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Application of lead-acid battery in the automobile field

A battery can be any device that stores energy for later use. The word battery identifies an electrochemical device that converts chemical energy into electrical energy through a galvanic cell.

A galvanic cell is a relatively simple system consisting of two electrodes (an anode and a cathode) and an electrolyte solution. The voltage of a lead-acid battery galvanic cell is typically 2 Volts. A battery consists of one or more galvanic cells.

The purpose of batteries is not to produce electricity but to accumulate or store electricity. Chemical reactions within batteries vary according to whether electrical energy is stored or released. Fortunately, this process can be repeated repeatedly in lead-acid rechargeable batteries. Typically batteries do not offer 100% efficiency but settle on lower values. This means some energy is lost as heat during the chemical charge and discharge reactions. For example, drawing 1000W (watts) from a battery could require 1200 to be fully recharged. To increase efficiency, low charge or discharge currents must be used (the famous slow charge or discharge).

For this reason, a battery with a nominal capacity of 180Ah, rated over 6 hours, could be rated at 220Ah over 20 hours or 260Ah over 48 hours. We will elaborate on this aspect later. Our batteries are rated on a 20-hour basis. The typical efficiency of lead cells is around 85-95%. For information, alkaline and NiCd batteries are on 65-70%

WHAT ARE THE MAIN TYPES OF LEAD BATTERIES?

Batteries can be classified in two ways, by type of application or by construction technique.

The two main distinctions by an application are Automotive and Deep-Cycle.

The automotive is intended for the automotive world of cars and motorcycles and is used for services and starting. Deep-Cycles include photovoltaic applications and electrical backup systems. The main construction types differ in how the liquid electrolyte is handled. There are Flooded (literally “flooded” or wet), Gel and AGM (Absorbed Glass Mat) ones. AGM batteries are also called “dry” as their glass fibre is 95% saturated with sulfuric acid, and the excess liquid is negligible. Flooded batteries can be the standard type with filler caps or the “maintenance-free” type.

Gel batteries can be Sealed or “Valve Regulated”, where a small valve maintains a slight positive pressure. Most AGM batteries are sealed (airtight) valve regulated (commonly referred to as VRLA – Valve Regulated Lead-Acid). This valve is important because gas can form inside the accumulator during recharging. Most valves are set to pressure from -1 to 4 psi at sea level.

WHAT IS AN AGM BATTERY?

It is an increasingly widespread type of sealed battery which does not require any maintenance. The peculiarity of these batteries is using a thick fibreglass fabric impregnated with electrolyte, which separates the lead plates. The material used is a very fine borosilicate glass fibre. These types of batteries have all the advantages of the Gel accumulator but better support any excess use. Like gel batteries, AGM batteries do not lose acid if the container breaks (in the event of particularly important damage to the external casing, I can have a very limited electrolyte dispersion).

WHAT ARE THE ADVANTAGES OF AGM BATTERIES?

AGM batteries do not leak even if damaged and do not require any maintenance. They are also defined as “recombination, ” as the oxygen and hydrogen in the water recombine inside the battery. In recombination accumulators, the gases generated by water electrolysis are recombined during the charging phase through the so-called “oxygen recombination cycle”, generating water again. The recombination efficiency is typically 99%, meaning the water loss becomes negligible.

The charging methods (voltages and currents) for most AGM batteries are the same as for a standard battery (at the same voltage and nominal capacity). Therefore, it is unnecessary to resort to a special solar battery charger or regulator.

The internal resistance is extremely low, and there is hardly any battery heating, even with high charge or discharge currents. AGM batteries have a very low self-discharge rate (1% to 3% per month). An AGM battery can be stored for a long time (typically up to 6 months) without recharging.

The assembly methods of the AGM plates make this type of battery able to withstand shocks and vibrations better than any standard battery.

WHAT DOES THE ABBREVIATION AH ON THE BATTERIES MEAN?

The acronym Ah, read Ampere-Hour and identifies the current capacity of a battery. It varies concerning the intensity of the current supplied. The nominal capacity (C) is conventionally defined at discharge in 20 h, with an end-of-discharge voltage of 1.75V per galvanic cell at a temperature between 20/25°C. Recall that in a lead-acid battery, the galvanic cell has a nominal voltage of 2V. The capacity of a battery (Ah) is expressed as the product of the discharge current (A) and the time (h) elapsed until the final discharge voltage is reached. “An ampere-hour is one ampere for one hour, or 2 amperes for half an hour, 4 amperes for a quarter of an hour, and so on.

For example, if you have a load that absorbs 20A (amperes) and is used for 20 minutes, the following formula is used to calculate the Ah used:

20 (Ampere) x 0.333 (hours) = 6.67 Ah.

Attention! To evaluate the capacity of the batteries, the 20h rate is almost always used. This means that, in the case of a 12V battery, it is discharged up to 10.5 volts in 20 hours, and the value in Ah that it can supply during this discharge period is measured. Some manufacturers report the discharge rate tables for different periods (e.g. 6 hours and 100 hours) in their battery datasheets, making comparing different models for different applications easier.

For example, in the case of industrial batteries, the 6h rate is used, which reflects the typical daily work cycle. Other times the 100h rate is used to indicate better performance than the real one, even if the 100h data is useful for understanding the battery capacity for long-term backup needs. As previously written, lead-acid batteries are more efficient with low discharge currents. For this reason, the longer the discharge time, the greater the resulting capacity.

DO THE BATTERIES DISCHARGE WHEN NOT IN USE?

All batteries, regardless of their chemistry, are subject to self-discharge. The self-discharge rate depends on the battery type and the battery’s storage temperature. In the case of marine batteries, a general self-discharge value to consider is around 1% – 3% per week, even reaching values of 2% per month in the case of GEL batteries; regardless of the type of battery, the importance of the temperature should be taken into consideration, the values indicated are normally evaluated with an ambient temperature of 20/25°C.

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