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Technical Information about Batteries

> Introduction

> Principles of battery operating

> Classification of lead acid batteries

> Battery construction types

> Battery lifetime


> Introduction
Battery technology has seen many changes over the years, driven by the need to reduce the dimensions and the masses of the batteries, keeping the same energy stored.
Although various types of batteries have been introduced to the market over the years (Nickel Cadmium, Nickel Metal Hydride, Lithium Ion, etc., etc.), lead acid batteries are still widely used in various sectors, after more than a century of honorable service.
In particular, for automotive and marine use (starting and deep-cycle batteries) such batteries are even irreplaceable, thanks to a very competitive price/performance ratio and to their extremely high reliability, when used properly.

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> Principles of battery operating
Qualsiasi batteria piombo/acido è internamente costituita da una serie di celle. Ad esempio, una batteria 12V si compone di 6 celle da 2V collegate in serie tra loro.
Ciascuna cella è a sua volta composta da un elettrodo positivo (barra di biossido di piombo), da un elettrodo negativo (barra di piombo) e da un elettrolita (soluzione di acqua distillata e acido solforico) contenente ioni di zolfo (carichi negativamente) e ioni di idrogeno (carichi positivamente).
Quando la batteria viene collegata ad un carico elettrico (ad esempio una lampadina), gli ioni si muovono verso i rispettivi elettrodi per cedere la loro carica elettrica. Pertanto, al crescere del livello di scarica della batteria, diminuirà la concentrazione di ioni nell'elettrolita.
Per contro, caricare la batteria inverte il processo di scarica sopra descritto, dal momento che il piombo solfatato viene convertito in piombo e in ossido di piombo rispettivamente sull'elettrodo negativo e positivo della cella piombo acido.

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> Classification of lead acid batteries
Below, an overview of the main fields of application of lead-acid batteries:

These batteries (also called SLI - Starting, Lighting, Ignition) are generally used for starting and operation of internal combustion engines. Therefore the main feature of these batteries is precisely to be able to provide strong currents (several tens of Amperes), for a relatively short time (few seconds) during the cranking of a combustion engine.
This type of battery is characterized by the presence of a large number of electrodes more or less subtle, in order to maximize the surface contact between the electrode itself and the electrolyte, to provide very intense electric currents for short periods.

Deep Cycle:
These batteries are specifically designed to be discharged up to 80% and find their main use in the field of electric traction (think for example to use electric carts on golf courses or forklift).
Their electrodes are generally mechanically more robust than the ones of starting batteries, since they have to bear a greater number of charge/discharge cycles and they can be discharged up to about 80%.

Many batteries for marine use are a sort of "hybrid" between SLI and deep-cycle batteries. The electrodes are generally more robust than the ones of starting batteries, though they are less thick when compared to deep-cycle batteries. Usually manufacturers recommend not to discharge them more than 50%.

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> Battery construction types
Below, an overview of the main types of lead acid batteries:

Standard (Wet) / Sealed (MF)
Most batteries are of this type and they are characterized by having both electrodes (positive and negative) immersed in a liquid electrolyte (a solution of distilled water and sulfuric acid). Standard batteries have removable caps to allow the restoration of the electrolyte level, while sealed (or MF - Maintenance Free) batteries are made up of ventilation holes which (usually) can not be removed and allow to any gas generated during battery charging to disperse in the air outside the battery.
Therefore, MF batteries are not totally sealed and it is also important to avoid overcharging, or the electrolyte will evaporate prematurely, causing a significant shortening of the life of the battery.
Some (not many) sealed batteries have special caps that can convert the hydrogen and oxygen generated during the charging process back into water, to reduce water losses of the electrolyte up to about 90-95%.
A good number of deep-cycle battery uses Lead-Calcium plates or Lead-Antimony plates, in order to increase the battery life and the mechanical strength of the electrodes.

Gel batteries contain gelified electrolytes (obtained by adding silica gel): this make the electrolyte (acid solution of distilled water and sulphuric acid) solid and gel-like.
The main advantage of this kind of batteries is that they can't release acid, even if they are turned upside down or if their case is broken.
Therefore, they are a kind of sealed batteries which doesn't require any maintenance (for example, the periodic re-filling of the electrolyte).
However, there are also any disadvantages in using gel batteries: they have to be recharged at lower currents and voltages, to avoid the creation of bubbles within the gelified electrolyte, which could permanently damage the battery. For this reason, it's particularly important to avoid using traditional fast chargers for the maintenance of gel batteries.

This kind of batteries contain a AGM solid electrolyte (Absorbed Glass Mat, that is a fine fiberglass imbued a solution of water and sulphuric acid) between the electrodes.
These batteries are very safe, since they can't release acid, even if they are turned upside down or if their case is broken.
Nearly all AGM batteries are also VRLA (Valve Regulated Lead Acid) type: this means that the battery has a small valve that maintains a slight positive pressure with respect to the external environment (atmosphere). These batteries are therefore slightly under pressure.
As one can easily imagine, these batteries have all the advantages of gel batteries but they do not present their limits, since they can withstand a higher charging current: the battery can be charged as a standard (or MF) battery.
Another important feature is that these batteries are "recombinant", i.e. the oxygen and hydrogen formed during charging are recombined again generating water (with efficiency greater than 90%) directly inside the battery, ensuring a very slight loss of water during the entire lifetime of the battery.

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> Battery lifetime
The lifetime of a battery can vary considerably depending on several factors. The most important, however, are a proper use and a proper maintenance (charging).
Among the main reasons that can bring a battery to premature death, there are the following:

  1. Leaving the battery inactive (sulfation for self-discharge).
    One feature not to be underestimated of all lead-acid batteries is that they self-discharge by their own, especially if they're not used for several weeks or even months.
    To make matters worse, electronic devices on board vehicles in today's day (control units of various types, alarm, radio, digital clock, etc., etc.) require a weak but continuous current to the battery, even when the vehicle is kept off and unused.
    For example, this happens very often for motorcycles: they are left in the garage in autumn - at the beginning of the bad season - and in spring they are no longer able to be put in motion since the battery is fully (or partly) discharged.
    In these cases it often happens that, despite trying several times to recharge it, the battery can not start the vehicle: the battery is in fact "sulphated", i.e. the electrodes are coated with crystals of lead-based sulfur, so large and deep that they can no longer be removed during the charging process.
    This process (sulfation) is practically irreversible and condemns the battery to a premature death, which would be easily avoidable keeping the battery always at 95-100% of its charge during the period of inactivity of the vehicle.
  2. Keeping the battery partially discharged (progressive sulfation)
    To leave a lead-acid battery partially discharged causes a progressive accumulation over time of lead-based crystal-sulfur clusters, sufficiently large and deep that thay can no longer be removed during the charging process. This process generated inside the battery is called sulfation and it has the characteristic to be virtually irreversible, condemning the battery to a premature death which would be easily avoidable keeping the battery always at 95-100% of its charge during the period of inactivity of the vehicle.
  3. Discharging the battery beyond its limit (electrodes' corrosion)
    The discharge of the battery beyond its limit is another serious "killer" because every time it is discharged in this way, a fraction of the electrodes corrode going to settle on the bottom of the battery.
    Obviously this causes a progressive lower surface of exchange between the electrolyte and the electrode inside the battery, together with a lower mechanical resistance of the electrodes themselves (since corroded).
  4. Overcharging (electrolyte's destruction)
    Overloading the battery is a serious and insidious error at the same time, because its effects are often not visible to the inexperienced user, who buys cheap battery chargers delivering a constant current regardless of the status of the battery. If the battery charger provides a too high current to the battery during the various stages of the charge, this will cause in a short time (hours) the evaporation of the electrolyte in standard batteries (sealed or MF), or the formation of cavities (gas bubbles) in the electrolyte in the case of gel batteries.

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