps, or. This will suck about one half amp hour from your battery daily at 80 degrees Fahrenheit. This draw, combined with the self-discharge rate, will have your battery 50 percent discharged in two weeks if the bike is left unattended and unridden.

Charging is a process that reverses the electrochemical reaction. It converts the electrical energy of the charger into chemical energy. Remember, a battery does not store electricity; it stores the chemical energy necessary to produce electricity. A battery charger reverses the current flow , providing that the charger has a greater voltage than the battery. The charger creates an excess of electrons at the negative plates, and the positive hydrogen ions are attracted to them.

The hydrogen reacts with the lead sulfate to form sulfuric acid and lead, and when most of the sulfate is gone, hydrogen rises from the negative plates. The oxygen in the water reacts with the lead sulfate on the positive plates to turn them once again into lead dioxide, and oxygen bubbles rise from the positive plates when the reaction is almost complete.

Many people think that a battery's internal resistance is high when the battery is fully charged, and this is not the case. If you think about it, you'll remember that the lead sulfate acts as an insulator. The more sulfate on the plates, the higher the battery's internal resistance. The higher resistance of a discharged battery allows it to accept a higher rate of charge without gassing or overheating than when the battery is near full charge.

Near full charge, there isn't much sulfate left to sustain the reverse chemical reaction. The level of charge current that can be applied without overheating the battery or breaking down the electrolyte into hydrogen and oxygen is known as the battery's "natural absorption rate. The battery may overheat, and the electrolyte will bubble. Actually, some of the charging current is wasted as heat even at correct charging levels, and this inefficiency creates the need to put more amp hours back into a battery than were taken out.

More on that later. There are many things that can cause a battery to fail or drastically shorten its life. One of those things is allowing a battery to remain in a partially discharged state. In between the fully discharged and charged states, a lead acid battery will experience a gradual reduction in the voltage.

Voltage level is commonly used to indicate a battery's state of charge. The dependence of the battery on the battery state of charge is shown in the figure below. If the battery is left at low states of charge for extended periods of time, large lead sulfate crystals can grow, which permanently reduces battery capacity.

These larger crystals are unlike the typical porous structure of the lead electrode, and are difficult to convert back into lead. The charging reaction converts the lead sulfate at the negative electrode to lead. At the positive terminal the reaction converts the lead to lead oxide. As a by-product of this reaction, hydrogen is evolved. During the first part of the charging cycle, the conversion of lead sulfate to lead and lead oxide is the dominant reaction.

However, as charging proceeds and most of the lead sulfate is converted to either lead or lead dioxide, the charging current electrolyzes the water from the electrolyte and both hydrogen and oxygen gas are evolved, a process known as the "gassing" of the battery.

If current is being provided to the battery faster than lead sulfate can be converted, then gassing begins before all the lead sulfate is converted, that is, before the battery is fully charged.

Gassing introduces several problems into a lead acid battery. Not only does the gassing of the battery raise safety concerns, due to the explosive nature of the hydrogen produced, but gassing also reduces the water in the battery, which must be manually replaced, introducing a maintenance component into the system. In addition, gassing may cause the shedding of active material from the electrolyte, thereby permanently reducing battery capacity.

For these reasons, the battery should not regularly be charged above the voltage which causes gassing. The gassing voltage changes with the charge rate. Lead sulphate is an insulator, and therefore the way in which lead sulfate forms on the electrodes determined how easily the battery can be discharged.

For most renewable energy systems, the most important battery characteristics are the battery lifetime, the depth of discharge and the maintenance requirements of the battery. This set of parameters and their inter-relationship with charging regimes, temperature and age are described below.

The depth of discharge in conjunction with the battery capacity is a fundamental parameter in the design of a battery bank for a PV system, as the energy which can be extracted from the battery is found by multiplying the battery capacity by the depth of discharge. Batteries are rated either as deep-cycle or shallow-cycle batteries. To achieve the same useable capacity, a shallow-cycle battery bank must have a larger capacity than a deep-cycle battery bank.

In addition to the depth of discharge and rated battery capacity, the instantaneous or available battery capacity is strongly affected by the discharge rate of the battery and the operating temperature of the battery.

However, high temperatures are not ideal for batteries either as these accelerate aging, self-discharge and electrolyte usage. The graph below shows the impact of battery temperature and discharge rate on the capacity of the battery. Figure: Relationship between battery capacity, temperature and discharge rate.

Over time, battery capacity degrades due to sulfation of the battery and shedding of active material. The degradation of battery capacity depends most strongly on the interrelationship between the following parameters:. The following graph shows the evolution of battery function as number of cycles and depth of discharge for a shallow-cycle lead acid battery. Figure: Relationship between battery capacity, depth of discharge and cycle life for a shallow-cycle battery.

In addition to the DOD, the charging regime also plays an important part in determining battery lifetime. Overcharging or undercharging the battery results in either the shedding of active material or the sulfation of the battery, thus greatly reducing battery life.

The final impact on battery charging relates to the temperature of the battery. Although the capacity of a lead acid battery is reduced at low temperature operation, high temperature operation increases the aging rate of the battery.

Figure: Relationship between battery capacity, temperature and lifetime for a deep-cycle battery. Constant current discharge curves for a Ah lead acid battery at different discharge rates, with a limiting voltage of 1. Longer discharge times give higher battery capacities. The production and escape of hydrogen and oxygen gas from a battery causes water loss and water must be regularly replaced in lead acid batteries.

Other components of a battery system do not require maintenance as regularly, so water loss can be a significant problem. If the system is in a remote location, checking water loss can add to costs. Maintenance-free batteries limit the need for regular attention by preventing or reducing the amount of gas which escapes the battery. However, due to the corrosive nature the elecrolyte, all batteries to some extent introduce an additional maintenance component into a PV system. Depending on which one of the above problems is of most concern for a particular application, appropriate modifications to the basic battery configuration improve battery performance.

For renewable energy applications, the above problems will impact the depth of discharge, the battery lifetime and the maintenance requirements. The changes to the battery typically involve modification in one of the three basic areas:. Flooded lead acid batteries are characterised by deep cycles and long lifetimes.

However, flooded batteries require periodic maintenance. Not only must the level of water in the electrolyte be regularly monitored by measuring its specific gravity, but these batteries also require "boost charging". Boost or equalization charging involves short periodic overcharging, which releases gas and mixes the electrolyte, thus preventing stratification of the electrolyte in the battery. In addition, boost charging also assists in keeping all batteries at the same capacity.

For example, if one battery develops a higher internal series resistance than other batteries, then the lower SR battery will consistently be undercharged during a normal charging regime due to the voltage drop across the series resistance. However, if the batteries are charged at a higher voltage, then this allows all batteries to become fully charged.

A flooded battery is subject to water loss from the electrolyte due to the evolution of hydrogen and oxygen gas. The specific gravity of the electrolyte, which can be measured with a hydrometer, will indicate the need to add water to the batteries if the batteries are fully charged. Alternately, a hydrometer will accurately indicate the SOC of the battery if it is known that the water level is correct. SG is periodically measured after boost charging to insure that the battery has sufficient water in the electrolyte.

The SG of the battery should be provided by the manufacturer. Gelled or AGM lead acid batteries which are typically sealed or valve regulated have several potential advantages:. For instance, how does a lead-acid battery work? For that matter, what exactly is a lead-acid battery? Are these batteries still efficient enough for certain uses or are they obsolete? How do they compare to other batteries? To start with, we can assure you that these batteries are not obsolete.

We also provide a comprehensive explanation about what a lead-acid battery is and how it works. Read on to learn all there is to know about lead-acid batteries. A lead-acid battery is a rechargeable battery that uses lead and sulphuric acid to function. The lead is submerged into the sulphuric acid to allow a controlled chemical reaction.

This chemical reaction is what causes the battery to produce electricity. Then, this reaction is reversed to recharge the battery. Believe it or not, this technology is over years old. However, it has been improved upon since its invention in and it now works more efficiently. When looking at how a lead acid battery works, it is necessary to look at the basic components.

The battery has comparatively few components - essentially there are four main elements:. These constituents are all contained within a plastic container which acts to keep the electrolyte in and the battery together. The overall battery will normally consist of several cells placed in series to give the required voltage as each cell is capable of providing an EMF of 2.

In order to enable the basic lead acid cell to produce a voltage, it must first receive charge. The voltage applied to provide this must be greater than the 2. If it were less than this, charge would actually flow out of it. Once charged, the cell or battery will be able to provide charge to external circuits, often operating over several hours dependent upon the drain on the cell or battery.

The self discharge characteristic of the lead acid battery is relatively good.