Higher charging speeds for lithium batteries generally lead to accelerated ageing. Pulse charging promises to achieve a long service life despite very high charging speeds. Whether this promise is realistic and how the start-up BAVERTIS wants to put this process into practice is presented in this article.
A Tesla Model 3 can recharge up to 275 km in 15 minutes at the fast charging station. A BMW i4 charges up to 164 km in 10 minutes. The Porsche Taycan charges to 80 % in 22 minutes. Fast charging is nothing new for electric cars. But fast charging doesn’t just have advantages: Of course, it helps enormously on long journeys, but the service life is considerably reduced by frequent fast charging with a high constant charging current. But can’t it be even faster? Maybe! Pulse charging is an alternative charging method designed to enable faster charging without ageing the battery prematurely.
How are batteries charged today?
The most commonly used charging method today is CCCV charging. CCCV stands for Constant Charging – Constant Voltage. The charging process is divided into two parts. In the first section, a constant current is charged until the upper voltage limit is reached. In the second section, the voltage is then kept constant and the current is reduced. Towards the end of the charging cycle, less and less power is transferred. This is why charging the last 20% of the charging process takes considerably longer. However, the reduction in current is necessary to prevent the battery from overcharging. Depending on how quickly the battery is charged, the CV section is reached sooner or later. With fast charging, the charging current is already very high at the start and the CV phase is reached earlier.
In practice, however, it is rarely possible to charge ideally with CCCV. Instead, other operating conditions such as the cell temperature must also be taken into account. The BMS (Battery Management System) is responsible for this. It monitors the charging process of each individual cell and communicates with the charger which charging currents are currently possible.
Anyone interested in what a charging process looks like in practice: A few years ago, Nextmove carried out measurements for a Tesla Model 3 SR+ on an IONITY fast charger and an 11 kW wallbox (CCCV). The CCCV charging curve is clearly visible here, especially for the 11 kW charge.
What happens during pulse charging?
In pulse charging, short rectangular current pulses with a very high amplitude are applied to the cell. There is a pause between the pulses. The length of the pauses and pulses then depends on the exact application, but is often <1s. In the figure on the left, a longer pulse duration has been selected for better visualization.
In order for the battery to be charged just as fast (or faster) with pulse charging than with conventional CCCV charging, the pulse current must be significantly higher. For example, instead of 10 A with CCCV, the battery is charged with 20 A pulse.
Why do you need pulse charging?
The current charging process is not well suited to charging a battery quickly. High constant charging currents, as occur with CCCV charging, lead to premature ageing of the cell.
To understand why this is the case, you have to delve deep into battery chemistry. What happens on a chemical level when charging a battery has already been analyzed here.
To summarize, CCCV fast charging results in a temporary shortage of Li-ions at the anode because the Li-ions penetrate the anode very quickly, but new ions from the cathode are slow to follow. The reduced concentration of lithium ions then leads to local overvoltages. This results in decomposition reactions of the solvent. The decomposition products are deposited as unwanted deposits on the surface of the active material (thickening of the SEI layer) and increase the internal resistance of the cell . Lithium plating is also favored. This is an undesirable side reaction in which lithium ions are deposited on the top of the anode and remain there permanently. This reduces the amount of lithium available and therefore also the capacity of the cell.
With pulse charging, the regular pauses between the pulses ensure that the Li-ion depletion layers can be reduced and local concentration gradients are equalized. This prevents overvoltage and reduces parasitic reactions.
It is even possible to create a discharge pulse instead of a pause. This is intended to correct the concentration gradients even faster. In addition, it should even be possible to partially reverse lithium plating so that a longer service life of the cell is achieved.
What are the challenges of pulse charging?
Pulse charging is currently still at an early stage of development. Research into this has only just begun. The following challenges still need to be solved:
- The few research results available to date are not consistent While some studies have shown a significant improvement in fast-charging properties, other studies have not identified any benefits.
- Studies on pulse charging are very complex: Pulse charging requires chargers that can apply high-frequency charging pulses. Extremely high currents are also required. However, many simple test devices do not have this function. The cells also have to be charged and discharged several thousand times to test their ageing behavior. This is very expensive and time-consuming
- Pulse charging uses pulses with a significantly higher current than CCCV charging methods. However, many cell manufacturers have not approved their cells for these high currents. The cells therefore have to be re-qualified at great expense, which extends the development cycle.
Due to Ohm’s law, there is more self-heating. The stronger the pulse, the higher it is.
How companies want to implement pulse charging (feat. BAVERTIS)
The number of companies working on the commercialization of pulse charging is currently still quite small. One of these companies is the Munich-based start-up BAVERTIS.
BAVERTIS is taking a new approach to building battery modules. Instead of fixed battery modules with e.g. 14 cells in series and 8 cells in parallel, each cell is connected via two separate MOSFET transistors so that each cell can be switched individually. During operation, the cells are then connected together in real time as they are actually needed.
This circuit concept also makes it possible to charge the cells via pulse charging. A complex charging infrastructure is then not necessary with this concept, as everything is controlled on-board. Bavertis currently assumes that this charging method, combined with an individual load profile for each cell, will increase the service life of the cells by 80%.
The concept envisages the creation of a digital twin of each cell, which is simulated online in a cloud. The exact charging/discharging profile of the individual battery cells is then calculated. There are a few degrees of freedom here, such as the frequency, on and off time, etc.
The company already validated its concept in a test vehicle 2.5 years ago. The 10 kW system works with up to 96 V. Bavertis is currently investigating which pulse frequencies can achieve the best service life.
Initially, the concept is to be used for special-purpose vehicles, later the system will be used for aircraft and then also for the automotive industry. However, it is not expected to be used in the automotive sector before 2032 due to the long product cycles.
Thanks to BAVERTIS and Dr. Manuel Kuder as well as Dr. Katarina Cicvaric for their support in creating this article.
Further information about BAVERTIS:
- Youtube: https://bitly.cx/XXAu2
- Website: https://bavertis.com/
