LM317T Solar Battery Charger Circuit

A baby solar console makes an accomplished array charger for AA and AAA rechargeable batteries. Only a few apparatus are appropriate and architecture is actual simple authoritative this a absolute aboriginal renewable activity project.

Rechargeable AA and AAA batteries accept a voltage of about 1.2 Volts back absolutely charged. Therefore 2 in alternation gives a absolute of 2.4 Volts, 4 in alternation 4.8 Volts. Common solar console achievement voltages are 3 Volts (for archetype the 3V 100ma solar panels in the REUK Shop), and 6 volts – absolute for charging 2 or 4 batteries respectively.

The Limitations of a Basic Solar Charger

Details on authoritative the simplest affectionate of Solar Array Charger are accessible here. Unfortunately this accoutrement has one austere limitation – the solar console has to be able-bodied akin to the batteries to be answerable or the batteries may be overcharged. If you after adjudge to allegation batteries with a altered capacity, you would allegation to change the solar panel.

Current

Current is the best important agency in any array charger. As continued as the voltage of the solar console is greater than the absolute absolutely answerable voltage of the batteries, the batteries will be answerable . If the accepted is too little, the batteries will allegation actual slowly. If the accepted is too aerial the batteries will be answerable too fast, are at accident of actuality overcharged, may overheat, and accept their accessible lifetime reduced. Therefore the abutting development footfall is to accomplish a accepted bound array charger.

Safe Array Charging Current

Since we are still authoritative a simple solar charger it will not automatically about-face off back the batteries are full. Therefore we allegation to accumulate the charging accepted low abundant that it will not accident the batteries alike back they are absolutely charged. A accepted of about 10% of array accommodation gives the appropriate antithesis of charging acceleration and assurance – for example, 2700mah AA rechargeable batteries should be answerable with a accepted of 10% of 2700 = 270ma. To allegation the batteries faster a college accepted could be used, but the adventitious of the batteries actuality overcharged would increase.

Limiting Accepted with an LM317T

The LM317T is a voltage regulator chip. It can additionally be acclimated with a acceptable resistor to adapt current. Abounding capacity on how this works are accessible actuality in our adviser to application the LM317T with LED lighting.

The amount of the resistor appropriate is accustomed by Ohm’s Law as 1.25V disconnected by the achievement accepted required. (The 1.25V is the adapted achievement from the LM317T’s ADJ output.) The college the ascribe voltage and current, the added calefaction will be generated by the LM317T back the achievement accepted is anchored and the added ability has to go somewhere. The LM317T will cope with currents of up to 1.5 Amps and so will accept no botheration at all with baby solar panels.

Choosing the Resistor for the LM317T Accepted Attached Circuit

Resistors are alone accessible in assertive ethics – e.g. 5.6 Ohms and 6.8 Ohms, but not 6.2 Ohms. Below is a table of accessible resistor ethics calm with the achievement accepted generated if anniversary resistor is acclimated in an LM317T accepted attached ambit (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 application the table aloft we can see that to allegation 1000mah AAA rechargeable batteries with a accepted of 100ma, a 12 Ohm resistor would be perfect. A 15 Ohm resistor would abate the accepted and apathetic bottomward charging, a 10 Ohm resistor would access the accepted and acceleration up charging.

Example Solar Array Charger with LM317T

In this archetype we will accomplish a solar charger application a 6 Volt 250ma Solar Panel to allegation four 800mah AAA batteries. The batteries can be put into a brace of 2 x AAA array holders and active in alternation (link the absolute achievement from one array holder to the abrogating of the other).

This gives us 4 x 1.2 = 4.8 Volts with a accommodation of 800mah – accordingly we appetite a charging accepted of about 80ma. According to the table above, a 15 Ohm resistor gives a anchored accepted of 83.3 milliamps which will be perfect.

The ability accident in the resistor is afresh accustomed by Ohm’s Law as I * I * R = 0.0833 * 0.0833 * 15 = 0.10 Watts, accordingly we can cautiously use a accepted 0.25 Watt 15 Ohm resistor.

For testing the ambit photographed beneath was congenital application prototyping breadboard. The ambit is affiliated amid the absolute achievement of the solar console and the one chargeless absolute advance of the array holders.

A agenda multimeter has been acclimated to admeasurement the absolute accepted achievement by the ambit – in this case 84.1ma is a little college than the accepted 83.3ma back resistor ethics are not exact, and the ADJ voltage achievement from the LM317T is not absolutely 1.25 Volts.

Complete Current Limited Solar Battery Charger Circuit:

The abrogating advance from the solar console should be affiliated to the chargeless abrogating ascribe of the array holders. The absolute advance from the solar console is affiliated to the absolute ascribe of the accepted attached circuit, and the achievement from that ambit affiliated to the chargeless absolute advance of the array holders.

You now accept an bigger solar charger which will accelerate a anchored bulk of accepted (sunlight permitting) to the batteries. Charging from collapsed would booty about 10 hours of sunlight in this formed archetype and you do not charge to anguish about overcharging your batteries.

A final accessory would be to absorb a simple array cachet adviser application a Zener diode to ablaze an LED back the array voltage alcove the adapted level.