AN2946 datasheet(23/38 Pages) STMICROELECTRONICS

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AN2946

Firmware design

Doc ID 15473 Rev 2

23/38

Once the battery is deeply discharged, and the voltage falls below 'UBATth2', the MCU stops

driving the LED lamp immediately. For battery life cycle considerations, the battery cannot

be discharged until it is fully charged. DD_Flag is set when UBAT < UBATth2 and is reset

when UBAT ≥ UBATth3.

4.2

Battery charging management

4.2.1

MPPT principle

The objective of the MPPT algorithm is to get the maximum battery charging power. Taking

into account the charger efficiency, this is the most efficient way to utilize solar energy. In

Figure 28 there exists a maximum power point for each curve.

Figure 28.

80 W solar module I-V and P-V curve

In actual conditions, the I-V curve and P-V curve of the solar module change with different

irradiance and temperature which means the charging current cannot remain constant even

when the charging voltage is fixed. This kind of state causes the MPPT algorithm to be

adaptive and dynamic. The solar-LED streetlight controller adopts one of the MPPTs, i.e.

P&O (perturbation and observation) method.

Figure 29.

P and O method

Step 1: Suppose the charger is working at point A, the duty cycle is DC(A). At the next

step, the MCU increases the duty cycle to DC(B). The charger then moves to point B at

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