The BYD battery self heating system in the Denza Z9 GT means that even in cold conditions it just adds 3 minutes to the charge time. Not quite as the quote is from 20% SOC in -30°C conditions and not the 10% SOC starting point when the battery is warm.
In the BYD European patent [2] they describe the system:
| A battery self-heating device, comprising: a bridge arm converter (3), an energy storage element (5), an inductor (4) and a controller. The controller is configured to control, in a preset state, the connection and disconnection of the bridge arm converter (3), such that a first power battery (1) and a second power battery (2) are respectively charged/discharged by means of the inductor (4), and each form a freewheeling circuit by means of the energy storage element (5), so as to realize the continuous heating of the first power battery (1) and the second power battery (2). Further disclosed are a self-heating method executed by the device, and a vehicle having the device. Two power batteries are provided, and an energy storage element (5) is provided for freewheeling, such that an oscillation current can be generated between each of the two power batteries and an inductor (4), so as to realize low-cost and high-efficient battery self-heating, and the stability of the total voltage of the two power batteries and the continuous flow of current in the batteries can also be ensured, thereby ensuring the safety of the batteries during the self-heating process. |
As BYD point out the batteries 1 and 2 could just be a tapping point in one battery series string. The inductance can be the electric motor and the switches the inverter. We haven’t looked at the resonant frequency of a typical system such as this, but the suggestion is the AC heating is operating between 100Hz and 500Hz.
The vehiclethermal.com [4] post states: The frequency is generally between 10Hz and 1kHz. 1kHz is the frequency range for testing the AC internal resistance of the battery. In this range, it is generally believed that no electrochemical reaction has occurred, which is harmful to the battery. Core performance is not affected. However, the internal resistance of high-frequency charging and discharging batteries is small, and the heating effect is poor; theoretically, the lower the frequency, the greater the internal resistance of the battery core, and the better the heating effect. However, pulse charging at low frequency is prone to side effects such as material particle fragmentation and lithium precipitation, so it cannot be too low.
A variation on this is proposed in the paper by Cui et al [1].
The battery pack consists of four modules; each module is divided into AB batteries. The designed circuit is connected to both ends of batteries A and B to facilitate quick preheating of the AB batteries. Battery A consists of n + 1 cells (a) connected in series, while battery B consists of n cells (b) connected in series. The number of batteries in battery A is greater than the number of batteries in battery B. The difference in battery count allows for the formation of a potential difference within the battery module.
The operation of this is: Consequently, the direction and magnitude of the current can be accurately adjusted, realizing voltage elevation conversion. High-frequency AC is applied to alternately charge or discharge within the battery modules, effectively dividing the battery into two parts without the need for additional power supplies or components. The battery A, the inductor L1, and the MOSFET tube S1 are turned on, and S2 is turned off, forming loop 1. And the battery B, inductor L1, and MOSFET tube S2 are turned on, and S1 is turned off to form loop 2.
Cui et al [1] build a model and correlate that to experimental data.
The initial 10°C rise in cell temperature takes just 100s.
The most important aspect though of this AC heating method is that the temperature rise is uniform across the cell. A very important aspect if you are going to fast charge the battery pack.
This post has been built based on the support of and sponsorship from: illumion, Custom Power, Quarto Technical Services, Eatron Technologies and About:Energy
If you have more details or battery temperature vs time data from a pack with this system in operation we would really like to see what other parameters we could infer.
References
- Cui W, Li Y, Ma ZY, Nie JX, Liu YC. High-Frequency AC Heating Strategy of Electric Vehicle Power Battery Pack in Low-Temperature Environment. ACS Omega. 2024 Mar 11;9(11):12753-12767.
- BATTERY SELF-HEATING DEVICE AND METHOD, AND VEHICLE,
EP4517933A1, EUROPEAN PATENT APPLICATION - Extreme Conditions BYD 2nd Generation Blade Battery and FLASH Charging Technology Testing, YouTube
- battery temperature too low solution-byd self-heating technology, vehiclethermal.com