Simple 1.2V AA Ni-MH Battery Solar Charger circuits

This is a simple 1.2V AA battery Solar charger circuit. Imagine, if you want to charge only one or two 1.2V AA Ni-MH batteries, and must be charged outdoor without home electricity. What could we do?

If what you have are plain components. Is it possible to use those components? to create a circuit according to this problem.

The simplest and cheapest version

This project is the easiest and cheapest because we use a few components: a solar cell 3V 160mA, and a diode.
Then, I measure a solar’s voltage as around 3.3V as below. It is enough for charging a AA battery.

And, I have an AA rechargeable battery, a “Nickel-metal Hydride” type at 1.2V 700mAh. Going forward it will be called the AA Ni-MH battery.

In addition, we need to have a diode. To prevent a reverse voltage from a battery to the solar cell when there is no sunlight.

We use a number 1N4007 diode because it is a high max voltage of 1,000V 1A more than enough in this case. But it is worth more than the lower voltage model.

As figure below, we connected them as a simple series circuit.

We will charge the battery for about 8-10 hours or about 8:00 to 17:00 o’clock. It will have full power after that and can be measured at its voltage of approximately 1.3V as below.

But this circuit has its own con, which is it cannot set the charging current to be constant. To extend the life of the battery. We should charge with a constant current and have a value of 0.1 of the battery’s maxed capacity or called 0.1C.

Practical example:

1.2-volt 700mAh AA Ni-MH rechargeable battery, we should charge it with a current of 700mA x 0.1 = 70mA for about 10 to 16 hours. In this way, the battery does not overheat while charging, therefore it lasts longer. It can be recharged up to 1000 times. It is really worth it.

Next, let’s look at the second circuit that solves this problem.

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1.2V AA Ni-MH battery solar charger circuit

This is the simple solar battery charger circuit. It is suitable for charging one or two 1.2V AA nickel-cadmium batteries or AA Ni-MH batteries.

Currently, this type of battery has increased capacity, but the price remains the same. For the worth, we should choose the proper battery, I chose the size 1900mAh to 2400mAh. Of course, the charging current should be increased to around 200mA at a constant level too.

This circuit can do well. Let’s see.

How simple solar Ni-MH battery charger works

Here is the circuit to convert the voltage from the general power supply or Solar cell. This circuit causes a voltage across the battery to be around 3V.

Important conditions

The solar cell normally doesn’t supply the voltage evenly, depending on sunlight. This circuit functions to maintain a constant current level, 250mA as above. But the solar cell voltage must be more than around 5V or 6V, at 500mA of current.

Here is a solar cell of 10 watts or at 12V 0.5A.

Look at the circuit, we use two transistors for controls a steady current to the battery.

First, the resistor R1and R2 are connected as the voltage divider circuit as a current bias for an NPN transistor-Q1 works.

Then, LED1 light up, and its voltage is steady at approximately 1.9V, all input voltage range. At the same, this causes the steady voltage at R1 and B-E of PNP transistor-Q2, which gets a bias current.

Therefore, Q2 keeps a constant current on the AA Ni-MH batteries.

This kind of circuit we call Constant current regulator. In the past, I used an NPN transistor. But in this case, we will try to use a PNP transistor, Which works well, too.

Which R4 determines the charging current. We can easily find R4 = 1.2V ÷ I_R4.

Note: I_R4 is charging current, For example, in this case, I set it to be 250mA or 0.25A.

Therefore, R4 = 1.2V ÷ 0.25A = 4.8Ω or 4.7Ω at the watts size = 1.2V x 0.25A = 0.3W or 0.5W

We can use  the Vin  as both 5V and 6V, which we need to change:
R1: 22Ω for 5V and R1: 27Ω for 6V.
R3: 270Ω for 5V and R3: 330Ω for 6V.

Parts you will needs
Q1: BC549, 45V 100mA NPN Transistor, Quantity: 1
Q2: BD140, 80V 1.5A PNP Transistor, Quantity: 1
LED1: 3 mm Green LED.
R1: 27Ω(6V), 0.25W Resistors tolerance: 5%
R2: 3.3K, 0.25W Resistors tolerance: 5%
R3: 330Ω(6V), 0.25W Resistors tolerance: 5%
R4: 4.7Ω, 0.5W or 1W Resistors tolerance: 5%
The Solar cell 10 watts, 12V 0.5A normally load.
PCB, Battery holder, Wires, Heat sink, others.

Testing and measuring the circuit

This circuit is very easy. Therefore, we can assemble it on a perforated universal PCB. My daughter soldered the leads of components directly on the bottom of the PCB copper plate. It is easier, no need to flip the PCB to put the component pins.

Then, test to charge the 1.2-volt 2,400mAh Ni-MH batteries and measure the voltage and measure the current at various points.

Look at the circuit diagram below. It show the level of voltage and current at various points.

• The solar voltage should be more than 5.5V.
• The battery current is 250mA in constant.
• The battery pack voltage is 2.9V while charging, but not over 3.2V.
• Measure voltage across LED1 is 1.9V fixed voltage.
• V_R4 is 1.2V at all times.
• V_{BE of Q2 is 0.7V}

The important point is that you should mount the heat sink for the Q2 because it gets a little hot while operating.

What is more? Not only that. Look:

• Simple nicad battery charger circuit by little part
• Universal nicd and nimh battery charger using LM317T
• Outdoor solar lights circuits
• Solar battery charger with overcharge protection

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