Is there a way to extend battery life, speed up charging, and increase safety? Yes: modular multilevel batteries – in other words, a battery pack with a modular multilevel converter (MMC)!
According to Dr. Manuel Kuder, creative mind and co-founder of Pulsetrain, modular multilevel batteries (batteries with an MMC) offer exactly these possibilities. If you look at today’s electric vehicles, they are typically equipped with an on-board charger (OBC), a battery junction box (a switching and distribution unit within the high-voltage system of an electric vehicle), a battery management system (BMS), a converter/motor controller with power electronics, and a battery pack. »With the multilevel inverter, all components can be eliminated except for the battery pack,« explains Kuder. Instead, each battery cell is equipped with power electronics consisting of four switches. These switches allow the battery cells to be dynamically connected and bridged if a cell is not working, so that this failure no longer results in a vehicle failure. Kuder: »This is the core feature of such novel battery packs: the probability of failure is significantly lower.« A MMC basically consists of power semiconductors (low-voltage MOSFETs) for the desired voltages and currents, and a master that coordinates the switching operations. »This combination allows the required voltage, capacitance, and power to be individually scaled,« says Dr. Kuder.
MMCs offer a wide range of benefits
»By turning cells on and off with software, we can create a perfect sine wave to power the engine, making it run more efficiently,« Kuder continues.
Another major advantage of modular multi-level batteries and dynamic connection of individual cells is that they equalize aging and extend service life by up to 60 to 80 percent. In addition to compensating for differences between cells, pulse charging plays a particularly important role. According to Kuder, initial indications suggest that pulse currents can influence or even partially reverse electrochemical processes such as plating. In addition, up to three times faster charging with less heating has been observed at a Tier 1 supplier. Kuder continues: »To further investigate these effects, battery cyclizers are being developed that will allow pulse charging to be explored for different cell chemistries. The era of pulse charging is dawning – with the potential for more lifetime, charging speed, and safety in e-mobility.«
He believes that battery manufacturers can also benefit from the MMC approach: »Even with a well-run battery production, there is still a 20 percent scrap rate,« explains Kuder. This high reject rate is due to the Gaussian distribution within a production. This means that a few »very good«, mostly »good«, and a few »bad« battery cells are produced during production. Today, however, all batteries in a system must be equally good; on average, they must have the same characteristics. Kuder: »But that’s not necessary at all, because we can turn battery cells on and off individually, so the weakest link in the chain is no longer the limiting factor in the chain of large series connections of battery cells.« Kuder points out another advantage: »One-third of the production costs of batteries are due to formation and aging.« During formation, new cells are charged for the first time and then stored for two to three months to see if a battery cell still has the right parameters. »With the MMC, this is no longer necessary, because even in this case, if a battery cell does not work, we bypass it. In this way, battery manufacturers can reduce their production costs by a third,« Kuder continues. And by eliminating some of the cost of manufacturing the battery, the battery itself can become cheaper. And since the battery is still the most expensive part of an electric vehicle, this should also have a positive effect on the cost of the vehicle.
“Electromobility 1.0 was the attempt to replace diesel with electricity. Electromobility 2.0 is the moment, when we finally stop improving stupid systems from the past and focus on real innovation.“
Battery Module with Pulsetrain Electronics. Source: Pulsetrain
But the MMC solves even more problems. As is well known, battery cells are not all the same, either in terms of manufacturing or in terms of operation – accordingly, battery cells typically have differences in capacity and internal resistance. And it is these variations that pose a problem, because in a series connection, as explained above, the cell with the lowest capacity limits the overall performance of the entire battery pack. If a cell has less capacity, it will reach the state of charge or discharge more quickly, resulting in an uneven load. This can cause the weaker cells to be overcharged or overdischarged, shortening their life, and compromising the safety of the entire system. This problem is solved in modern battery systems by balancing. There are two approaches: active balancing, which deliberately transfers energy from a more fully charged battery cell to a less fully charged cell; and passive balancing, which dissipates excess energy in the form of heat via resistors. Due to complexity and cost, the automotive industry has favored passive balancing. The modular multilevel converter (MMC) approach allows proactive balancing by selectively charging and discharging individual battery cells.
In addition, the MMC and the cell’s pulse response can be used to accurately determine the state of the battery cell. »The previous BMS provided estimated values, but our approach is much simpler. And if a cell is too full or too empty, we just turn it on and off.«
The Pulsetrain system also allows different battery chemistries to be mixed. »This has been a problem in the past, but in the future the multilevel converter will make it possible to combine sodium and lithium, for example, because we can set an individual profile for each battery cell,« continues Kuder.
Source: Pulsetrain
The MMC also eliminates the high-voltage problem. Kuder: »Today, repair shops need employees who have completed high-voltage training, otherwise they are not allowed to repair electric vehicles. With our MMC, this problem is eliminated because we work with 4 V; anything below 60 V is safe,« Kuder continues. This is also important in the event of an electric vehicle fire, because not every firefighter has high-voltage training. Kuder: »There are no regulations or standards for this yet, but in cooperation with the car companies and their lobby, the problem should be solved quickly. The system cannot emit dangerous voltages when it is switched off.«
Controlling each individual cell also has the effect of slowing down thermal runaway of the entire battery pack. If one cell starts to burn, the adjacent cell and the next cell can be discharged as quickly as possible. According to Kuder, tests have shown that the MMC can extend the thermal runaway time by 80 seconds. And every second counts!
Kuder also explains that Pulsetrain uses simple low-voltage silicon-based MOSFETs for the power electronics. While there is a lot of discussion about SiC or GaN, and their advantages are undeniable, »high frequencies are not important for our MMCs,« which obviously has a positive effect on cost. And these lower frequencies also have a positive effect on EMC behavior.
Automotive understands »slowly but surely«
Despite its advantages, Pulsetrain’s MMC has not yet been mass-produced for the automotive market. However, the company is currently working with automotive customers to develop prototypes. Kuder is also convinced that the first multilevel vehicles will soon be coming to Europe from Asia. The reason why development has not progressed further is that the industrialization of MMCs for the automotive industry costs a lot of money and has not yet been completed for all of them. On the other hand, the automotive industry typically tends to wait and see when it comes to completely new technical approaches. Modular multilevel battery systems also have one disadvantage: they lack the classic 400 V or 800 V DC intermediate circuit. Kuder: »In applications that absolutely require such a voltage – for example, in multi-engine vehicle architectures – the system can be used as a highly integrated BMS. All the benefits in terms of safety, lifetime, and cell management are retained, even if the charging and motor control is done conventionally.« However, in many markets, such as construction equipment, motorcycles or boats, this DC decoupling is not necessary. »This is where the system really comes into its own – with maximum integration, safety, and efficiency,« continues Kuder. Accordingly, construction equipment manufacturers are more open to the new technology, and Pulsetrain has already been able to sell its MMCs in this area, according to Kuder.
Others are more open to the technology: Pulsetrain successfully closed a seed financing round of €6.1 million. The funds will be used to further develop the technology and to scale and industrialize the product. Vsquared Ventures, Planet A, and Climate Club participated in the financing round.
