Deciphering Charging Dynamics: Part 1
Why Does It Take So Long To Charge My
Battery?

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Vineet Dravid

Founder & CEO oorja

Deciphering Charging Dynamics: Part 1
Why Does It Take So Long To Charge My Battery?

Introduction

Li-ion batteries are ubiquitous. Everything is powered by them, from our phones and watches to our vacuum cleaners and power tools to our cars and buses. Therefore, understanding the different aspects of charging these batteries becomes crucial.

In this blog series, we will discuss the different aspects to consider while charging your batteries. We will cover various topics, from understanding the bottlenecks during charging to what fast charging is and why it is so challenging. We will also include the safety aspects of charging. For instance, did you know that the EV FireSafe database shows about 18% of fires occurred when vehicles were charging and 2% within an hour of disconnecting from the charger? But that’s a discussion for another day.

Today, we will talk about the basics of charging. How do I decode battery terminology, and how long does it take a battery to charge? Batteries come with a labelled C rating. For example, a C rating of 5 Ah means discharging the battery at 5A would completely drain it in 1 hour, and discharging it at 10A would drain it in half an hour. The same holds for charging the battery as well. Theoretically, this is how fast you can completely charge your batteries if you pump in your currents:

C Rating Time
10C
6 minutes
5 C
12 minutes
2C
30 minutes
1C
1 hour
0.8C
1 hour 15 minutes
0.5C/ C/2
2 hours
0.2C/ C/5
5 hours
0.05C / C/20
20 hours

It sounds simple, right? The more current you pump in, the sooner you can charge. Not really. The challenge is that as you start pumping in more current, it damages the battery, puts it at a higher risk of explosions, and reduces its life.

So, what’s the best way to charge the battery? We will take this up in later blogs. Today, let s take the case of low C rate charging. Any charging C rate < 1C can be loosely defined as low C rate charging and is generally a much safer regime, though caveats still apply.

However, when I charge at 0.5C, will the battery charge in 2 hours? Not really. This is because a battery also has a rated voltage: the maximum and minimum voltage (Vmax and Vmin, see Figure 1) window within which a battery works. For a regular NMC cell, this is usually between 4.2 V and 2.6 V. As you keep charging, the cell eventually reaches Vmax. Beyond this, you cannot keep charging at the same C rate, as this will damage the cell. You now move to a lower current while maintaining the voltage at Vmax, increasing the charging time.

We have run some analyses to explain how long it takes to charge when you charge at different C rates:

Fig. 1 shows the CCCV charging profiles for the LGM50 NMC 4.61Ah cell at different C-rates. Ideally, we expect charging time to decrease with increasing C-rate, which is true, but at higher C-rates, the CV stage becomes longer as the SOC achieved at higher C-rate is less than that at lower C-rate, as shown in Fig. 2. One of the key inferences is that the actual time for the charge to complete is more than what is calculated

theoretically.

You can see that 1C charging takes much more than 1 hour and so on. Note that this is still a theoretical estimate since, in real life, your device maker will never give you access to the entire SOC window to protect the cell from abuse. Yet another blog topic!

In addition to time, another aspect to consider is the heat rise in temperature. Cells are extremely sensitive to temperature. Here is the temperature increase in cells as you keep charging them, as described above.

You will notice that above 0.5C, you can reach temperatures as high as 70℃. This can be extremely dangerous for your packs, especially since charging vehicles are stationary and may not be effectively cooled.

This is another aspect that must be considered when devising a charging strategy, even at low C rates.

To conclude, we hope you better understand how long it takes to charge batteries. In the subsequent blogs, we will discuss the challenges with fast charging of batteries and different strategies for battery charging, protecting batteries from electrochemical, thermal, and electrical abuse while charging.

To explore how you can use oorja to optimise fast charging strategies, contact us at info@oorja.energy

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