Battery Charging | Capacity of the battery | Battery Efficiency


There are different types of batteries available in the market namely the lead-acid battery, Nickel-Cadmium battery, and the sealed maintenance free (SMF) battery. However, the conventional lead-acid battery is most used in many systems.

In this lecture, we are going to see the battery charging method, capacity of the battery, battery efficiency. So let’s discuss each topic one by one.

The capacity of the Battery

The capacity of the battery is expressed in ampere-hour (AH).

It is defined as the product of a constant discharge current and the time duration beyond which the battery voltage falls below a voltage level called “final discharge voltage”.

The battery voltage should not be allowed to fall below the “final discharge voltage”, otherwise battery life will be shortened.

The lead-acid battery consists of a number of cells connected in series to give desired terminal voltage. Some of the important battery voltages are given in the below table:

Important Battery voltage for a lead-acid battery:

Sr. NoParameterValue per cell
1.Normal Voltage2.4 volts
2.Nominal Voltage2.0 volts
3.Final (minimum) discharge voltage1.8 volts
4.Float/trickle voltage2.15 volts
5.Boost voltage2.6 volts
6.Extended charging voltage2.65 volts

Also Read: UPS, Types of UPS & Application of UPS

Battery Charging

The completely discharged battery is first charged at a constant charging current rate as shown in the below figure.

This will cause the terminal voltage of the battery to increase considerably. When the battery terminal voltage reaches a sufficiently high value, constant voltage charging is adopted.

The battery charging current then decreased. This mode of charging is known as float or trickle charging.

Typically the voltage at which this changeover from constant current to constant voltage mode takes place is 2.15 volts.

The trickle charging is necessary to keep the battery fully charged always. The trickle charging current is very low and it helps to compensate for the loss of charge due to the internal discharge of the battery.

Battery Efficiency

There are two different values of battery efficiency:

  1. Ampere-hour efficiency
  2. watt-hour efficiency

1. Ampere-hour efficiency:

The ampere-hour efficiency is defined as the ratio of ampere-hours taken from the battery to the ampere-hours supplied to it while charging.

A'H efficiency = \frac{A-H \;during \;discharge}{A-H \;input \;while \;charging}

The typical value of AH efficiency is 90 to 95%.

5 to 10% reduction is due to the losses taking place in the battery. The ampere-hour efficiency takes into account only the current and time but it does not consider the battery terminal voltage at all.

2. watt-hour (energy) efficiency:

The watt-hour(WH) efficiency is defined as follows:

WH \;efficiency = AH \;efficiency \times \frac{Average \;cell \;voltage \;while \;discharging}{Average \;cell \;voltage \;while \;charging}

The above equation we can rewrite as,

WH \;efficiency = \frac{Energy \;supplied \;by \;battery \;while \;discharging}{Energy \;supplied \;to \;battery \;while \;charging}

The WH efficiency is less than AH efficiency. Typically it is 75 to 80%

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