EV Charging 102

Fast and slow charging

It is a universal fact that Fast and Slow are relative in nature. Charging of batteries is generally a slower process than refilling a fuel tank, which can be done in a few minutes. Inevitably, this is also a disadvantage for an electric vehicle owner. Hence, it may appear that the solution lies in creating a faster charging process. But it is not so easy.

At heart of every electric vehicle is a battery, which is a complex device where electrical energy is stored as chemical energy. Battery can be made from a myriad of chemical compositions- Lead acid, Lithium Iron Phosphate (LFP), Lithium Nickel Manganese Cobalt (NMC), Lithium Titanium Oxide (LTO) etc. The material of the battery inherently determines how much energy it can store. It is also decides how slow or fast the battery can be charged. For example, in case of a lead acid battery, then the charging process of the battery is very slow. It might take 8-10 hours to recharge any lead acid battery.

On the other hand, Lithium batteries can be charged relatively faster. Even within the family of Lithium batteries there are different chemical compositions, and some of them can be charged faster than others. As an example the LTO can be charged at faster rates in comparison with LFP. That is, the charging time of any battery is very much dependent on the intrinsic characteristics of the material.

Battery Capacity

How 'big' the battery is, i.e. the capacity of the battery, is the next thing of importance here. The capacity of a battery is generally expressed in terms of energy it can discharge in kilo-Watt-hours (kWh). In my opinion, a better way of expressing battery capacity is in terms of Ampere-hour (Ah) with the corresponding Voltage (V). In fact, representing battery in V and Ah makes better sense to plan its charging. As an example, its easier to make out that 72 V, 100 Ah (7.2 kWh) battery needs a charger with an output voltage around 72V. On the other hand if the same battery pack was a 48 V, 150 Ah (7.2 kWh again) the charger should be of 48 V. As a matter of fact, charging a low voltage battery with a high voltage charger can be harmful. In summary, charger is always customised according to the battery.

Before exploring the nuances of the battery charging, it is necessary to see how big EV batteries are. As you would expect the size of a battery is not the same for an electric car and an electric bus. When we were researching for "Charging India's Bus Transport" , we found that electric buses have huge batteries ranging from 100-350 kWh. The battery of a typical electric bus at 200 kWh is 5 times that of Hyundai Kona, the newest entrant in the Indian market. Additionally, two different electric vehicles might not have the same battery capacity, for example the Hyundai Kona has a battery of ~40 kWh, where as the battery of the Mahindra e-Verito or Tata e-Tigor is less than half of that. Tesla cars have battery as huge as 80 kWh. Translating it into charging time, it seems that the bigger battery will take a larger time to charge than the smaller one. What is missing out here is the "power" at which the battery is charged.

Charging Power

Power is simply the rate at which the energy is transferred, which is expressed in kilo-Watt (kW). It has to be looked from two perspectives, power the battery can accept and the power the electricity grid can supply. Between the two we already saw that battery is the king, and the acceptance limit of the battery is the most important when it comes to charging.

Say, there is a 200 kWh electric bus battery, that is made from Lithium Titanium Oxide (LTO) battery. LTO is a battery that can be discharged and charged at very fast rates. Hence let's look at it from what power needed from the grid then. The following is the approximation on the power needed to charge this battery.

• To charge a 200 kWh battery in 1 hour, we might need a power of 200 kW
• To charge a 200 kWh battery in 2 hours, we might need only half the power i.e. 100 kW
• To charge a 200 kWh battery in 0.5 hour, we might need double the power i.e. 400 kW

In reality, a battery is never charged from 0-100% as it is never discharged or charged fully. This is because batteries are expensive and it is important to preserve the health of the battery. Every battery has a maximum possible depth below which it is not discharged. Secondly, every mobile phone user knows that it takes a long time to charge from 90-100%. Hence, the power needed to fully charge would be lower. However, for the sake of simplicity a full charge is assumed here. A deep dive into the State of Charge will be attempted later.

The C-rate

We have discussed that it is vital to see if the battery can actually accept this power. An easy way to pinpoint this is using the 'C-rate' of the battery.The C rate of a battery is a technical term which can describe the speed of charge or discharge of a battery of any capacity. It should be noted that C rate is actually defined for discharging of the battery, and that the charging and discharging characteristics of the battery is not the same. However, it is a good proxy as any to explain charging, and the maximum possible C-rate for a battery is decided by the battery chemistry. A shortcut to understanding C-rate is

• 1 C means charged in an hour
• 2C means charged in (1/2) of an hour
• 0.5C means charged in 2 hours

The C-rate is applicable for any capacity of the battery. That is 1C for 200 kWh implies 200 kW charging power, and 1C for 40 kWh becomes 40 kW. Similarly 2C for 200 kWh is 400 kW and for 40 kWh is 80 kW. As you would have already figured out, the way to estimate this is by dividing the battery capacity (in kWh) by power (in kW). But, let's dig a bit deeper.

Power = Voltage x Current

Most of us already know that, electric power is expressed as a product of voltage and current, i.e. P= V* I. The same power can be delivered by a high current, low voltage system (100=2V*50A) or a low current, high voltage system(50V*2A). But what matters when deciding power is what the battery voltage is. It is not possible to deviate much from the inherent voltage of the battery when charging. What I am trying say is that current is the hero in the charging game, who decides slow and fast. Voltage of charging is an intrinsic function of the battery chemistry and arrangement.

We can try to work out the charging power needed for DC charging in an hour. First we focus on voltage of the battery pack. For example the first generation electric cars in India, Mahindra e2O, eVerito and Tata eTigor has battery packs of voltage 72V. Hence the DC chargers needed for them is also around 72V. Now lets see how to charge this battery.

• A 15 kWh battery pack at 72V, is ~208 Ah.
• To charge at 1C rate, (i.e. to charge in an hour) we need around 200 A
• We need to put the voltage a little bit more than 72V, say 75V to charge
• The power needed is 75V* 200A = 15 kW

The charger designed for these cars are under a standard called Bharat DC 001. You might have seen these chargers in the news.

Let's do the same exercise for Hyundai Kona, which has a battery of ~40 kWh. The battery pack voltage is 327V, higher than that of the first generation cars and hence the chargers for this car has to be atleast around 330V. That is these cars cannot be charged using the 72V Bharat DC 001 chargers. The charger power estimation to charge in an hour is below.

• A 40 kWh battery pack at 327V, is ~120 Ah.
• To charge at 1C rate we need 120 A current
• Say we charge the car at 330V
• The power needed is 330*120= 40 kW

Hence the chargers needed for these cars are distinct, from the voltage and current perspective. As you can see, the current needed here to charge 120 A, is lower than the current of Bharat DC 001. If truth be told, if we were to supply 200 A at 330 V (66 kW) it would be possible to charge the car in 35 minutes. That's provided the battery can accept the 1.6 C rate. This cars follows the CCS protocol for charging. In the next article we will talk more about the CCS and CHAdeMO chargers.

Let's not forget AC

Moving on to AC charging, imagine two AC chargers with input 230V, 6A (1.2 kW) and 230V, 16 A (3.3 kW). It is easy to make out that the second one is three times faster than the first. We have seen in "EV charging 101" that in case of AC charging the charger is on-board the EV. The capacity of that charger will determine the maximum power the electric vehicle can be charged with. In simple words if the charger is only 1 kW (230V, 6A), even if the wall socket outlet is of 3.3 kW (230V, 15A) the charging will be at 1 kW.

Typically AC chargers are designed to charge the battery at very slow rates ranging from 2 to 8 hours. In terms of C rate this will translate to 0.125 C to 0.5 C. To full charge a 15 kWh battery at 0.25 C the charger needed is around 3.75 kW.

Another type of charger you might have observed in news lately is a Bharat AC charger . The Bharat AC charger has three 3.3 kW outputs, which allows three electric vehicles to charge at the same time. The on-board charger on electric vehicles are generally so small and suited to the power that a domestic plug points can deliver. Now you understand why the Bharat AC 001 charger has a 3.3 kW output. Remember that the same can be achieved from a 230V, 16 A plug outlet at home.

If we try to charge 15 kWh battery at this power, it is going to take around 5 hours for full charge. At the same time if we try to charge the 40 kW Hyundai Kona at this power, it is going to take 12 hours for a full charge.That is the AC chargers are useful to charge a car when you have a lot of time.

Obviously, it does not make any sense to charge a 200 kWh bus battery at 3.3 kW power, because it is going to take 2.5 days for the charging. Read more about bus charging options in our report "Charging India's Bus Transport", and let me know if you have any queries/ suggestions/ feedback in the comments section.

So then what is fast and slow?

You have seen that the game of fast and slow is not that easy. The three key take aways are

• There is limit on the speed of charging depending the battery chemistry
• The bigger the battery, the more the charging time and power needed
• The faster you want to charge, the more the charging power needed

Still it is possible to attempt to create frameworks that can help understand what is fast and slow. Using C rates we will try to define a baseline for Fast Charge for Lithium Ion batteries. Remember that 1C corresponds to one hour, we can call it a base line. Lets say any charging done under 1 hour is fast, i.e C rate of 1C and above is Fast. Anything below 1C is slow. Next time when someone says fast charging you can quickly demonstrate your expertise. These are the steps to do that.

• Find out the battery capacity and charger power
• Run a quick division in your head to find the C rate
• Use 1C as the baseline and see if it is Fast or Slow

Voila, now you have mastered the art of Fast and Slow charging. To be one step ahead, you can look into the battery chemistry and find out the maximum C rates possible for charging that chemistry. For instance the LTO can go up to 6C charging rate, which means it can be full charged in ten minutes.

The importance of thermal management

On a concluding thought, one should keep in mind that fast is not always great, because it has implications on the health and life of battery. Additionally, it is important to remember that heat is generated during battery charging and it is a potential fire hazard, if not managed. This is important in the case of India where summer temperature can soar above 45 degree Celsius. In electric cars the air conditioning comes on when the car is charging, and this helps in cooling the battery. On the other hand, for three wheelers and two wheelers this possibility does not arise. Hence the Fast charging rates associated with these vehicles are typically lower than the electric cars. Eventually, thermal management will determine how fast you can charge a battery. Let's conclude that sometimes a slow pace in life and battery charging is also great.

Link to other articles in this series

Link to Part 1: EV Charging 101

Link to Part 3: EV charging 103

Link to Part 4: EV Charging 104

Link to Part 5: EV charging 105

Link to Part 6: EV charging 106?

Note: I am an electrical and electronics engineer and not an expert in EV or batteries. However, my current area of research is EV charging, and my autodidactic tendencies are lately on an overdrive with my focused reading. The article is a summary of my notes from multiple sources. A fellow EV enthusiast may find it useful, though caution is advised. Additionally, this article is part of process for improving my writing skills, please feel free to point out any grammatical errors you may find.