BATTERY BASICS AND ITS RATINGS

Hello folks,

Let us talk about some real world voltage sources. The most commonly used voltage source is the battery, which we all must have frequently used in many of the electrical equipment and gadgets. The battery is the DC voltage source which means voltage is 1) constant 2) Independent of load current 3) Constant with respect to time. There exist many diverse types and versions of battery. In practice, battery has 2 terminals, one is positive terminal and other is negative terminal. [Fig. 1]

Inside this ideal battery, there exists a chemical process. Consider a container that contains two electrodes i.e. positive electrode (anode) and negative electrode (cathode). This container is filled with an electrolyte. Among all the batteries, the most commonly used is Lead Acid Battery that is used in almost all the automobiles. In this battery, the anode is Lead Oxide (PbO2), cathode is Lead (Pb) and dilute sulphuric acid (dil. H2SO4) is used as an electrolyte. The electrons get generated at the cathode, flow to the anode, then through external conductor they flow towards external circuit and then again back to the cathode. In this process, water is generated in the lead acid battery and it dilutes the battery and hence the charge goes down. So repeatedly the charge goes down and battery gets discharges. We need to recharge the battery to its original capacity i.e. the concentration is brought back and now battery is in the charged state. [Fig. 2]

                               Fig 1. Battery Symbol       Fig.2 Practical Battery

In practical battery, the electrolyte is in gel form. On any practical battery we can see name plate ratings. For e.g. Let’s consider the 2 most important ratings in case of battery

i)                Terminal voltage (Nominal DC voltage) across 2 terminals Vt  and

ii)              Ampere hours (Ah) rating

Let Vt = 12 V         Ah = 7

7 Ah means we can draw 7A for 1 hour. But it’s impossible in actual practice. So it implies that each battery has maximum discharge current capability. Let that battery has 10 cells and now we can draw 7/10=0.7A for 10 hours. This is 7 Ah.

Energy capacity of battery = 12 V * 7 Ah = 84 Wh

Energy stored in the battery is called battery capacity which is measured either in Wh (Watt-hours), kWh (kilo Watt-hours), or Ah (Ampere-Hours). The most common measure of battery capacity is Ah, defined as number of hours for which a battery can provide a current equal to the discharge rate at nominal voltage of the battery.

The charging/discharging rates affect the rated battery capacity. If battery is being discharged very quickly (discharge current is high), then the amount of energy that can be extracted from the battery is reduced and the battery capacity is lower. Alternately, if the battery is discharged at a very slow rate using a low current, more energy can be extracted from a battery and the battery capacity is higher.