Black, crimson, gold – these colours point out the charging standing of a battery cell. During charging, lithium ions are deposited in the graphite anode, and the graphite modifications its coloration in line with the charging standing. When discharged, graphite seems to be black; charged midway, it turns crimson; and when totally charged, graphite seems golden. “This lets us see and assess the charging status,” explains Dr. Bernhard Rieger, cell knowledgeable on the Audi battery testing center in Gaimersheim. When electrodes are charged, they don’t accomplish that evenly. An electrode will cost probably the most shortly the place the best variety of lithium ions are current. This outcomes in a charging standing gradient. Ideally, all the things could be the identical coloration. But that might require an especially skinny electrode that might in flip have a neglectable capability. “When it comes to fast charging lithium ion cells, the art lies in precise current control to prevent overcharging the anode in the golden areas. That would result in premature aging,” says Rieger. “This means our task lies in ensuring the ideal operation of lithium-ion cells to achieve the best possible range and charging performance.” For Audi clients, this ensures a battery electrical car with each wonderful range and charging efficiency. “Once the basic need for range has been met, charging capacity becomes more relevant,” provides Rieger.
Balance between excessive vitality density and fast charging instances
The Gaimersheim cell technicians thus face conflicting objectives. “We have a limited package available to us, into which we have to pack as much energy as possible,” feedback Rieger. “But we also want to be able to recharge it as fast as possible.”
The battle lies in the truth that the larger the vitality density, the longer it takes to cost the cell. This means the cell technicians are going through a balancing act that requires them to a minimum of hold charging instances at a relentless whereas rising vitality density. In order to cost a whole lot of vitality shortly, charging capability is an early level on the agenda for mission growth. In this space, a minimum of two Audi fashions are setting requirements as we speak: The cells in the 93 kWh high-voltage battery used in the Audi e-tron GT quattro can cost from 5 to 80 % with as much as 270 kW of energy in solely 22.5 minutes in ultimate circumstances. The Audi e-tron, which first hit the market in 2019, remains to be thought of the benchmark as we speak with its distinctive plateau-like charging curve, the place the charging energy of as much as 150 kW is out there for a big a part of the charging course of.
“We invest a great deal of time in developing ideally controlled cells and current control to achieve as fast a charging time as possible coupled with high efficiency and a long service life,” explains Rieger. So in Gaimersheim, the main target isn’t restricted to battery cells alone: The complete battery system with its electronics, thermal administration system, and high-voltage periphery are equally related for the Audi fast charging idea. “Our systems are designed from day one with fast charging in mind because it is very difficult to increase charging capacity later on. Only a perfectly balanced total package where every detail counts from the earliest phase will reach the desired charging properties.”
From check stands to pre-series manufacturing – all the things beneath one roof
In addition to the standards of vitality density and charging capability, the battery cells additionally meet the best service life and security requirements. To this finish, the person cells and the battery system as a complete must go quite a few assessments on the roughly 4,400-square-meter center in Gaimersheim. Each car mission entails quite a lot of service life and fast charging assessments on a number of hundred cells.The cells must run by completely different charging and load profiles that the Gaimersheim technicians carry out in climatic chambers at temperatures between -30 and +60 °C. “Our tests and checks begin at an early sample phase about four years before the start of production to allow us time to make readjustments where necessary”, explains cell knowledgeable Rieger. In order to evaluate the cells’ getting older phenomena, they’re additionally uncovered to excessive temperatures for the house of a few 12 months. This permits Audi to simulate an in-car service lifetime of as much as 15 years. Service life check stands in Gaimersheim additionally enable the technicians to hurry up reconstructed mileage eventualities of about 300,000 kilometers. Other assessments embrace crash and overcharge testing in addition to varied security assessments. Diverse driving eventualities could be simulated in order to optimize the high-volt parts’ working technique or thermal administration. “This application allows us to perform the best possible fine-tuning on our batteries,” says Rieger. “New findings from cell measurements are rerouted right back to the virtual battery models so we can observe the effects on the vehicle.” Gaimersheim can also be residence to a building facility for prototype batteries; right here, workers construct the high-voltage batteries from the bottom up all the best way to pre-series manufacturing.
“At Gaimersheim, we have the competency to fine-tune our cells’ design and operational strategy to achieve properties that are perfect for Audi,” sums up Rieger. “How can I operate and build a battery to get the most use out of it? That’s a question we answer every day at the Gaimersheim battery testing center.”
