Basic Terms Related to Battery

Battery Cell

A cell is the smallest, packaged form a battery can take and is generally on the order of one to six volts.

Battery Modules

A module consists of several cells generally connected in either series or parallel.

Battery Packs

A battery pack is the assembled by connecting modules together, again either in series or parallel.

C-rate

A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity. A 1 C rate means that the discharge current will discharge the entire battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100 Amps. A 5 C rate for this battery would be 500Amps, and a C/2 rate would be 50 Amps.

E-rate

An E-rate describes the discharge power. A 1 E-rate is the discharge power to discharge the entire battery in 1 hour.

Secondary and primary battery

A primary battery is one that cannot be recharged. A secondary battery is one that is rechargeable.

Battery Conditions

1. State of Charge (SOC)(%)

An expression of the present battery capacity as a percentage of maximum capacity. SOC is generally calculated using current integration to determine the change in battery capacity over time.

2. Depth of Discharge (DOD) (%)

The percentage of battery capacity that has been discharged expressed as a percentage of maximum capacity. A discharge to at least 80 % DOD is referred to as a deep discharge.

3. Terminal Voltage (V)

The voltage between the battery terminals with the load applied. Terminal voltage varies with SOC and discharge/charge current.

4. Open-circuit voltage (Voc)

The voltage between the battery terminals with no load applied. The open-circuit voltage depends on the battery state of charge, increasing with state of charge.

5. Internal Resistance

The resistance within the battery, generally different for charging and discharging, also dependent on the battery state of charge. As internal resistance increases, the battery efficiency decreases and thermal stability is reduced as more of the charging energy is converted into heat.

6. Energy or Nominal Energy (Wh)

The “energy capacity” of the battery, the total Watt-hours available when the battery is discharged at a certain discharge current (specified as a C-rate) from 100 percent state-of-charge to the cut-off voltage. Energy is calculated by multiplying the discharge power (in Watts) by the discharge time (in hours). Like capacity, energy decreases with increasing C-rate.

7. Battery Cycle Life (number for a specific DOD)

The number of discharge-charge cycles the battery can experience before it fails to meet specific performance criteria. Cycle life is estimated for specific charge and discharge conditions. The actual operating life of the battery is affected by the rate and depth of cycles and by other conditions such as temperature and humidity. The higher the DOD, the lower the cycle life.

8. Specific Energy (Wh/kg)

The nominal battery energy per unit mass sometimes referred to as the gravimetric energy density. Specific energy is a characteristic of the battery chemistry and packaging. Along with the energy consumption of the vehicle, it determines the battery weight 

9. Specific Power (W/kg)

The maximum available power per unit mass. Specific power is a characteristic of the battery chemistry and packaging. It determines the battery weight required to achieve a given performance target.

10. Specific Power (W/kg)

The maximum available power per unit mass. Specific power is a characteristic of the battery chemistry and packaging. It determines the battery weight required to achieve a given performance target.

11. Energy Density (Wh/L)

The nominal battery energy per unit volume sometimes referred to as the volumetric energy density. Specific energy is a characteristic of the battery chemistry and packaging. Along with the energy consumption of the vehicle, it determines the battery size required to achieve a given electric range.

12. Power Density (W/L)

The maximum available power per unit volume. Specific power is a characteristic of the battery chemistry and packaging. It determines the battery size required to achieve a given performance target.

Excercise

Question: What is primary cell and secondary cell?

Answer. Battery which can be discharged only once is called primary cell ex: Manganese Zinc and alkaline battery cell.
Secondary cells are the battery which can be charged and discharged for a number of times ex: Lead acid and Nickel-cadmium battery cell.

Question: Do temperature have the effect on battery?

Answer. Temperature is one of the major factors that have an impact on battery performance, shelf life, charging and battery voltage control. At higher temperature batteries will have more chemical activity compared to batteries which operate at the designed temperature.

Question: How positive and negative terminals of a battery are identified?

Answer. Positive terminal is identified painted with red colour and negative terminal is identified painted with blue colour on the top of the battery

Question: What is the end of life of the battery?

Answer. If a battery cannot able to hold a proper charge or if the battery capacity falls below 80% of its rated capacity then it is considered as the end of life of the battery. Many factors affect the life of the battery such as

  1. Charging and discharging cycles
  2. Temperature
  3. Leakage of the gases or electrolyte
  4. corrosion of the plates
  5. over discharge rates

Question: What is stratification in the battery?

Answer. During charging of the battery acid is produced at the plate surface and during discharging of the battery acid will be consumed, due to this nonuniform concentration heavy parts of the electrolyte settles at the bottom and lighter parts of the electrolyte float on the top. This phenomenon is called stratification. Stratification affects the life of the battery.

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