# Few more words about energy stored in batteries

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Continued from technical article: Different types of battery used for auxiliary power supply in substations and power plants

## Can batteries of different ampere hour ratings be used together in series?

The ampere hour ratings of a battery indicate the rate at which the battery can be loaded. For Example a 20Ah battery indicates that the battery can supply a current of 1 ampere for 20 hours.

Smaller batteries have their discharge rates indicated in mAh (milli ampere hour).

When batteries of different ampere hour ratings are connected in series, it is necessary to ensure that none of the batteries is discharged beyond its capacity. This is because if a battery connected in series in a battery bank is discharged to its maximum, the voltage of this battery will become zero.

This will cause a voltage from the other batteries to be applied across the battery in the wrong direction. This can cause damage to the battery.

## How energy stored in battery is related with Ampere hour capacity of battery?

The Capacity of a battery is the quantity of energy a battery can store and deliver. The capacity of a battery is usually indicated in terms of Ampere hours or Ah. This is also known as the rated capacity of a battery.

The total energy stored in a battery can be calculated by multiplying the Ampere-hour rating with the battery voltage. Thus a 10Ah battery with 110V can store energy of 1100Wh or 1.1kWh.

The duration of a battery is thus dependent on its discharge rate. Manufacturers provide discharge charts for their batteries. Charging and discharging rates have a major impact on the life of a battery. A battery that is discharged at a faster rate may not deliver the same amount of energy as a battery with a lesser discharge rate. Some manufacturers specify the capacity of a battery as a function of the discharge rate.

Temperature, too, influences the capacity of a battery. Batteries at high temperatures will have a better capacity that batteries loaded at low temperatures. Extremely high temperatures, however, can cause damage to the battery and greatly reduce capacity.

In addition to these factors, age is also a factor that determines the capacity of a battery. As a battery ages, its capacity reduces.

## Why electrolyte level in battery should be inspected regularly?

The electrolyte in the battery is a mixture of sulphuric acid and water. The amount of water in a battery can fall due to electrolysis or evaporation. This may cause in a drop in the level of the electrolyte and consequently a drop in the battery output.

Hence, it is necessary to periodically inspect the level of electrolyte in the battery. If the level of the electrolyte falls below the minimum level, it can be topped up by adding water. Only distilled water should be added as ordinary water may contain a lot of impurities and ions which may contaminate the electrolyte.

The level of electrolyte in the battery tends to fall as the battery gets discharged and tends to rise as the battery gets charged.

Hence, water should be added to the electrolyte only when the battery is fully charged. If the water is added to the battery when it is in the discharged condition, the level can increase beyond the limit when the battery is fully charged and may overflow.

The acid used as the electrolyte is extremely corrosive and should be handled with extreme care. Proper protective outfits should be worn while handling them. Water can be added to a container of acid. However, acid can never be added to container of water as the heat generated can cause splashing.

## Which law governs the discharge of lead-acid batteries?

In 1897, the German physicist Peukert proposed that as the rate of a lead-acid battery’s discharge increases its available capacity. This is known as Peukert’s law.

It can be mathematically represented as Cp = IK t

where:
Cp is the capacity of the battery in ampere hours
K is the Peukert’s constant
I is the current and
t is the time

The Peukert’s Constant K is specified by the manufacturer of the battery and is usually in the range of 1.2 to 2.

Thus the time a battery can provide sustain a certain current without any appreciable drop in voltage would be given by:

t = Cp / IK

Thus a 100Ah capacity with a Peukert’s constant of 1.2 will be able to supply a current of 5A for 14.5 hours.

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#### Asif Eqbal

Bachelor of Engineering in Electrical & Electronics engineering, from Manipal University, (Karnataka), India in 2006. Presently involved in the design of EHV outdoor substation and coal fired thermal power plants for more than seven years. Motto of joining EEP as a contributor is to share my little engineering experience and help the budding engineers in bridging the conspicuous gap between academics and Industrial practice. “If you have knowledge, let others light their candles with it, so that people who are genuinely interested in helping one another develop new capacities for action; it is about creating timeless learning processes".

1. Lakshminarayana
Mar 11, 2015

hello sir, how to calculate battery bank capacity. i know no of cells 240 then each cell voltage is 2v .

2. samuel
Mar 08, 2015

very educative especially in engineering field

3. Daniel
Nov 30, 2014

Very educative article.This article contains simple electrical facts which most engineers don’t know. As an engineer I recommend engineer to download this article.

4. Patil Gaurav Ashok
Apr 01, 2014