BATTERIES

Definition – a container consisting of one or more cells, in which chemical energy is converted into electricity and used as a source of power. Batteries are of two types – Lead Acid Battery  &  2) Sealed Maintenance Free (SFM) Battery

  • LEAD ACID BATTERY – A lead acid battery is also known as tubular battery. A battery consists of four main parts – a) a container which can hold liquid acid, b) Positive Electrode – Led PerOxide(PbO2) c) Negative electrode – Sponge Lead (Pb) & d) Electrolyte (Sulfuric Acid (H2SO4). The container is filled with Sulfuric acid (H2SO4)& major portion of positive & negative electrodes are dipped in the sulfuric acid & some portion of these electrodes are left out which are used to connect the load.

When we buy a led acid battery, say, 12V, it comes in two parts – 1) battery with a container &two electrodes fixed with the container, 2) Sulfuric acid which comes in a separate container. Now let us prepare battery so that it can be sued. Let us follow this procedure – first fill the container with sulfuric acid (H2SO4). Sulfuric acid with water in 3:1 ratio is called electrolyte.Connect battery with charger & leave it for 8-10 hrs. After sufficient charging, check battery voltage between positive (P)& negative (N) terminals. If 12V available between P&N terminals battery is ready for use, if voltage is less than 12V, then again charge it till voltage reaches up to 12V. The operation of a battery works on chemical reactions which can be understood from following diagrams-

The above process is called discharging of battery.

Now, Battery charging is done as per following procedure –

  • Connect discharged battery with charger.
  • Sulfuric Acid (H2SO4) remains in the solution of PbSO4 in the form of H+ & SO4 – -,
  • As H+ ion is positive so it will go to negative side of the battery which is connected with negative terminal with DC source & H+ ion becomes H atom by receiving electrons from it.
  • These hydrogen atoms attract PbSO4 and form Lead & Sulfuric Acid,
  • SO4 – – ions will attract towards positive terminal of battery &give up extra electrons become SO4 radicals,
  • These radicals react with PbSO4 & form PbO2 and H2SO4.

So, above process – 1)increases gravity of sulfuric acid&potential difference between two terminals. 2) Lead sulfate of negative plate gets converted into lead peroxide & lead sulfate of positive terminal gets converted into pure lead.

  • Following chemical reactions take place during discharging of batteries -H2SO4 =   H+(ions)  + SO4 – – (ions)
  • PbO2 + 2H =  PbO + H2O,
  • PbO + H2SO4 =  PbSO4 + H2O,
  • PbO2+ H2SO4 + 2H =  PbSO4 + 2 H2O,
  • Pb + SO4 – – = PbSO4

IMPORTANT POINTS TO REMEMBER –

  • Voltage & specific gravity – A fully charged cell has a voltage of 2.2V & specific gravity of 1.250 respectively.
  • Longer life of battery –Forlonger life of battery, a cell should not be allowed to be discharged till voltage 1.8V & 1.1level of specific gravity.
  • Ampere Hours (AH) – An ampere hour is the amount of energy charge in a battery that will allow oneampere of current to flow for onehour.
  • Series connection of batteries – Voltage of each battery adds up & current remains same. Total voltage& current of batteries when connected in series V = V1+V2+V3+…, Total Current I = I1 = I2 = I3 =

So, to make a 12V battery, 6 cells should be connected in series.

Example – When 2nos 12V, 100AH batteries are connected in series, total voltage would be 2X12V = 24V & total ampere would be 100AH only.

  • Parallel connection of batteries – Voltage remains same & current add up. So, in order to increase number ampere hours batteries should be connected in parallel.

Example– 2 nos of 12V, 100AH batteries are parallel connected, then total voltage will be 12V but ampere hours would be double 200AH (=100+100 AH)

  • When batteries are made, they are first charged by electrical supply.
  • Hydrogen gas – When water (H2O) decomposes during charging of batteries, oxygen bubbles collects at positive plate & hydrogen bubbles at negative plate. This reaction is producing hydrogen gas which is highly explosive when mixed with air. This in turn loss of electrolyte.
  • Battery Recycling –Roughly 95% are batteries are recycled. A lead acid batteries contain 60-80% lead & plastic, so these two items are used again & again by battery manufacturers. So batteries have great contribution towards environment.
  • Electrolyte –A chemical compound that ionizes when dissolved or molten to produce an electrically conductive medium. In lead acid battery, solution of Sulfuric Acid & water is an electrolyte.
  • Avoid deep discharges – The deeper the discharge, the shorter the battery life will be. A brief charge on a 1–2 hour break during heavy use prolongs battery life.

MAINTENENC OF BATTERIES

  • Over Charging of battery –Cells should not be over charged, If cells are overcharged, physical properties of Lead Sulfate (PbSO4) gradually changes which decreases the specific gravity of electrolyte that hampers the rate of chemical reaction.

The color of sulfated plate becomes lighter & its surface becomes harsh & gritty.To avoid this, charging of battery should be done at low rate of charging current.

  • Refilling of lead acid battery –The level of electrolyte (mixture of water & sulfuric acid) in battery goes down due to two reasons – it evaporates naturally & secondly, hydrogen & O2 are produced in small amount during the operations.

If level of water goes down below the top the plates, the exposed plates sulfate& become inactive. So, batteries should be refilled with distill only (not from tap water & sulfuric acid). Because, the minerals of tap water affects the performance of batteries and acid increases the specific gravity which causes excessive corrosion.

  • Ventilation in battery rooms–As there is a production of hydrogen gas (in small amount) during charging/discharging process & there may be chances of spillage of electrolyte, these two things may pollute the atmosphere of battery room. Hence, battery room should be specious & proper ventilated.
  • Acid Stratification –When electrolyte concentrates at the bottom of plates & upper portion remains less concentration, this situation is called acid stratification.

This happens when battery remains at low voltage, never receives full charge & has shallow discharge. This causes following disadvantages – a) light acid on top plate limits activation, promote corrosion & reduces performance, b) high acid concentration on bottom makes the battery appear more charge (than actual). Battery shows an artificial full voltage, c) high concentration on bottom induces sulfating on lower half of plates.

Solution – Allowing the battery to rest for a few days. Doing a shaking motion or tipping the battery on its side helps correct the problem. Applying an equalizing charge by raising the voltage of a 12-volt battery to 16 volts for 1–2 hours also helps by mixing the electrolyte through electrolysis. Avoid extending the topping charge beyond its recommended time.