How can we confirm the voltage level from the Solar panel ? A 12 Volt Solar panel generates around 20 V DC Open circuit voltage in bright sunlight. The voltage gradually increases from 6 am and peaks in the mid noon between 11 am and 3 pm. Then the voltage from the Solar panel drops to zero around 6 p.m. So we get sufficient voltage for charging the battery for 4-5 hours only in a day. The 12 V battery requires more than 14 V for proper charging. Its current should be above 500 mA. The current rating of the Solar panel is represented in terms of Watts. As a general rule, for each watt, the current will be 80 mA. So a 10 watt Solar panel gives 800 mA current. This depends on the availability of sufficient voltage from the Solar panel. Many factors like corrosion of contacts, loose contacts in connections, dust on the panel etc will affect the current flow from the panel. So how can we monitor the voltage level from the Solar panel. A volt meter can do but we can make a simple LED Meter for this. Its cost is less than Rs.80. It has three LEDs. The Green LED lights, if the voltage from the Solar panel is above 13 V. Yellow LED lights if the voltage is between 9V and 11 V and Red LED lights, if the voltage is below 9V. Remember, the Solar panel is efficient only when the Green LED lights between 11 am and 3 pm.
You need the following components
1. IC LM 3915
2. 1K Resistor
3. 470 Ohms resistor
4. 1M Preset
5. 1 uF 25V Electrolytic capacitor
6. Green LED
7. Yellow LED
8. Red LED
LM 3915 is the popular Monolithic display IC that gives the LED indications in a Logarithmic fashion. It has 10 outputs which are “current sinking”. That means, the LED connected to its output lights only when the output turns off. So at power on, all the outputs remains high. When its input gets 125 mV as signal, the first output (Pin 18) turns low and the LED connected to it lights. So in this way, for each increment of 125 mV, the outputs from 18 sinks current in a descending order. Since the input (Pin 5) of IC requires only very low input voltage, a Preset or Pot is a must to control the voltage. The LEDs do not needs current limiting resistors, since the IC itself control the LED current though the resistor connected to its pin 7.
Assemble the circuit on a common PCB. Fix it near the battery to which the panel is connected. See image, the Diode is a must to prevent the backward flow of Battery voltage to the Solar panel as well as the circuit. So the LED lights only when the Solar panel outputs voltage. Connect the LED Meter to the outputs of Solar panel. Place the panel in bright Sunlight. Adjust Preset till Green LED lights. When the voltage drops below 13V, Green LED turns off and the Yellow LED lights. When the voltage drops below 11 V, Yellow LED turns off and Red LED lights.
To get more info about LM 3915, LM3914 and LM 3916 IC, see the link.