In my previous blog post I talked about the failure that happened during the dynamometer test. It was concluded that a short-circuit in the high-voltage compartment of the bike caused the motor to block. This short-circuit only lasted for a moment, but it was quickly noticed that something was damaged in the battery pack as well.
The battery pack consists of a few hundred Lithium Polymer (LiPo) cells connected in series in order to establish a DC voltage of 700 volt. This type of cells can be dangerous if not operated correctly. It has the largest energy density and could ignite by itself when charged to a too high voltage, charged from a too low voltage, drawing too much current or heated up too much. When the dyno-failure happened, it was observed that one single cell converted a lot of its energy into heat. Its voltage was gone and temperature was way up compared to the average of the battery pack. We think this cell was already weak and the short-circuit was too much to handle.
A stack of 27 cells. Finding out which cells are broken (less than 3 volts) and which are not (around 4 volts)
This broken cell cannot be used anymore, as charging it again would be dangerous and the cell itself was probably damaged because of the intense heat generation. The battery pack has to be disassembled to find and remove this cell. Due to the high voltage but also the fragility of the LiPo cells, this task must be executed with great alertness and care. Accidentally dropping a screw in the pack or touching the wrong pieces of metal could impose a health risk, to put it mildly.
Enough with the scaremongering, because tinkering on the battery pack is also fun, if done in a safe way. You get to wear rubber high-voltage gloves that can withstand 1000 volt. The tools we use are also isolated and we stand on an insulating mat. The LiPo cells remain on a moveable cart so in case of fire the cart can be removed quickly. Because the cells can not be “turned off” intrinsically in any way, the first step is to detach all wiring between “stacks” of cells. This reduces the maximum voltage in the pack to at most 40 volts, the maximum voltage of a stack, so it falls within the limits of a safe low voltage.
All safety measures pictured. High voltage gloves, safety cart, high voltage stick to pull away someone that is getting an electric shock, insulating mat, fire extinguisher and most important: never work alone.
Localizing the broken cells is then just a matter of measuring each stack voltage. Luckily, there was only one stack that gave a reading that was 4 volt lower than expected. We were taught that bad lithium-based cells will get soft and swollen when they get bad. (If the battery of your phone does this, replace it right away!) We could clearly see and feel this phenomenon on the broken cell and so it will be removed.
Nicolaas (left) and me (right) are both Electrical Engineering students working in the powertrain department of Nova Electric Racing.
In prevention of future cell failures, all cells will be tested for strength. A weak cell drops a lot in voltage when drawing power from it, a strong cell only a little. As replacing these cells now requires the cells to be desoldered and therefore heated, it might do more damage than good to the overall battery pack. We have encountered some other safety risks as well of this battery pack. This led to the decision to start in December with the design of a new battery pack, taking into account all our acquired knowledge and points of improvements learned from the previous design.