Battery : “Energy can neither be created nor destroyed !! But can be stored :D”

Electricity played a monumental role in the development we have achieved. We are all aware of the “Law of conservation of energy”. This law states that “energy can neither be created not be destroyed. It can be only transferred from one form to other”. This was about energy. Now you have heard about electrical power. Power when consumed over a time interval results into energy consumption. A similar law exists for power as well. At any instant, power sent from source must be equal to power at sink if we consider a loss less system. Let me explain it with a diagram (Sorry that my drawing and handwriting is really bad :P)

project_1492119294099

Now in the above image, P_G is the power generated at generating stations. P_{loss} is the power lost in form of heat during transfer from one end to other and P_L is the power delivered at load or consumer end. So as per the statement told,

P_G=P_{loss}+P_L

If we consider that the transmission is done using superconductors i.e. no losses, the P_{loss}=0. Therefore, P_G = P_L. Therefore, if the power is generated, it must be consumed at load end. But one may ask, what happens when the power at source and sink mismatches?. Well this situation will create other problems in the system and is beyond the scope of this article. Hence, for simplicity, we can note that instantaneous power is balanced. It would have been great if generation is constant and we can use as per our requirement. But dear friends, for large power systems this is not yet possible. But fear not! At lower power levels, there is someone to help us out and provide freedom of usage of power as we want. Ladies and Gentlemen, that someone is none other than Battery!

Battery is meant to be used in a situation when direct power or electricity is not accessible. Thanks to chemistry that allows us to understand the process involved in conversion of chemical energy to electrical energy. So in simple terms, when an electric equipment is connected across electrodes of a battery, free electrons as a result of chemical reaction, flow from cathode to anode constituting electric current in opposite direction. This electric current when flowing between a certain potential difference as determined by the battery, runs that electrical equipment.

Earlier we used to have non-rechargeable batteries which used irreversible reactions. But with hard work of scientists and engineers, we now have rechargeable batteries. When you use the rechargeable battery as energy source i.e. discharge, the reaction which occurs can be reversed when you charge them back. You don’t have to buy set of new batteries after discharge! Isn’t it really great? In spare time, you can go ahead with charging them and use them when you do not have access to power cord. But it should not be considered that once you buy a rechargeable battery you can do the back and forth process of chemical reactions for eternity. Therefore, every battery has a lifetime determined by its End of Life (EoL) value. EoL is defined by value of time or cycles after which it will cease having properties of a battery. Let us understand few basic and important terminologies when battery is considered. Rechargeable batteries are considered for the discussion.

Every battery capacity is determined by the term Ampere-hour (Ahr). If a battery is connected across two terminals and it can provide 10 A of direct current (DC) for 1 hour, then the battery is said to have 10Ahr capacity. Other term which is important is “Cycle”. When a battery is charged using a source and discharged to a load then a cycle is said to be complete. Therefore, EoL is in terms of number of cycles, e.g. 100 cycles means battery can undergo 100 full charging and discharging processes after which it cannot power the equipment at rated power. Currently the most common forms of batteries which we use are power-banks, phone batteries, laptop batteries, electric vehicles, so on and so forth. So battery charging and discharging has become an integral part of our lives. As batteries store energy, the duration of support provided by battery depends on the power. For example, in mobile phones a mode named “Battery Saver Mode” is present. What it does is it stops the most applications thereby reducing the current drawn. As explained earlier, if less current is drawn the capacity being constant, time taken to discharge the battery will be elongated.

There are a lot of common questions/problems all of us face regarding batteries:

“How much battery backup this phone provides? I charged it today, and see now only its charge reduced to 40%”, “I had just changed by inverter battery, now it is again unable to provide backup for long time” etc.  With these questions or complaints have we really ever thought that what can we possibly do to elongate the battery life? Many times people complain, “Don’t buy a phone from X company, they do not provide good battery backup”. But, have we ever questioned ourselves that, “Have I used the battery in an appropriate manner?” So, we have to understand that we too have a part to play in battery energy utilization! Let us first try to understand what happens during charging and discharging more closely to get a firm picture of the involved phenomena.

The charging phenomena can be depicted as shown below in the image.

2017070323572358a6174d19c2739a

(Above image depicts the charging process. Either it can be voltage or current or combination of both.)

The battery has a potential which is determined by its State of Charge (SoC). There is some resistance present due to leads, contacts etc. The charging can be either done through a current source or a voltage source. Hence the name current and voltage charging respectively. Combination of both voltage and current is also possible. Hence, broadly three ways the battery can be charged –

a) Constant Voltage (CV)

b) Constant Current (CC)

c) Constant Power (CP)

cc_cv
CC CV Charging

(Image Description: The image shows the two modes CC and CV. Initially constant current and when the voltage reaches to particular level, then mode is switched to Constant voltage . Slowly, the current reduces and hence the power also reduces.)

The first two methods CV and CC charging not very efficient. As explained above, battery holds the charge within itself. Using either CV or CC will damage the battery if not taken care of. Using CC mode, we try to put constantly charge in the battery. This increases the voltage across it.  If the charge dumping is constant, then eventually the voltage across the battery will be very high and battery cells would be damaged. A battery is made up of many cells, connected in series and parallel. Using just the CV mode do not ensure equal charging to every cell and uniformly. This is due to the fact that every cell is unique. Due to different internal resistances the charge will be different and it will not ensure effective charging. Now, we have some limits in a battery which should be taken care of. These limits are on voltage and current across and through the battery respectively.  If we try to ensure that the current limit or voltage limit does not exceed its limit then the battery charging will be slow and this is not desirable. Therefore, CP charging is considered to be the efficient approach for charging. In this method, initially the current fed into the battery is high enough to ensure fast charging (CC mode). As the charging process is done, the current is reduced and the process shifts to CV mode. As the battery voltage is reached then charging process is stopped. The maximum cell voltage in a battery is known as its GASSING Voltage (Typically 2.35-2.4 V/cell). Beyond this voltage is cells are charged then they start deteriorating.  Afterwards, research shows various combinations of charging like CCCV, CPCV etc. The details about charging can be accessed through various literature available based on your interest.

The story above tell us up to the charging process. Now here is the doubt, if everything is controlled in a way that battery is safe and charged and discharged properly what precautions do we have to take. The answer being, yes now the chargers are smart and researchers are trying hard to make things as simple as possible and reduce damage to batteries, still there is a room where we can be careful.

We know that if we kept the charger on forever, the charger will feed current slowly in a very very little amount. That little amount rises the battery temperature. This temperature rise is one of the cause of battery deterioration. The chemical reaction is efficient at low temperatures. Therefore, one thing we need to be careful when charging batteries is temperature should not reach very high value. That is why if you see in laptops, if you turn on the battery lifespan increment mode,  it stops charging the battery after 80% to remain within limits of temperature and gassing voltage.

The other critical aspect of battery life is its discharging phenomena. We have talked above up to the charging and various ways of charging. Some more ways are there as mentioned earlier and they are specific to particular applications. Nowadays, Dash charging, Turbo charging they are invented to provide a very fast and effective way of storing charge inside the battery. After charging, the way we discharge it is also very important. A term called Deep Discharge is coined for that.  Deep discharge means the SoC of battery upto which we should go before charging the battery again. There are various batteries and their own deep discharge limits. This will be covered as a separate article soon along with some popular batteries used in our day to day lives. Till then keep your hunger alive to know more. !!

This article is about the battery, its charging process and some common points to take care. Please feel free to have manthan on this.

Last but not the least, I would like to thank Nikunj and Sugoto for providing me crucial technical inputs in reforming the blog.

References –

  1. https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0ahUKEwj6zMSH2ePUAhUdS48KHSXCCloQFghzMAY&url=http%3A%2F%2Fwww.ti.com%2Flit%2Fan%2Fsnva557%2Fsnva557.pdf&usg=AFQjCNF7a0woFDaG5Sq_xK1ot6Ej74qO8w&cad=rja
  2. http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

Image Source : https://source.wustl.edu/2012/08/32-million-to-develop-battery-management-system-for-electriccar-batteries/

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