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Photovoltaic Battery Charge Controller

A charge controller is one of the main components of a solar battery charging system (Fig. 1). Its main function is to fully charge a battery without permitting overcharge or reverse current flow (generally during night). If a solar array is connected to a lead-acid battery with no overcharge protection, the battery life is adversely affected.

Fig. 1: Block diagram of a typical solar battery charging system
Fig. 1: Block diagram of a typical solar battery charging system

Ordinary controllers consist of a relay that opens the charging circuit when a preset high-voltage point is reached and closes the circuit again when a preset low-voltage limit is reached, allowing charging to continue. More sophisticated controllers utilise pulse-width modulation (PWM) or maximum power-point tracking (MPPT) to ensure that the battery is being fully charged in the most efficient manner.

Fig. 2: Charging and discharging set points at 25°C
Fig. 2: Charging and discharging set points at 25°C

The charge controller design described here incorporates the following features:
1. Based on PWM technique
2. Battery temperature compensation
3. Battery charge/discharge mode indication (through LED)
4. Reverse-current protection for the PV module
5. Limited protection against lightning and wrong polarity
6. Over-current protection (fuse and automatic load disconnect)
7. Low-voltage load-disconnect indication
8. Load-on indication (through LED)

PV panel selection
A photovoltaic (PV) panel or module comprises a number of PV cells in various series-parallel combinations in order to generate the required power (voltage×current). Typically,a PV cell built using a 9×9cm2 (81cm2) wafer, when subject to sunlight, could produce around 0.5V, which could source around 2.5A of current into a load. Note that a PV cell or module is not damaged when its terminals are shorted. Key specifications of a PV panel are:



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