Design and Function [3 of 4]

Self-discharge





Example of self-discharge (for open battery type) depending on battery temperature and discharge time
- A. Acid density in g/cm3
- B. Number of days that the battery was not under load
- C. Acid density at different battery temperatures.
There is always some self-discharge in a battery, when the battery is not in use and during both charging and discharging. If a battery is not used for a longer period, there is considerable self-discharge. The acid density falls and the active material in the plates is converted to lead sulphate. Excessive discharge must be avoided because otherwise there is an increased risk of sulfation. Sulfation may cause permanent damage to the battery. Regular charging of the battery will prevent sulfation. See Sulfation. There is an increased risk of damage from freezing in a heavily discharged battery. See Deep discharging.
The speed of discharge depends on the temperature, time, the condition and construction of the battery. The temperature is particularly influential. The rate of self-discharge is faster at higher temperatures. Batteries should be stored for prolonged periods in a dry, cold place, preferably below freezing.
Ensure that the battery is fully charged if it is to be left unused for a long period. No further charging will be required if the battery is in good condition and is stored in a dry cold place. If the battery is being stored in a warm place, it may require regular charging.
The illustration shows an example of how quickly a battery (of open type) can self-discharge, depending on the temperature of the battery. Note how the density of the acid reduces with time and how the self-discharge speeds up as the temperature increases. For an explanation of the density of the acid, see Acid density.

Acid density





Example of the variation in the stand-by voltage and in the density of the acid with the state of charge in a battery(of open type) at +25°C (77°F) (measured after approx. 2 hours charging or discharging)
- A. Stand-by voltage in V
- B. Acid density in g/cm3
- C. State of charge, SOC, in %
- D. Variation in the stand-by voltage with the state of charge
- E. Variation in the density of the acid with the state of charge.
The density of the acid is a unit showing the concentration of sulphuric acid in the electrolyte. The density of the acid is a measurement of the battery voltage and State of charge, SOC. The density of the acid is measured in g/cm3. Sulphuric acid is required for the chemical processes in the battery.
The higher the value of the acid (i.e. high concentration of sulphuric acid), the higher the voltage and state of charge. A low acid density value means a correspondingly low concentration of sulphuric acid, low voltage and a reduced capability for providing current. The electrolyte in a fully charged battery has a density of 1.28 g/cm3 at +25°C (+77°F). The density of the electrolyte in a fully discharged battery is 1.10 g/cm3 or lower depending on the type of battery.
The illustration shows how the stand-by voltage and the density of the acid drops as the state of charge of a battery reduces.

Hint: For maintenance-free as well as sealed batteries (AGM) the battery acid cannot be accessed and thus its density cannot be measured.

State of charge, SOC
The state of charge (SOC) is expressed as the amount of electrical energy that is stored in the battery at any given time, in relation to how much energy can be stored in a fully charged battery. The state of charge is listed as a percentage of full charge.

Capacity
The capacity of a fully charged battery is its ability to give a constant current during a certain time and is stated in the unit ampere-hours (Ah). The time for discharging varies depending on the battery's purpose.
In principle, the capacity is controlled by the plates' area as well as thickness. In a battery there is a big area on the plates, on which chemical reactions can take place, and also where a great amount of electrons (current) can be generated between the plates. This gives them a higher cold-start current value (CCA = Cold Cranking Amperes).
A start battery has big area but thin plates.
For a deep cycle battery, e.g., battery for leisure use, it is important that the chemical process can continue for a long time with lower current use, that is why the plates have to be thick. This gives them many ampere-hours or "spare minutes". Since the plates are thick, they need longer time to charge so that the chemical reaction also takes place deep inside the plates.
For start batteries, 20 hours' capacity (K20) is mentioned. This refers to how much current the battery can deliver during 20 hours in an ambient temperature of +25°C (+77°F) without the terminal voltage going below 10.5 V.
Example: A battery (of open type) with a stated capacity of 70 Ah should be able to provide a constant current of a max. 3.5 A (3.5 A x 20 h = 70 Ah) for 20 hours.
The capacity of a battery is not only dependent on the size and construction of the battery, but also varies considerably for batteries of the same type with the size of the discharge current. The lower the discharge current the higher the capacity and vice versa. The capacity of the battery is also affected by the temperature and the age of the battery. The nominal value is given at +25°C (+77°F). The capacity of the battery reduces considerably at low temperatures.
The table shows the difference in capacity in a battery at +25°C (+77°F) and -18°C (-0.4°F).






Battery service life




The service life of the battery depends primarily on its construction, maintenance and operating conditions. The following factors may shorten the service life of the battery:
- High temperatures
- Cycling
- Low electrolyte level
- Deep discharging
- Incorrect charging
- Sulfation
- Corrosion
- Vibrations
To maintain the best possible service life and capacity, the battery must be maintained and charged in accordance with Volvo's recommendations.

High temperatures
A high ambient temperature speeds up the chemical processes in the battery during charging and discharging. For every 10°C (18°F) increase in temperature, the reaction speed of the processes doubles. The risk of corrosion, self-discharge and sulfation increase at a high temperature and the service life of the battery is reduced. The service life of a battery improves in colder surroundings.

Cycling
Cycling means all the discharging and charging in a battery. A battery is always cycling. Discharging, or cycling, can have varying degrees of depth. Deeper discharge is more damaging than lesser discharge. Each discharge results in stress to the plates which deteriorate accordingly. Each instance of cycling cause the material in the plates to become more fragile and ultimately some of this material will separate from the plates. Cycling results in a reduction of capacity.
A lead battery cannot tolerate infinite cycling. Deep discharges should be avoided in order to maintain as long a service life as possible.