Accupack

18.1.2016 | author Adam Podhrázský, Jan Kosina, František Pech | translated by Mark Gurevich

There is a black box located behind the pilot’s back that, honestly, doesn’t attract much attention. Well, that is actually a prime mover of the whole one-seater. Here is stored all the energy it needs to function. However, the influence on dynamics is not only in providing the needed amount of energy to motor controller, from where it is distributed to motors, but also in quite considerable size and weight of acupack, substantially affecting free installation space and weight distribution. Just like majority of other components of one-seater, this one is designed according to Formula Student competitions rules. So, an innovation of battery pack became a major challenge for us.

Previous open-wheelers’ battery packs consisted of lithium-polymer (Li-Pol) prismatic cells. Last two open-wheelers: FSE.02 and FSE.03, had the same battery pack with a total energy density per unit mass of 103 Wh/kg or 138 Wh/l for energy density per unit volume. With a total weight of 85 kg that corresponds to 8,8 kWh. Designing the new battery for FSE.04x we aimed to increase the energy density and reduce the weight of the pack. That kind of a change surely required a completely new approach to the research.

First thing to do was the choosing of battery cells that would be better than the previous in terms of energy density. After a thorough research we decided to go for 18650-type cylindrical Li-Ion NCA secondary cells, featuring energy density parameter of 211 Wh/kg and a maximum current load of 11C. These cells are considered top notch by the energy density per unit mass parameter, which made the choice a no-brainer.

Next, in order to limit the energy density drop, cells had to be placed as compactly as possible. For that task, we used market available plastic holders, able to connect with each other in any order, allowing us to design larger structures, which then would carry battery cells. Thus, six similar, so-called, “stacks”, were created.

The energy is divided evenly between each other of them, as required by the competion rules. Each stack consists of cells, connected with each other by thin nickel strips in series-parallel circuit, using spot welding technology of Omnitron company. The ends were welded on thicker copper bars to create positive and negative poles of the stack. Due to the competition rules, then they were covered by the non-flammable composite material (FR4 fiberglass), which eventually ensured stacks’ integrity. After that, poles of every stack were connected in series by copper bars and equipped by safety nuts and washers, designed with help of the Department of Machining, Process Planning and Metrology using a 3D printing technology. 

Each and every stack is equipped by a circuit board called BMS (battery management system), that monitors temperature of stack and voltage of every parallel group precise to the nearest tenth of a millivolt. Then it transmits the data to its control unit - ECU-A. Which, in its turn, communicates with other units of the car, ensures that the car’s control board and acumulator’s cooling system stay powered, as well as it is responsible for the safety state evaluation with an option of traction power emergency shutdown. These units, just as all other, were designed and brought to life by our team members.

Last (but not the least) stage was the making of a strong, robust, however lightweight case to carry the stacks and all control electronics. Hence, the choice fell upon carbon composite, which was used to laminate shaped sandwich panels. With help of stiffness analysis the case was designed so that it could withstand the load of stacks of 40g lengthwise and diagonally and 20g in vertical direction. Thus it would endure a possible crash of the car. 

As a result, we've got before us an accumulator pack with a weight of 45 kg, which is 48% less than the previous model. Regarding of the weight of one-seater, capacity was decreased to 7,4 kWh, eventually featuring energy density per unit mass of 164 Wh/kg and energy density per unit volume of 183 Wh/l. Taking a look on the competitors' versions, it’s hard not to admit - this is a success, and an achievement one should try really hard to beat.