Solar power is a great way to get electricity out to a remote project site. One simple way to do this is to use a solar panel to charge the batteries of your cordless power tools. In this project, I am going to show you several ways you can do that.

Safety Note: Any time you use a DIY battery charger instead of a commercial charger, you are accepting a certain amount of risk, so always use caution. Don’t over charge the battery and don’t try to charge it faster than the manufacturer recommends. There are many different kinds of batteries and they all need to be treated differently. These instructions are designed for batteries that are made of Nickle-Cadmium cells. I make no guarantee that these instructions will be appropriate for other battery types such as NiMH or lithium batteries.


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Project Steps

Background: Cordless Power Tool Batteries and How They are Charged

There are many different styles of cordless power tool batteries and many different styles of chargers. Most cordless tool battery packs are made up of several smaller batteries that are wired together in series. In the most basic models, the charger will connect directly to the end terminals of the battery pack at 2 exposed pins. The charger then applies a small direct current and slowly charges the battery pack over several hours.

In more advanced models, the battery pack may have a number of internal sensors. For example, a Craftsman 19.2V battery has a thermal fuse and an internal temperature sensor. To accommodate these sensors, the battery has 2 additional terminals (a total of 4). The charger connects to the battery through the thermal fuse. If the temperature of the batteries ever goes above the maximum safety threshold, the thermal fuse will disconnect the battery from the charger. The temperature sensor allows the charger to actively monitor the temperature of the batteries while they are charging. This lets the charger automatically adjust the current to maximize the charge rate without overheating the batteries.

If you want to use a DIY charger to charge the battery, your primary concern should be safety. The safest way to charge a battery is to do it slowly. This will sacrifice some efficiency but it will guarantee that you will not destroy the battery.

Identify the Terminals of the Battery

The first thing you need to do is identify and label each of the terminals on the battery. If your battery has 2 terminals then you can easily use a multimeter to identify which is positive and which one is negative.

If the battery has more than 2 terminals, then there are several things that you need to check. Start by noting the position of the terminals on the power tool. A power tool will usually connect directly to the positive and negative terminals of the battery. You can then use a multimeter to measure the voltage of the corresponding terminals on the battery to determine which is positive and which one is negative. Any additional terminals will probably be a thermal fuse or a temperature sensor.

A thermal fuse is usually connected to the positive terminal of the battery. If you measure its voltage with a multimeter, it will have the same voltage as the positive terminal of the battery (or slightly less). The thermal fuse also connects to the positive output of the charger. Once you have identified the thermal fuse, any remaining pin will be the temperature sensor.

If you want to double check to make sure that you have correctly identified the terminals, you can carefully remove the cover of the battery pack and check to see where each wire is connected. Do not disconnect any wires, or move any of the sensors!

After identifying the terminals you can label them with tape, a permanent marker, or by scratching a note into the side of the plastic housing.

Select an Appropriate Solar Panel

The next thing that you need to do is select a solar panel that is well suited to charging your battery. The panel needs to have an open-circuit voltage (voltage measured with no load) that is higher than the voltage of the battery when it is fully charged. You can find the open-circuit voltage of the panel by connecting your multimeter directly to the panel when in bright sunlight. To find the fully charged voltage of the battery, simply charge it with the commercial charger and then use a multimeter to measure the voltage between the positive and negative terminals. This will typically be higher than the rated voltage that is listed on the side of the battery. For example, a 12V battery may measure 14V when fully charged.

The other thing that you need to be concerned about is the charge rate. To be safe, I recommend using a panel that will charge the battery more slowly than the original charger that came with the battery. The easiest way to determine this is to compare their wattage ratings.

Here is a simple example: My “12.0 Volt” drill battery came with a charger that lists its output as 15V DC at 200mA. This means that its wattage rating is 3 watts (15V x 0.2A = 3W). So you would want to find a solar panel that is rated for less than 3 watts; I chose a 12 volt panel that was rated for 1.5 watts.

Charge Controllers

It is possible to charge the battery by connecting the output of the solar panel directly to the terminals of the battery. However, if you are not carefully monitoring the voltage of the battery, then you run the risk of overcharging it. This is why you usually want to use a charge controller.

A charge controller is any circuit that limits the current that is sent to the battery in order to prevent overcharging. You can purchase a commercial charge controller at most stores that sell solar equipment, or you can build your own.

In a previous project, I showed how you can use an Arduino as a charge controller. Because the Arduino has multiple inputs, you can use it to monitor and control the charging of multiple batteries. Check out the link to see how I did it:

You can also make a charge controller with a simple 555 timer IC. These chips have an internal comparator that toggles the output on and off depend on the input voltages. This can be used to connect and disconnect the battery with a low power relay. This control circuit is discussed in more detail in the next step.

If you plan on leaving the battery connected to the solar panel overnight, then you want to make sure that either the panel or the charge controller has a blocking diode. This is a diode that is placed between the panel and the battery to prevent the battery from discharging through the panel when it is dark. A Schottky diode gives the best performance for this.

555 Control Circuit Design

Here is one simple example of how to make a DIY charge controller, built around a 555 timer IC. This chip has 2 inputs (pins 2 and 6). It compares these input voltages to a set of reference voltages that are based on the supply voltage. If the voltage at pin 2 drops below 1/3 of the supply voltage, then the output at pin 3 goes HIGH. If the voltage at pin 6 goes above 2/3 of the supply voltage, then the output at pin 3 goes LOW.

By using the 7805 voltage regulator, we can fix the supply voltage to 5 volts. So 1/3 of the supply voltage will always be 1.66V and 2/3 of the supply voltage will always be 3.33V.

The input voltages at pins 2 and 6 are dependent on the voltage of the battery. Each input has a voltage divider that is made of 2 resistors. The ratio of the 2 resistors determines what percentage of the battery voltage is sent to the input pins. In this example, the pin 2 voltage divider uses a 68 kohm resistor and a 10 kohm resistor. This means that the voltage at pin 2 will always be 12.8 percent of the battery’s voltage. Similarly, the voltage divider at pin 6 uses a 33 kohm resistor and a 10 kohm resistor. This means that the voltage at pin 6 will always be 23.2 percent of the battery’s voltage. This is only approximate because all resistors will vary a little from the indicated value.

When the battery’s voltage goes above 14.4 volts, the output of the 555 goes LOW and activates the relay. This disconnects the battery from the solar panel. When the battery’s voltage drops below 13 volts, the output of the 555 will go HIGH and deactivate the relay. This resets the system for another charging cycle. In the case of cordless power tool batteries, this will happen when you disconnect the battery. But in a typical solar system, the battery stays connected and the voltage will drop as the power is used.

If your battery has a different operating voltage, you can change the voltage setting of the charge controller by using different values for the resistors. The 33 kohm resistor can be replaced using the formula R = (3 x Vcutoff) – 10. The 68 kohm resistor can be replaced using the formula R = (6 x Vreset) – 10.

Assemble the Circuit

First, prototype the circuit on a breadboard and make adjustments as needed. Once it is working, solder the components onto a breadboard.

In this case, I needed to make some modifications to the board. The pins on the relay were positioned in such a way that there was no place that I could mount it on the board where the pins could be isolated from each other, so I mounted it in the middle of a set of columns. Then I used a knife to cut the conductors in the middle. This separated the halves and let me access the pins independently.

I also used a Dremel to trim the edges of the circuit board. This let me easily fit the board in a small plastic project enclosure.

Cut Slots in the Project Enclosure

The circuit board has 4 wires that connect to it, so we need to cut 4 slots in the side of the project enclosure to accommodate these wires. I just used a knife to cut 2 lines in the side of the housing, then broke off the tab of plastic with a pair of needle nose pliers. Do this for all 4 locations on the top and bottom of the housing.

Test the System

Now you need to test the system to make sure that everything is working properly. The easiest way to test it is to switch the battery and the solar panel. The battery is connected to the input of the charger where the solar panel would normally be connected. Then the solar panel is connected to the output of the charger where the battery would normally be. Keep the polarities of the positive and negative wires the same.

The battery supplies a stable power source for the charge controller circuit and the solar panel acts as a variable voltage source. Start by covering up the solar panel. You can either put something over it or turn it face down. Then, slowly uncover the panel. As more light hits the panel, its output voltage will go up. When the voltage reaches 14.4V, you should hear a click from the relay activating. Then slowly cover the panel. When the voltage drops to 13V, you should hear another click from the relay turning off. If everything is working properly, then you are done.

Hook Up Your Finished Solar Charger

Now you are ready to connect all the components together. First, connect the output of the charge controller to the battery. Then connect the solar panel to the input of the charge controller. The relay may turn on and off a few times while the circuit is powering up.

Now you have your own solar charger. This will let you charge your batteries anywhere there’s sunlight. This can be really useful for working with power tools at remote project sites.