At present, safety, range, charging and price are the main factors limiting the development of electric vehicles. All four of these factors are batteries related. It is not difficult to see that the power battery occupies an important position in the whole vehicle, and is one of the key components of electric vehicles.
The process of charging and discharging batteries is a chemical reaction, related to temperature. This paper analyses the existing low temperature battery heater and the strategy and finds that there are some problems and risks. After the analysis, the low temperature AC heating strategy is improved, and the charging heating strategy of the new battery heater can effectively solve the existing risks.
1. Power battery thermal management system
In order to solve the failure of the heating mode to the charging heating mode during low temperature of the power batteries. This can cause problems with charging. A heating strategy including battery heater, charging heating and electric mode is proposed to optimize the precharging strategy during mode conversion.
Under low temperature conditions in winter, battery activity deteriorates, and graphite's ability to insert lithium into the cathode material decreases. With high-current charging, lithium precipitation will occur and puncture the diaphragm, affecting charging safety and efficiency.
Therefore, in order to improve the low temperature charging performance of the power battery, it is necessary to use the battery heater to heat the batteries. Once the battery thermal management fails, the damage may occur. There are three common heating methods: electric heating film, PTC heating and liquid heating.
● Electric heating film
The heating film and PTC belong to the resistance heating method, which generally encapsulates the metal heating wire in the insulation layer, and the heat generated by the metal wire can heat the battery heater system after being energized.
● PTC heating
PTC battery heater is also a type of resistance heater. However, its resistance will increase with the increase of its own temperature, so as to achieve the effect of constant temperature heating.
Liquid heat is a method of heating the coolant to a certain temperature through the heating parts of the PTC battery heater of the entire vehicle, and using the active liquid cooling system to heat the battery heater system.
2. The PTC heating design
When the temperature is low, the electrochemical reactivity of batteries are reduced. The low characteristics of power batteries hinder their development in low temperature areas. But some batteries, such as lifepo4 battery also have excellent performance at low temperatures. In order to ensure that the batteries can be charged normally under low temperature conditions, it is necessary to develop a low temperature charging strategy and a suitable battery heater.
In the power supply system, the heating circuit is mainly composed of heating sheets, battery heater and fuse. The voltage detection at both ends of the circuit is an additional part of the subsequent optimization circuit, mainly to determine whether the battery heater is sticky.
The circuit of the battery heater is connected in parallel with the charging and discharging circuit of the power battery, and for the battery pack, the charging circuit and the discharging circuit are not distinguished.
3. Traditional strategy of battery heater
After entering the charging mode, the BMS determines the batteries temperature and enters the charging mode when the minimum temperature of the single battery is greater than or equal to 5°C. When the minimum temperature of the single cell is less than or equal to 0 °C, it enters heating mode.
BMS controls the heating relay to close, requests charging voltage and current, and the requested voltage is 166 volts according to the maximum pack voltage of the power battery pack. The onboard charger responds to the voltage and current requests of the BMS and outputs them at smaller values. When the minimum temperature of the single battery meets 0 °C, it enters the charging heating mode.
The BMS request current is the sum of the allowable charging current of the power battery and the current consumed by the heating plate, and the working voltage of the heating plate should be equal to the battery pack voltage. However, the risk is that in the heating mode, the battery's temperature reaches 1°C after the charging temperature reaches 1°C.
The BMS request voltage is (pack voltage add 5 V), the current is the pack voltage/heating plate resistance, and the onboard charger responds to the BMS voltage and current request and outputs it according to the smaller value. The initial goal of the strategy is to omit the precharge process, but there is a possibility that the precharge will fail.
Causes conversion process to fail during the heating transition to the charging heating mode and the charging stops. The basic reason is that the heating plate increases with the temperature, the resistance increases, and the BMS request current changes with the resistance of the heating plate.
When the request current is compared with the voltage, the charger responds to the current request, the output voltage is small, and the precharge fails. For this risk, the original low temperature heating strategy and battery heater are optimized.
4. New strategy of battery heater
To prevent the battery heater from stopping charging due to precharge failure when switching from heat mode to charging heat mode, the following strategy is optimized. In heat mode, the temperature of the single cell reaches 1 °C, and the BMS disconnects the heating relay. The requested voltage and current are all 0, so that the charger is awake but no output.
The BMS controls the precharge relay to close the precharge process, and this policy does not cause precharge failure due to changes in heater resistance or changes in pack voltage. In addition, the original strategy runs the risk that the output of the power battery will be supplied to the battery heater due to the lack of charger output during charging and heating.
The following protection strategy is added to the new strategy. During the conversion of heat mode and charging heat mode, BMS detects the output of 2A current from the on-board charger before closing the heat relay. At the same time, in order to avoid heat relay failure, increase the heat relay fault judgment strategy, after closing the heat relay, it means that the heat circuit is disconnected. Turn off the charger and perform a power failure.
If the heat circuit is closed normally, enter normal mode and heat the batteries. During the discharge process of the power battery, if the heat relay sticks, it will cause the power battery to continue to supply power to the heating plate. The original strategy did not add how to solve the battery heater temperature reduction during charging and heating, and the protection strategy was added to the optimized strategy.
During the heating process, the plate works at full power, and this shows an upward trend. During the charging and heating process, because the plate is connected in parallel with the power battery, the plate is subjected to the battery heater voltage and cannot work at full power. If the outside temperature is too low, there will be a situation where the heat pad is working, but the batteries temperature drops.
To achieve this strategy, when the temperature of the monomer falls below a certain value, it is re-entered into heat mode. The over-temperature alarm of the heat plate was not mentioned in the original battery heater strategy, and this protection strategy related to car battery voltage was added after optimization. During the heating process, the temperature of the heat plate is greater than 59 °C, the maximum allowable charging voltage is adjusted to 144 volts, and the power is reduced.
After heat plate is less than 40°C, the maximum allowable charging voltage of 166 volts is restored and the required current is 8 A. The temperature of heat plate is greater than 67 °C, the maximum allowable charging voltage is 0, the charging current is 0, and S2 is continuous, keeping the heating relay closed. After the heat plate is less than 40 °C, the maximum allowable charging voltage is 144 volts and the charging current is 8 A.
If heat plate exceeds 70 °C, stop heating. Disconnect the heating relay and power down the fault. If the heating sheet is greater than 59 °C during the heating process while charging, and the temperature is too high for the Level 1 alarm, only the alarm is not processed.
The temperature of heat plate is greater than exceeds 67 °C, the temperature is too high, the 2-level alarm, stop heating, disconnect the heat relay, and only charge. The temperature of the heat plate is greater than 69 ° C (is too high 3 level alarm), disconnect all relays, charge error.
This paper analyzes the heating of traditional low temperature battery heater, points out the risk of precharging failure from heating mode to charging heating mode, and proposes a new low temperature AC charging heating strategy.
The results show that a novel strategy is adopted to disconnect the heating relay when the heating is converted to charging and heating modes. Power on according to charging and power-up processes to avoid pre-charge failures. The new battery heater strategy effectively avoids the risk of mode conversion failure during low-temperature charging, and ensures the charging stability and safety of electric vehicles under extreme belts.