Plugged-in: Understanding Rechargeable Batteries

We currently live in a world where battery-powered devices surround us. There is a high probability that the device you are using to read this blog is running on battery power. Such is the omnipresence of solid-state chemical cells that are literally running the world around us, from humble smartphones to Electric Vehicles and all the way up to the satellites in space.

Now, traditional batteries themselves can power a system till they drain off all the stored energy. Still, these need to be constantly replaced which is not feasible for all kinds of applications, especially if a substantial current drawing load is to be driven. So to overcome this issue, rechargeable batteries of different chemical compositions were invented.

Rechargeable Batteries

As the name suggests, rechargeable batteries are cells that can be recharged to their full capacity once discharged thus providing a device with uninterrupted power for seamless functioning. This negates the hassle of constantly needing to swap batteries and therefore makes it much more convenient to put into intelligent devices.

Now, there are multiple chemistries that these batteries could be based upon like Nickel-Cadmium (NiCd), Nickel-Metal Hydride (NiMH), but for our explanation, we will stick with the most prevalent of them all, i.e., Lithium-Ion (Li-ion).

Your phone’s heart (Lithium-ion Cells)

Almost all modern battery powered are using some variation of a Lithium-ion cell, from your humble smartphones all the way up to electric Vehicles.

Now we won’t go into the extreme intricacies of what exactly happens inside a Li-ion cell. But to explain it briefly, it is made up of the said element which is naturally found. During discharging the electrons flow from the anode (positive side) to the cathode (negative Side) and vice-versa during charging. The entire process of first charging and then discharging a battery cell is known as a cycle. This is important because a life of a Li-ion cell is determined by the number of cycles it goes through.

How do you recharge them?

Now, since these batteries are complex devices, they just can’t be charged directly with a DC (Direct Current) source. There is a certain protocol that needs to be followed while charging them known as the charge profile. To achieve this Li-ion battery packs are usually paired up with a charge controller system known as a BMS or a Battery Management System.

Battery Management System (BMS)

As the name suggests the Battery Management System is charge-controlling hardware that governs the charging, discharging and protection of Lithium-ion cells. The charge profile of these cells is explained in the diagram below.

This might look a bit complex at first sight, but it explains how the battery should be charged. Let’s take an easier example of filling an empty glass with water.

Now, imagine the glass is an empty battery cell, and the water is voltage. The rate at which you pour the water is current. If the glass is empty to start with and you pour water at a humungous rate, there is a high probability that the glass might topple over by the sheer weight of the water. Similarly in a Li-ion cell, if you charge it with a substantial current when it’s fully discharged, it might permanently damage the cell.

On the flip side, if you slowly pour water in the beginning and gradually keep on increasing your rate of pour and slow down in the end, the glass would not topple over and would be safely filled to the brim as well. This is the main concept explained by the diagram previously shown, when the battery is fully discharged, it is first charged to its nominal (starting) voltage (usually 3.6~3.7V) using a very low current. This is known as Trickle Charging or pre-charging. Once the nominal voltage is achieved, max current is applied till the full voltage is reached. This is known as fast-charge. Finally, at max voltage, the current is reduced and the BMS goes into something called constant voltage mode. These are the basic steps to charging a Li-ion cell and hence why a battery management system is essential.

Now, Battery Management systems like any other piece of modern hardware can come in shapes and sizes depending on their application. From tiny fitness trackers to family EVs.

Like the charging profile, Li-ion cells also have a discharging profile that dictates a safe discharging of the battery to the load without damaging itself. Here is a pictorial description of the discharge profile.

Conclusion

As we reach the end of this blog, I hope that you have been able to understand the basic idea behind how rechargeable cells work and hopefully be more enthusiastic when you come across a device that uses them. But since we only talked about the positives of Li-ion batteries, It would be fitting for me to share with you a few negatives. The two big problems being faced by the world right now in regard to these cells are the efficient and safe disposal of Li-ion cells and the Unethical mining of Cobalt which is the main component in its construction. Big firms like Tesla have taken a pledge toward solving these issues and we hope that in the electric future that we all envision, we would be able to overcome these hurdles and thus play an important part in solving the climate change crisis.

On an ending note, I would leave you with the father of DC current and the founder of GE (General Electric) Corporation, Thomas Alva Edison, because of whom our smartphones and TV remotes work without being constantly plugged into a power source. I encourage you to watch one of the countless videos on DC power and battery cells currently on YouTube to further advance your knowledge.

Till next time! Thanks!!!!