Solar Battery Charger with LM317
 
Solar Battery Charger with LM317


 

Solar Battery Charger with LM317

 


This is a solar panel battery charger schematic for AA and AAA rechargeable batteries. A small solar panel makes an excellent battery charger for AA and AAA rechargeable batteries. Only a few components are required and construction is very simple making this a perfect first renewable energy project. Rechargeable AA and AAA batteries have a voltage of around 1.2 Volts when fully charged. Therefore 2 in series gives a total of 2.4 Volts, 4 in series 4.8 Volts. Common solar panel output voltages are 3 Volts for example the 3V 100ma solar panels, and 6 volts - perfect for charging 2 or 4 batteries respectively. Solar panels should be well adapted to the battery to be charged or the battery may be overcharged. If you want to charge batteries with different capacities, then you need to change the solar panels. Since this is a simple solar battery charger that does not automatically turn off when the battery is full. So we need to maintain the charging current is low enough that will not damage the battery even when they are fully charged. An LM317T voltage regulator chip that can be used with a suitable resistor to regulate current. See solar AA battery charger


Solar Battery Charger with LM317

The Limitations of a Basic Solar Charger
Details on making the simplest kind of Solar Battery Charger are available here. Unfortunately this set-up has one serious limitation - the solar panel has to be well matched to the batteries to be charged or the batteries may be overcharged. If you later decide to charge batteries with a different capacity, you would need to change the solar panel.

Current
Current is the most important factor in any battery charger. As long as the voltage of the solar panel is greater than the total fully charged voltage of the batteries, the batteries will be charged . If the current is too little, the batteries will charge very slowly. If the current is too high the batteries will be charged too fast, are at risk of being overcharged, may overheat, and have their usable lifetime reduced. Therefore the next development step is to make a current limited battery charger.

Safe Battery Charging Current
Since we are still making a simple solar charger it will not automatically turn off when the batteries are full. Therefore we need to keep the charging current low enough that it will not damage the batteries even when they are fully charged. A current of around 10% of battery capacity gives the right balance of charging speed and safety - for example, 2700mah AA rechargeable batteries should be charged with a current of 10% of 2700 = 270ma. To charge the batteries faster a higher current could be used, but the chance of the batteries being overcharged would increase.

Limiting Current with an LM317T
The LM317T is a voltage regulator chip. It can also be used with a suitable resistor to regulate current. Full details on how this works are available here in our guide to using the LM317T with LED lighting.

Current limiting with the LM317T

The value of the resistor required is given by Ohm's Law as 1.25V divided by the output current required. (The 1.25V is the regulated output from the LM317T's ADJ output.) The higher the input voltage and current, the more heat will be generated by the LM317T since the output current is fixed and the extra power has to go somewhere. The LM317T will cope with currents of up to 1.5 Amps and so will have no problem at all with small solar panels.

Choosing the Resistor for the LM317T Current Limiting Circuit
Resistors are only available in certain values - e.g. 5.6 Ohms and 6.8 Ohms, but not 6.2 Ohms. Below is a table of available resistor values together with the output current generated if each resistor is used in an LM317T current limiting circuit (R = resistance, I = current).

R (Ohms) 3.9 4.7 5.6 6.8 8.2 10 12 15 18 22 27 33
I (mA) 321 266 223 184 152 125 104.2 83.3 69.4 56.8 46.3 37.9

Therefore using the table above we can see that to charge 1000mah AAA rechargeable batteries with a current of 100ma, a 12 Ohm resistor would be perfect. A 15 Ohm resistor would reduce the current and slow down charging, a 10 Ohm resistor would increase the current and speed up charging.

Example Solar Battery Charger with LM317T
In this example we will make a solar charger using a 6 Volt 250ma Solar Panel to charge four 800mah AAA batteries. The batteries can be put into a couple of 2 x AAA battery holders and wired in series (link the positive output from one battery holder to the negative of the other).

2 AAA battery holders wired in series

This gives us 4 x 1.2 = 4.8 Volts with a capacity of 800mah - therefore we want a charging current of around 80ma. According to the table above, a 15 Ohm resistor gives a fixed current of 83.3 milliamps which will be perfect.
The power loss in the resistor is again given by Ohm's Law as I * I * R = 0.0833 * 0.0833 * 15 = 0.10 Watts, therefore we can safely use a standard 0.25 Watt 15 Ohm resistor.

For testing the circuit photographed below was built using prototyping breadboard. The circuit is connected between the positive output of the solar panel and the one free positive lead of the battery holders.

Testing the LM317T current limiting circuit

A digital multimeter has been used to measure the actual current output by the circuit - in this case 84.1ma is a little higher than the expected 83.3ma since resistor values are not exact, and the ADJ voltage output from the LM317T is not exactly 1.25 Volts.

Complete Current Limited Solar Battery Charger Circuit

Completed solar battery charger with lm317t current limiting circuit

The negative lead from the solar panel should be connected to the free negative input of the battery holders. The positive lead from the solar panel is connected to the positive input of the current limiting circuit, and the output from that circuit connected to the free positive lead of the battery holders.

You now have an improved solar charger which will send a fixed amount of current (sunlight permitting) to the batteries. Charging from flat would take around 10 hours of sunlight in this worked example and you do not need to worry about overcharging your batteries.

A final enhancement would be to incorporate a simple battery status monitor using a Zener diode to light an LED when the battery voltage reaches the desired level.




Accurate LC Meter Capacitance Inductance Meter with 16F628 and LCD
Volt Ampere Meter with 16F876 Microcontroller and LCD display
 
Accurate LC Meter

Build your own Accurate LC Meter (Capacitance Inductance Meter) and start making your own coils and inductors. This LC Meter allows to measure incredibly small inductances making it perfect tool for making all types of RF coils and inductors. LC Meter can measure inductances starting from 10nH - 1000nH, 1uH - 1000uH, 1mH - 100mH and capacitances from 0.1pF up to 900nF. The circuit includes an auto ranging as well as reset switch and produces very accurate and stable readings.
PIC Volt Ampere Meter

Volt Ampere Meter measures voltage of 0-70V or 0-500V with 100mV resolution and current consumption 0-10A or more with 10mA resolution. The meter is a perfect addition to any power supply, battery chargers and other electronic projects where voltage and current must be monitored. The meter uses PIC16F876A microcontroller with 16x2 backlighted LCD.

50MHz 60MHz Frequency Meter / Counter with 16F628 & LCD
1Hz - 2MHz XR2206 Function Generator
60MHz Frequency Meter / Counter

Frequency Meter / Counter measures frequency from 10Hz to 60MHz with 10Hz resolution. It is a very useful bench test equipment for testing and finding out the frequency of various devices with unknown frequency such as oscillators, radio receivers, transmitters, function generators, crystals, etc.
1Hz - 2MHz XR2206 Function Generator

1Hz - 2MHz XR2206 Function Generator produces high quality sine, square and triangle waveforms of high-stability and accuracy. The output waveforms can be both amplitude and frequency modulated. Output of 1Hz - 2MHz XR2206 Function Generator can be connected directly to 60MHz Counter for setting precise frequency output.

BA1404 HI-FI Stereo FM Transmitter
USB IO Board PIC18F2455 / PIC18F2550
BA1404 HI-FI Stereo FM Transmitter

Be "On Air" with your own radio station! BA1404 HI-FI Stereo FM Transmitter broadcasts high quality stereo signal in 88MHz - 108MHz FM band. It can be connected to any type of stereo audio source such as iPod, Computer, Laptop, CD Player, Walkman, Television, Satellite Receiver, Tape Deck or other stereo system to transmit stereo sound with excellent clarity throughout your home, office, yard or camp ground.
USB IO Board

USB IO Board is a tiny spectacular little development board / parallel port replacement featuring PIC18F2455/PIC18F2550 microcontroller. USB IO Board is compatible with Windows / Mac OSX / Linux computers. When attached to Windows IO board will show up as RS232 COM port. You can control 16 individual microcontroller I/O pins by sending simple serial commands. USB IO Board is self-powered by USB port and can provide up to 500mA for electronic projects. USB IO Board is breadboard compatible.

RF Remote Control 433MHz Four Channel
100m 4-Channel 433MHz Wireless RF Remote Control
 
200m 4-Channel 433MHz Wireless RF Remote Control

Having the ability to control various appliances inside or outside of your house wirelessly is a huge convenience, and can make your life much easier and fun. RF remote control provides long range of up to 200m / 650ft and can find many uses for controlling different devices, and it works even through the walls. You can control lights, fans, AC system, computer, printer, amplifier, robots, garage door, security systems, motor-driven curtains, motorized window blinds, door locks, sprinklers, motorized projection screens and anything else you can think of.
100m 4-Channel 433MHz Wireless RF Remote Control

Four button RF remote is used to turn ON / OFF four different devices independently. Any of the four outputs can be configured to work independently in either toggle or momentary mode. Outputs are buffered by BC549 NPN transistors and can drive low voltage devices directly or be connected to either 5V or 12V relays (or motors) to control appliances that use 110V / 220V mains voltage or any voltage of your choice. Multiple remote systems can be used independently to control more than four appliances in the same location by changing the address code on 433MHz receiver and remote. It is also possible to use several remotes to control the same appliance such as garage door.
 

Electronics-DIY.com © 2002-2014. All Rights Reserved.