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83% of India’s vehicles are two-wheelers and three-wheelers (including small goods vehicles) and 13% are economy cars, costing less than Rs. 1 million, thus constituting over 95% of India’s vehicles. These vehicles tend to be small and affordable. As these vehicles are electrified, the affordability requirements will keep the batteries small, charging at lower voltage and lower power. Consequently, the chargers would need to have low output power and voltages and the potential costs would have to be in tune with this affordability. The premium cars are only 2%. They would use higher power / higher voltages and higher cost chargers. The chargers should be deployed in public space, in proportion[1] to the number of electric vehicles in each category. Further, the standardisation of the chargers and charging infrastructure needs a balance between these 95% economy vehicles and the 2% premium vehicles. In this blog, we will look at the nature of the batteries that different category of Indian vehicles would have and the charging standards dilemma.
Battery-sizes
The Indian two-wheelers will have batteries in between 1 kWh and 3 kWh, constrained both by the space available and affordability. Three-wheelers would have batteries from 2.5 kWh to about 8 kWh. Once again, they will be constrained by costs and space available. Electric rickshaws and small autos will make do with 2.5 kWh to 3 kWh batteries, whereas the seven to eight-seater large autos and three-wheeler goods vehicles may have batteries in between 5 kWh to 8 kWh. As batteries with its cooling systems would cost[2] close to Rs. 22,500 per kWh, a 15 kWh battery would cost about Rs. 340,000. Thus, economy cars costing close to Rs. 1 million may limit its battery size to be 15 kWh, whereas the cars costing about Rs. 0.5 million are unlikely to have batteries of more than 12 kWh. Thus, the affordable segment of vehicles, constituting 95% of the Indian vehicles would have battery size in between 1 kWh to 15 kWh. Of-course the premium cars are not cost-constrained, and their battery size would begin with 30 kWh and could go up to 75 kWh or even more.
Battery Voltages
World-over, two-wheelers and three-wheelers[3] tend to use 48V or 72 V. On the other hand medium to large sized cars today invariably use close to 350V (or 380V). Still larger sized vehicles like buses use 750 Volts. What are the reasons for these varying voltages? Can all vehicles not use single voltage, say 350V? Let us answer this by looking first at 48V usage.
48V is the most popular voltage used for most two/ three wheelers. Voltage below 60V is considered to be basically Safe-voltage[4], whereas voltages above 60V is considered to be unsafe. For vehicles, where battery voltage may exceed 60V, would need a complete isolation between all electric systems and anything that passenger / drives are likely to touch and certified thus. A 48V battery, when fully charged may be 56V to 58V. Thus any battery above 48V nominal would require isolation. Small vehicles thus use 48V battery. This is not so only for two-wheelers, three-wheelers and small four wheelers, but even the larger sized mild-hybrid cars with 5 kW to 10 kW battery uses 48V.
The second reason is that it is easy to build a 1 kWh battery at 48V and practically impossible to build this at 350V. A 1 kWh battery made at 350V will require will require about 100 cells with less than 3 Ah rating, connected in series. Such low energy cells cannot be pouch cells or prismatic cells, commonly used in EVs. The only option would be cylindrical cells, which are known to have lower life-cycles. Same will be the case for battery-sizes of up to 5 kWh. Only when battery size exceeds 10 kWh, 350V could be a viable option.
Thus the smaller vehicles avoid electrical isolation by using 48V batteries. The low-cost batteries do not normally use more than 1C charging, as discussed later in this blog, and use no more than 200 Ampere current, thus enabling a 10 kWh battery. This could be stretched to 15 kWh battery with 300 Ampere charging. Sometimes, small vehicles use 72V and 200 AH modules (cells in parallel) allowing one to have 15 kWh batteries. While 72V is not inherently safe voltage and isolation is indeed required, much of the commonly used electronic circuits at 48V could be used for 72V. As the battery pack size increases, higher voltage is desirable. 350V to 380V is widely used for batteries larger than 25 kWh and up to about 100 kWh. 750V or thereabout is chosen for larger battery.
Is DC001 really the Chinese GBT Charger?
Some multinational companies and some experts in Delhi have often spoken that DC001 specified by Department of Heavy Industry in 2017 is not an Indian standard at all, but it is the Chinese GBT Charger. How true is this? First of all DC001 is a Fast Charger for small vehicles (small and medium sized-cars, tempos and large autos), having a battery of size between 5 kWh and 15 kWh. As discussed in this blog, these are part of 95% of Indian economy-vehicles. Drive-train for these vehicles is at 48V or 72V. The Chinese GBT charger (ISO reference) is not defined for 48V or 72V, but defined for voltages between 200V to 750V? How foolish is that DC001 is same as Chinese charger?
Secondly, what is Chinese about DC001? What are we importing from China? What royalty are we paying to China? Are the chargers being manufactured in China? Are they over-priced and not affordable?
The communication protocol used in DC001 is indeed based on GBT (as stated in the standard), but has extra commands and parameters. The software is fully written and owned in India. There is no royalty payable for anyone outside India. The charger itself is made by several companies in India (including some start-ups) and costs about ?150,000 in volumes, which make setting up of such chargers economically viable. The connector used is indeed GBT connector costing less than $100 and is currently imported (but then are not many components for vehicles imported). Further there are companies who are ready to manufacture the connector in India, as soon as requirements scale. DC001 is a unique fast charger, especially created in India for its small and affordable electric vehicles. The tirade against DC001 is actually against electrification of these vehicles.
Current constraints in batteries
Batteries can be charged or discharged at certain maximum currents, depending upon the kind of cells the Ah rating of modules connected in series. The charging rate of a Li Ion cell is defined as C-rate of cells. Charging or discharging at 1 C implies that a cell can be charged from totally discharged state to fully charged state in 1 hour. 2C rate means charging / discharging at twice the 1C-rate and therefore charging from totally discharged state to fully-charged state in half an hour. Similarly, a 0.5C rate means charging / discharging at half the 1C-rate and therefore charging from totally discharged state to fully-charged state in two hours. Now most cells get the best life when charged / discharged at around 0.5C and at ambient temperature of 25°C. Higher rate charge / discharge and higher temperature charging / discharging bring down the life-cycles for which battery can be used. Most cells tend to therefore limit charging rate to 1C or 1.5C. The extent of deterioration of life-cycle, however, depends upon the cells. There are more expensive cells, where charging rate can be 2C, 3C or even 8C.
Thus economy vehicles with battery sizes of up to 15 kWh, would limit the charging rate to 15 kW even while fast-charging. The fully discharged battery would therefore take one hour to fully charged. But since battery is usually used with a Depth of Discharge (DoD) of nearly 80% to have maximum life, this 80% charging can be done at 1C in 48 minutes. Vehicles rarely come to a charging station when they are fully empty; thus the vehicle, when it comes to the charger, in a nearly discharged state could be charged nearly full in 40 to 45 minutes. DC001 is Fast Charger[5], defined at 48V and 72V with a maximum charge rate of 15 kW, to cater to these economy vehicles.
The premium vehicles, on the other hand, may use slightly more expensive batteries. They may charge at 1.5C and could therefore be nearly fully charged (considering DoD and SoC when it comes to charger) in nearly 30 minutes. Thus a vehicle with 40 kWh battery could be fully charged (considering DoD) in 30 minutes using a 60 kW charger.
Thus, low to medium cost battery today dictates that they are not charged above 1C rate most of the time[6] to preserve battery life. The economy vehicles are likely to use low-cost batteries, which could be fully charged (considering certain DoD and the SoC when they start charging) in about 45 minutes at 1C charging rate. The fast chargers for these vehicles need a maximum charge rate of 15 kW. The premium vehicles may use slightly batter batteries, which could be charged at 1.5C and therefore near-full charge in about 30 minutes. The chargers could now have up to 75 kW output. One should however never forget that the batteries get their best life when they are charged slowly, in about 3 hours to 6 hours. 230V 3 kW AC slow charging would be ideal for this.
Excerpted from http://electric-vehicles-in-india.blogspot.in