Solar Battery Charger With LM317T
Solar Battery Charger With LM317T
Find out how to make a current controlled solar battery charger using an LM317Tsolar | education | electric circuit
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
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
in the REUK Shop
), and 6 volts - perfect for charging 2 or 4 batteries respectively.
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.
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
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
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
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
|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).
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.
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
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.
Article Last Modified: 09:34, 30th Mar 2007
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