Simple Solar light circuit version II using Li-ion battery

Previously, we had built the Automatic Solar Light circuit and had been using it for many months. It has fair efficiency for everyday lighting. But this circuit has two problems: a dim light level and batteries that are hard to find and quite expensive.

But now my daughter has built this Simple solar light circuit version II. It has a better performance than the first one.

Solar light circuit

  • It got far brighter because she used two white SMD LEDs.
  • Use a 3.7V Li-ion battery instead, which is widely used. Making it easier to find than the 1.2V Ni-MH battery.
  • And the circuit is also simpler.

And of course, we also use recycled components. Thus saving us money and helping us learn more about using them.

simple solar bright led lighting
Comparing the new simpler and brighter circuit to the old one.

How it works

Now let’s look at the block diagram of this circuit. It will help us visualize the circuit we would need.

First, let’s say it’s daytime.

daytime-circuit-operation block diagram

An electrical current from the solar cell charges the battery, and some current also goes to the control, turning the LEDs off.

Simple Solar Li-ion battery charger circuit

This is the simplest Solar Li-ion battery circuit, consisting of only three components:

Simple Solar Li-ion battery charger circuit
Simple Solar Li-ion battery charger circuit
  1. Free 3.7V Li-ion Battery

Nowadays, we prefer to use Li-ion batteries over other types of batteries because they have higher efficiency. It supplies a voltage of around 3.7V (up to 4.2V). Similar to a lead-acid battery, it doesn’t need to run out of power before recharging; it also maintains power for a long time, among other advantages. Which we will learn about more later.

BL-5C, 3.7V Li-ion Battery in cell phones
The BL-5C, 3.7V Li-ion Battery in cell phones

This type of battery can usually be found in old mobile phones and other similar devices. They come in 3.7V 800mAh to 1000mAh. Alternatively, you can get a BL-5C for about $1, which is cheaper than a 1.2V 950mA Ni-MH battery.

It has similar properties to lead-acid batteries in that when the power is full, there will be a voltage of about 4V. While charging, be careful not to let the voltage exceed 4.2V and should charge with a low current.

Recommended: Recycle Free Li-ion battery from E-waste

6V 1W Solar cell

Another important component of this circuit is the solar cell panel, which should be capable of supplying a voltage of about 5V to 6V with a size of 1W to 2W. It will supply a current of about 100mA.

When exposed to sunlight for about 5 to 7 hours, it should have charged the battery to 80% or more. It would not exceed the voltage rating of the 4.2V battery, complying with battery specifications.

This low current does not make the battery heat up at all. We have tried this method many times and in many circuits. It has worked well and is suitable for small and economical circuits.

6V 1.2W Solar Cell
We use a 6V 1.2W Solar Cell

Assume we use an LED (load) that consumes about 60mA of current, but the battery has about 600mA of current; it can provide light for about 10 hours, almost the entire night.

Read Also: Simple Li-ion Battery circuit with automatic cut-off

  1. 1N5819 Diode

We only use a single diode to prevent reverse current from flowing from the battery to the solar cell.

In the circuit above, the current from the solar cell flows through D1 to charge the Li-ion battery. When there is less sunlight, the higher voltage from the battery cannot flow back to the solar cell.

Because there is a D1 blocking it, the current can flow only one way. The energy in the battery is stored and gradually increases until it is full.


In contrast, at night or without sunlight, there is no power from the solar cell, as shown in the block diagram below.

At night circuit operation block diagram.
At night circuit operation block diagram.

So, there is no power from the charger as well. The control detects this state. It then switches to driving the power from the battery to light up the LED instead.

Free Super bright LED

We are going to use this super bright LED we got from recycling a white SMD LED from the broken T8 tube. It is very bright; for two LEDs, it uses only 60mA of current. We connected them together in parallel and connected the current limiter resistor for each LED.

super bright white smd LED
Two white SMD LEDs from e-waste.

When used with the 800mAh Li-ion battery, it can remain on for more than 10 hours, or almost the entire night.

Turning it into circuit diagram

Next, we have to come up with the circuit according to the block diagram above.

During the day

Current charging circuits at daytime
The Current flowing and charging circuits during daytime

(1) The solar cell receives sunlight, generating electricity to charge the battery through D1.

(2) At the same time, some current will flow through R1 as a biased current to the base of Q1. It causes Q1 to conduct current or turn on.

(3) The main current flows through D1, R2, and the collector-emitter of Q1 to the negative. Thus, there is no biased current to the base of Q2, turning off Q2 and the LEDs as well.

During the Night

Nighttime battery current drive LED light up
At night, the battery current will power the LEDs, causing them to light up.

Now there is no power from the solar cell at all. So, there is no current flowing through R1, and Q1 also turns off.

(1) The battery is fully charged, so the current flows back out. But it cannot flow through R1 because it is blocked by D1, which is in a reversed bias state.

Forcing the current to flow through R2 instead, as a biased current to the base of Q2.

(2) and (3) are in play after the Q2 is turned on and drives a main current through both LEDs. So, LED1 and LED2 light up.

simple solar LED light circuit using 3.7V li-ion battery
simple solar LED light circuit using a 3.7V li-ion battery

Next, we take each part and assemble them together into a complete circuit.

Read more: Simple Circuits

How to build

My daughter built this project on a solderable breadboard PCB and a wooden board because it’s simple and economical.

Solderable breadboard PCB layout of solar light
Solderable breadboard PCB layout of solar light
Mounting PCB and battery-on plywood
Mounting PCB and battery-on plywood


After we have created this second version of the Simple Solar light circuit, we know that it is more efficient and easy to build than the previous one. Because the SMD LED is very bright and the 3.7V Li-ion battery provides higher power with a smaller size.

In the future, we will try to build a larger circuit, and we hope it will have more use for you.

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