How to design a cost-effective, highly efficient e-scooter contactless solution?

In the last article, we introduced TE03, the contactless charging solution for e-scooters. In this article, we will discuss why TE03 is the ideal solution for e-scooters in more detail.

We need to understand the architecture of the e-scooter charging dock. The figure below shows the difference between contact and contactless charging is the additional contactless power module pair. This addition brings convenience and reliability but increases the cost and energy loss. Therefore, designing a low-cost and highly efficient contactless power module pair is the key to whether contactless charging can replace contact charging.

The contactless power module pair mainly converts electricity and magnetic fields, using the magnetic field to transfer power wirelessly. We list one of the high-frequency solutions and our approach for this wireless power transfer ( abbreviations as WPT hereafter), as shown below. Type-1 is the high-frequency resonant WPT architecture, and Type-2 is Tseetech's loosely-coupled inductive WPT architecture. Type-1 uses fixed magnetic field frequency, so it needs a pre-regulator in front of the power amplifier for magnetic field strength adjustment. It usually applies an out-band method like BLE for Tx/Rx communication. Type-2 uses varied magnetic field frequencies, so no pre-regulator is needed, and the Tx/Rx communication can proceed directly between coils.

First, we need to know that the available magnetic field frequencies for wireless power transfer are limited. According to the regulatory rules published in each country on earth, there are three frequency bands available, those are, 79KHz to 90KHz, 100KHz to 148.5KHz, and 6.78MHz.

For a long time, there have been debates about which frequency band is best for magnetic field coupled wireless power transfer. 6.78MHz has the advantage of less metal heating effect and lower coil inductance to achieve long-distance power transfer, so Type-1 often uses 6.78MHz as the fixed frequency. In the above figure, we show the state-of-the-art efficiency of each function block of Type-1. As a result, the best end-to-end efficiency is around 75%, which matches the performance number of several leading resonant WPT companies.

As for Type-2, Tseetech uses 79KHz to 90KHz as the magnetic frequency band, the same as the worldwide EV wireless charging standard. With Tseetech's expertise in each block, eventually, TE03 achieves near 93% end-to-end efficiency at a maximum.

93% and 75% mean a huge heat difference in the system. For example, to provide 4A charging current to a 48V (13S) battery, the Rx needs to output up to 220W to the load. 93% means the system generates 16W heat, and 75% means the system generates 73W heat. Assuming the heat is separated equally in both the Tx and Rx side, Type-2 has 8W heat to dissipate, and Type-1 has 36W heat to dissipate. Apparently, Type-1 is very challenging for the thermal design, and lots of active cooling methods are necessary, which adds cost and complexity. Instead, the 8W heat generated in Type-2 is easy to handle with a simple passive cooling method, which is more practical for mass deployment.

Some might argue that traditionally it's harder to achieve long-distance power transfer with a lower frequency, with nature's physic. We can use a factor to clarify this question, which is called the diameter-distance factor. The diameter means the Tx and Rx coil diameters, assuming they are equal. The distance means the maximum coil-to-coil power transfer distance where the coil-to-coil efficiency is higher than 90%. For 6.78MHz, this factor is around 0.5 to 1, depending on the coil topology. For 79KHz to 90KHz, this factor is about 0.3 with Tseetech’s technology. The small diameter-distance factor might cause issues like additional weight or no available space for small or light devices like consumer drones. However, for e-scooter applications, the larger and heavier coils caused by the small diameter-distance factor are not a problem. 15mm to 35mm coil-to-coil distance is enough for e-scooter parking dock design, and that coil size and height are acceptable.

In this article, we have compared the high frequency resonant WPT and Tseetech's loosely coupled inductive WPT. We can see the latter is obviously superior to the former in the e-scooter application. And what we didn't mention is due to fewer components needed, the cost of Tseetech solution is also dramatically less than the  high frequency resonant WPT solution.

We look forward to cooperating with operators and solution providers to help you run a profitable business with our technology. Don't hesitate and contact us now!