Lithium iron phosphate battery charging curve drawing method

Abstract

The invention discloses a lithium iron phosphate battery charging curve drawing method, which comprises the following steps: carrying out a charging and discharging test on a lithium iron phosphate battery, recording the charging and discharging time of the lithium iron phosphate battery under the condition of the same charging and discharging current, and calculating the capacity of the lithium iron phosphate battery. According to the characteristics of the lithium iron phosphate battery, the current of the constant-current main charging stage is determined, and meanwhile, the time for converting the constant-current pre-charging stage into the constant-current main charging stage is calculated according to the charging voltage U1 of the constant-current pre-charging, the measured charging voltage U2 of the constant-current main charging, the capacity Q of the lithium iron phosphate battery and the current of the constant-current pre-charging stage. And the time Tpre of the stage of converting the constant current pre-charging to the constant current main charging can be obtained without adopting a complex algorithm, so that the production cost is reduced.

Claims

1. A method for drawing a charging curve of a lithium iron phosphate battery, characterized in that: It includes the following steps: S1. Use the host computer to electrically connect the charger to the lithium iron phosphate battery; S2. The first charging stage: The host computer controls the charger to output a current I1 to perform constant current charging on the lithium iron phosphate battery. The host computer detects the output voltage U1 when the charger outputs a current I1, and the host computer detects the current of the lithium iron phosphate battery during the charging process as I2. When I2 < i, the host computer determines that the lithium iron phosphate battery is fully charged and controls the charger to stop charging; where i is the current threshold during the charging stage of the lithium iron phosphate battery; S3. The first discharging stage: The host computer controls the charger to discharge the lithium iron phosphate battery in the fully charged state in step S1 with a current I1 and feedback it to the power grid. The host computer detects that when the discharging current I1 of the lithium iron phosphate battery is 0, the discharging time of the lithium iron phosphate battery is t1, and the lithium iron phosphate battery completes discharging. Calculate the capacity Q1 of the lithium iron phosphate battery, and the capacity Q1 of the lithium iron phosphate battery = I1t1; S4. The second charging stage: The host computer controls the charger to charge the lithium iron phosphate battery in step S3 again with a current I1. The host computer detects that when I2 < i, the charging time of the lithium iron phosphate battery is t2, determines that the lithium iron phosphate battery is fully charged, and calculates the capacity Q2 of the lithium iron phosphate battery. The capacity Q2 of the lithium iron phosphate battery = I1t2; S5. The host computer calculates the average value Q of Q1 and Q2, that is, Q = (Q1 + Q2) / 2 = (I1t1 + I1t2) / 2, where Q is the capacity of the lithium iron phosphate battery; S6. The second discharging stage: The host computer controls the charger to discharge the lithium iron phosphate battery in step S4 again with I1 and feedback it to the power grid. The host computer detects that when the discharging current I1 of the lithium iron phosphate battery is 0, it represents that the power of the lithium iron phosphate battery is exhausted; S7. The host computer controls the charger to perform constant current pre-charging on the lithium iron phosphate battery in step S6. The current during the constant current pre-charging stage is I1, and the voltage at the initial moment when the lithium iron phosphate battery enters the constant current pre-charging stage is U1; when the host computer detects that the voltage of the lithium iron phosphate battery rises from U1 to U2, the host computer controls the charging of the lithium iron phosphate battery to change from the constant current pre-charging stage to the constant current main charging stage. The current during the constant current main charging stage is I, I = C*Q, C is the charging rate of the lithium iron phosphate battery, and U2 is the trickle charging voltage threshold of the lithium iron phosphate battery; S8. When the lithium iron phosphate battery is in the constant current main charging stage, the host computer detects the voltage change of the lithium iron phosphate battery. When the host computer detects that the voltage of the lithium iron phosphate battery gradually rises and reaches U3 and remains constant at time t3, the host computer controls the charging of the lithium iron phosphate battery to enter the constant voltage charging stage, and the host computer controls the output current of the charger to gradually decrease from I to 0, and the constant voltage charging stage of the lithium iron phosphate battery ends; S9. Draw a current curve based on the capacity Q of the lithium iron phosphate battery calculated in step S5, the current I1 and voltages U1 and U2 in the constant current pre-charging stage determined in step S7, the current I in the constant current main charging stage, t3 detected in step S8, and the current change during the process of the charger's output current decreasing from I to 0; draw a voltage curve based on the capacity Q of the lithium iron phosphate battery calculated in step S5, the voltages U1 and U2 in step S7, the voltage U3 in step S8, and time t3.

2. The method for drawing a charging curve of a lithium iron phosphate battery according to claim 1, characterized in that: In steps S3 and S6, the method of discharging the lithium iron phosphate battery and feeding it back to the grid is: using an inverter method to feed back the lithium iron phosphate battery electric energy to the grid.

3. The method for drawing a charging curve of a lithium iron phosphate battery according to claim 1, characterized in that: The current curve drawing in step S9 includes: Calculate the time required for the voltage U1 to rise to U2: Tpre=Q(U2-U1) / I1; Draw a current curve of the constant current pre-charging stage of the lithium iron phosphate battery according to the current I1 and Tpre of the constant current pre-charging stage determined in step S7; Draw a current curve of the constant current main charging stage of the lithium iron phosphate battery according to the current I of the constant current main charging stage determined in step S7 and the time t3 detected in step S8; A current curve of the constant voltage charging stage is drawn according to the current change during the process in which the output current of the charger decreases from I to 0 in step S8.

4. The method for drawing a charging curve of a lithium iron phosphate battery according to claim 3, characterized in that: The voltage curve drawing in step S9 includes: Draw a voltage curve of the lithium iron phosphate battery in the constant current pre-charging stage according to the voltage change of the lithium iron phosphate battery during the process of rising from U1 to U2 and Tpre in step S7; According to the voltage change process of the lithium iron phosphate battery gradually rising from U2 in step S8 and reaching U3 at time t3, a voltage curve of the lithium iron phosphate battery in the constant current main charging stage is drawn; A voltage curve of the lithium iron phosphate battery in the constant voltage charging stage is drawn according to the voltage U3 in step S8.

5. The method for drawing a charging curve of a lithium iron phosphate battery according to claim 1, characterized in that: The charger comprises a control unit (1) and a charging unit (2), wherein the control unit (1) is electrically connected to a host computer; the charging unit (2) is electrically connected to the control unit (1); when the lithium iron phosphate battery is charging, the charging unit (2) is connected to the power grid and the lithium iron phosphate battery, the host computer controls the control unit (1) of the charger, the control unit (1) controls the AC current of the power grid to flow into the charging unit (2), the charging unit (2) rectifies the AC power of the power grid into DC power, and the DC current flows to the lithium iron phosphate battery to charge the lithium iron phosphate battery; when the lithium iron phosphate battery is discharging, the charging unit is connected to the power grid and the lithium iron phosphate battery, the current of the lithium iron phosphate battery flows into the charging unit (2), the host computer controls the control unit (1), the control unit (1) controls the charging unit (2) to invert the DC power into AC power, and the AC power is fed back to the power grid.

6. The method for drawing a charging curve of a lithium iron phosphate battery according to claim 4, characterized in that: The charging unit (2) includes a first H-bridge part (21), an LLC part (22) and a second H-bridge part (23), wherein the first H-bridge part (21), the LLC part (22) and the second H-bridge part (23) respectively have a terminal 1, a terminal 2 and a terminal 3, wherein the terminal 1 of the first H-bridge part (21) is electrically connected to the power grid, the terminal 2 of the first H-bridge part (21) is electrically connected to the terminal 1 of the LLC part (22), the terminal 2 of the LLC part (22) is electrically connected to the terminal 1 of the second H-bridge part (23), the terminal 2 of the second H-bridge part (23) is electrically connected to the lithium iron phosphate battery (3), and the terminals 3 of the first H-bridge part (21), the LLC part (22) and the second H-bridge part (23) are respectively electrically connected to the control unit (1); When charging the lithium iron phosphate battery, the control unit (1) controls the first H-bridge unit (21), the LLC unit (22) and the second H-bridge unit (23) to operate, and the AC power of the power grid is rectified and filtered by the first H-bridge unit (21) to be converted into DC power, and then inverted into AC power by the LLC unit (22) and stepped down. The stepped-down AC power is rectified into DC power by the second H-bridge unit (23) to charge the lithium iron phosphate battery (3); When the lithium iron phosphate battery is discharged, the control unit (1) controls the first H-bridge unit (21), the LLC unit (22) and the second H-bridge unit (23) to operate. The direct current output by the lithium iron phosphate battery is inverted into alternating current by the second H-bridge unit (23), then enters the LLC unit (22) to be boosted into high-voltage direct current for phase adjustment, and finally inverted and fed back to the power grid by the first H-bridge unit (21).

7. The method for drawing a charging curve of a lithium iron phosphate battery according to claim 5, characterized in that: The first H-bridge part (21) comprises a first filtering module (211), an energy storage module (212) and a first H-bridge (213); the first filtering module (211) is electrically connected to the power grid and the energy storage module (212), respectively; the energy storage module (212) is electrically connected to the first H-bridge (213), and the first H-bridge (213) is electrically connected to terminal 1 of the LLC part (22) and the control unit (1), respectively; When the lithium iron phosphate battery is charged, the alternating current is filtered and noise is eliminated by the first filtering module (211) and the energy storage module (212), and the control unit (1) controls the first H-bridge (213) to rectify the alternating current, and after rectifying the alternating current into direct current, the direct current is input into the LLC unit (22) to be inverted into alternating current and step down the voltage; When the lithium iron phosphate battery is discharged, the high-voltage direct current output by the LLC unit (22) is boosted and passes through the first H-bridge (213). The control unit (1) controls the first H-bridge (213) to invert the high-voltage direct current into square-wave alternating current, and then smoothes the square-wave alternating current into sinusoidal alternating current through the first filtering module (211) and the energy storage module (212) to feed back to the power grid, thereby reducing the interference of harmonics on the power grid.

8. The method for drawing a charging curve of a lithium iron phosphate battery according to claim 5, characterized in that: The LLC part (22) includes a second filter module (221), a conversion H-bridge (222) and a transformer module (223), the second filter module (221) is electrically connected to the No. 2 terminal of the first H-bridge part (21) and the conversion H-bridge (222), the conversion H-bridge (222) is electrically connected to the transformer module (223) and the control unit (1), and the transformer module (223) is electrically connected to the No. 1 terminal of the second H-bridge part (23); When charging the lithium iron phosphate battery, the alternating current of the power grid is rectified and filtered by the first H-bridge unit (21) to be converted into direct current, and then the noise is eliminated by the second filtering module (221). The control unit (1) controls the conversion H-bridge (222) to invert the direct current into alternating current and uses the transformer module (223) to reduce the voltage. The alternating current after the voltage reduction is rectified into direct current by the second H-bridge unit (23) to charge the lithium iron phosphate battery (3). When the lithium iron phosphate battery is discharged, the direct current output by the lithium iron phosphate battery is inverted into alternating current through the second H-bridge unit (23), and then enters the transformer module (223) for voltage boosting. The control unit (1) controls the H-bridge (222) to rectify the alternating current into high-voltage direct current, which is then phase-adjusted through the second filter module (221). The high-voltage direct current is finally inverted and fed back to the power grid through the first H-bridge unit (21).

9. The method for drawing a charging curve of a lithium iron phosphate battery according to claim 7, characterized in that: The transformer module (223) comprises a resonant cavity (2231), a transformer Tr (2232) and a third filtering module (2233); the resonant cavity (2231) is electrically connected to the conversion H-bridge (222) and the transformer Tr (2232), respectively; the transformer Tr (2232) is electrically connected to the third filtering module (2233) and the second H-bridge portion (23), respectively. When charging the lithium iron phosphate battery, the control unit (1) controls the conversion H-bridge (222) to invert direct current into alternating current, and generates a resonant current through the action of the resonant cavity (2231). The circuit thus operates at a resonant frequency, thereby improving the efficiency of the transformer module (223) in reducing the voltage. The reduced alternating current is filtered by the third filtering module (2233) and then enters the second H-bridge part (23) to be rectified into direct current to charge the lithium iron phosphate battery (3). When the lithium iron phosphate battery is discharged, the direct current output by the lithium iron phosphate battery is converted into alternating current by the second H-bridge unit (23), and then enters the third filter module (2233) to filter out noise. The transformer module (223) boosts the alternating current, and the boosted alternating current passes through the resonant cavity (2231) to eliminate harmonics of the alternating current, and then enters the conversion H-bridge (222) for inversion.

10. The method for drawing a charging curve of a lithium iron phosphate battery according to claim 5, characterized in that: The second H-bridge portion (23) comprises a second H-bridge (231), a fourth filtering module (232) and a protection module (233); the second H-bridge (231) is electrically connected to the LLC portion (22), the fourth filtering module (232), the protection module (233) and the control unit (1); the fourth filtering module (232) is electrically connected to the lithium iron phosphate battery (3) and the protection module (233); and the protection module (233) is electrically connected to the second H-bridge (231) and the fourth filtering module (232); When charging the lithium iron phosphate battery, the LLC unit (22) outputs a stepped-down alternating current, which is rectified into direct current by the second H-bridge (231) and then filtered by the fourth filtering module (232) to charge the lithium iron phosphate battery (3). The protection module (233) performs a current limiting function to prevent the output direct current from being too large and causing damage to the lithium iron phosphate battery (3). When the lithium iron phosphate battery is discharged, the direct current output by the lithium iron phosphate battery is subjected to noise elimination by the fourth filtering module (232), the direct current is inverted into alternating current by the second H-bridge (231), and then enters the LLC unit (22) to be boosted into high-voltage direct current and phase-adjusted. The protection module (233) prevents the direct current from damaging the second H-bridge (231).

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