Battery Voltage
A fully charged battery has a voltage of at least 12.6 V. At 12.2 V a battery is half full and at 11.9 V almost empty. The battery should be charged by voltage from 14.2 V to 14.4 V (not higher than 14.7 V). The battery should not be (deeply) discharged so that its voltage drops below 10.8 V (1.8 V per cell).
Battery current
The battery should be charged with a current strength of 10-20% of its capacity amount. Therefore, a 10 Ah battery should be charged with a current of 1 A to 2 A, but not a stronger current!
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| Yuasa 12AL-A2 |
Battery operation
The battery has two electrodes (two poles) where one pole (cathode = positive + pole) is connected to lead dioxide (PbO2) plates while the other pole (anode = negative - pole) is connected to pure lead (Pb) plates and they are immersed together in sulfuric acid (H2SO4 with concentration of 1.28 kg/l for a fully charged battery).
When discharging the battery, the sulfur deposits on the plates of both electrodes (process of sulfation) and crystallizes (lead sulfate PbSO4 is formed). Consequently by sulfur deposition the sulfuric acid concentration drops (from 1.28 kg/l for fully charged battery to 1.1 kg/l for empty battery).
The chemical reaction to discharge a battery is described by the following chemical relation:
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| The figure taken from the site https://www.schrack.hr |
PbO2 (cathode) + 2H2SO4 (sulfuric acid) + Pb (anode) --> 2PbSO4 (lead sulfate) + H2O (water)
A reverse process occurs when the battery is charged, the crystallized sulfates from the plates break down (desulfation) and the dissolved sulfur increases the concentration of sulfuric acid.
Thus, electrolyte density is a reliable parameter for determining the amount of energy in a battery. In practice, electrolyte density measurement is carried out very simply with a bometer, refractometer, etc.
Battery discharge
The battery naturally discharges even when no power supplies are connected: eg GEL batteries discharge 2% per month at 20°C. For each additional 10°C the discharge increases by double.
When the battery is fully charged there should be no lead sulfate on the electrodes. However, with each charge and discharge cycle, a little amount of sulfate still remains on the electrodes, and this sulfate is also a major cause of battery aging, so batteries should always be kept full. If an empty battery is stored and not recharged for a long time, this sulfate crystalline structure will harden so much that it will no longer be able to dissolve it later. The same thing will happen if the battery is not fully charged for a long time, so that part of the surface of the plates will remain irreversibly sulfated and such deposits cannot be converted to pure lead and lead dioxide by any battery charging. The result is a smaller surface area of the plates exposed to electrolyte, which permanently loses battery capacity.
When the battery is fully charged there should be no lead sulfate on the electrodes. However, with each charge and discharge cycle, a little amount of sulfate still remains on the electrodes, and this sulfate is also a major cause of battery aging, so batteries should always be kept full. If an empty battery is stored and not recharged for a long time, this sulfate crystalline structure will harden so much that it will no longer be able to dissolve it later. The same thing will happen if the battery is not fully charged for a long time, so that part of the surface of the plates will remain irreversibly sulfated and such deposits cannot be converted to pure lead and lead dioxide by any battery charging. The result is a smaller surface area of the plates exposed to electrolyte, which permanently loses battery capacity.
Deep discharge
Deep discharges on classic batteries can be considered at some 50% of discharged capacity. Therefore, a battery with a capacity of 60 Ah should not consume more than 30 Ah. Deep discharges are not good for a classic battery. There are special (deep cycle) batteries that are tailored for deep discharges and can discharge up to 80% of their capacity (but they should not be completely discharged either). Deep discharge damages the battery in such a way that the active material is permanently removed from the lead plates, reducing their conductivity. The black/brown deposit at the bottom of the battery is the first indication that the battery has been frequently discharged deep.
Plate erosion and corrosion
During normal operation and regular and regular filling, it is impossible to avoid that part of the lead or lead dioxide does not fall off from the electrodes, so the mass of the electrodes decreases over time - this is the erosion of the plates. Enhanced or accelerated erosion of panels in a battery is called plate corrosion.
The erosion of the material is deposited at the bottom of the battery container and the deposit becomes thicker with time. As it is an electrically conductive material, a thick layer of precipitate can bind the electrodes and cause a short circuit between the battery panels.
The erosion of the material is deposited at the bottom of the battery container and the deposit becomes thicker with time. As it is an electrically conductive material, a thick layer of precipitate can bind the electrodes and cause a short circuit between the battery panels.
Normal erosion is controlled by limiting the discharge depth, reducing the number of discharge cycles, and taking care of the battery operating temperature. Intense corrosion, in turn, is caused by long overcharging of the battery. In contrast, charging with too little current will result in ions in the electrolyte being unable to dissolve lead sulfate in a high-quality and fast manner.
Battery stratification
Stratification (separation into layers) is a process that happens if the battery is not charged with sufficient current (or if it is never charged to more than 2/3 of its capacity). At low charging currents, low ion flow will not mix acidic electrolyte enough, so water and acid will separate so that heavier acid sinks to the bottom and water remains at the top. The concentration of acid at the top becomes too low, and at the bottom too high which causes the top of the plates to sulfate and the bottom of the plates to intensify.
Stratification is avoided in batteries with a liquid electrolyte by deliberately shorter charging pulses at higher voltage and by deliberately induced intensified extinguishing in which gas bubbles thoroughly stir the electrolyte. This is used by smart chargers with pulse charging modes.
In closed batteries (non-liquid electrolyte, eg GEL), no stratification occurs.
Peukert's law
Peukert's law is about how the capacity of a battery depends on the
amount of current the battery empties. Specifically, the battery
capacity is not linear with the discharge current. Eg. a battery with a
rated capacity of 60 Ah implies that it is discharged with a current of 3
A (20 times less than the capacity output) and will thus withstand 20 h
(3 A × 20 h = 60 Ah). But the same battery if emptied by a stronger
current of, for example, 10 A will not deliver this current for 6 hours
(although linearly mathematically it is also 10 A × 6 h = 60 Ah). Due to
the actual processes occurring in the battery (heating, etc.), higher
charging currents will not achieve the same efficiency and hence the
rated capacity of the battery.
Starter batteries
Starter batteries are a special type of battery designed to give the extremely high currents (of several hundred amps) required when starting a vehicle. These batteries have thin foamed lead plates and are sensitive to deep discharges. Commonly used so-called AGM (Absorbent Glass Mat) batteries. Such batteries are used on new vehicles and are quite expensive.
AGM batteries
AGM (Absorbed Glass Mat) batteries are a type of lead-acid batteries that are different from classic lead-acid batteries because they have glass wool separators that completely absorb electrolyte (no electrolyte topping). The absorbed electrolyte cannot even leak if the battery case breaks and the batteries can operate normally in all positions. Up to 50% more power than standard batteries (per unit volume / mass).
This type of battery is expected to last up to 10 years or 3,000 ignition cycles, withstands low temperatures, is vibration-proof, and self-discharge is 1-3% of capacity per month. They have less electrical resistance which enables fast charging.
GEL batteries
GEL batteries contain a silica gel containing an electrolyte in the form of a thick paste, which prevents leakage in the event of a rupture. They do not require special handling and are extremely vibration resistant and work in all positions. They give slightly less power (per unit volume / mass) than AGM batteries. They tolerate low temperatures well, but not as AGM batteries, and self-discharge is 2-6% capacity per month. They are sensitive to wrong charging - the battery charger must be designed for GEL batteries only.
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