Portable Battery Charger

At one of the amateur radio sites I saw a scheme for charging portable Ni-Mn and Ni-Cd batteries with an operating voltage of 1.2-1.4 V from a USB port. With this device you can charge portable rechargeable batteries with a current of approximately 100 mA. The scheme is simple. It will not be difficult to assemble it even for a beginner radio amateur.
Scheme simple
Of course, you can buy ready-made memory. On sale they are now a great many and for every taste. But their price is unlikely to satisfy a novice radio amateur or someone who can make a charger with their own hands. I decided to repeat this scheme, but to make a charger for charging two batteries at once. The output current of USB 2.0 is 500 mA. So you can safely connect two batteries. The modified scheme looked like this.
 Scheme of advance
Just wantedto be able to connect an external power source with a voltage of 5 V. The scheme contains only eight radio components.
 radio elements
The tool will require a minimum set of radio amateurs: soldering iron, solder, flux, tester, tweezers, screwdrivers, knife. Before soldering radio components, they must be checked for proper operation. For this we need a tester. Resistors are very easy to check. We measure their resistance and compare with the nominal value. On how to check the diode and the LED there are many articles on the Internet. For the case used a plastic case size 65 * 45 * 20 mm. The battery compartment was cut from a children's toy "Tetris".
 tetris
I will tell you about the remaking of the battery compartment in more detail. The fact is that initially the pros and cons of the battery power terminals are set opposite. But I needed to have two insulating plugs at the top of the compartment, plus terminals, and one common minus at the bottom. For this, I moved the lower plus terminal upward, and cut out the common minus of the tin, soldering the remaining springs.
battery compartment
 battery compartment
 case of the charger
As a flux when soldering springs used soldering acid in compliance all safety regulations. The place of soldering must be rinsed in running water until all traces of acid are completely removed. Wires from the terminals soldered and passed through the drilled holes into the case.
 charger charge
The battery compartment is secured to the case cover with three small screws. The board cut from the old modulator of the game console "Dandy". Removed all unnecessary parts and tracks of printed wiring. Left only the power socket. As a new track used thick copper wire. I drilled holes in the bottom cover for ventilation.
 charger
The finished board is densely seated in the body, so I did not fix it.
the charger is ready
After installing all the radio parts into place, check the correctness of the installation and clean the card from the flux. Now let's unmount the power cord and setting the charging current for each battery. As the power cord used a USB cable from an old computer mouse and a piece of power cord with a plug from the Dandy.
plug connectors

Power cord need to pay special attention. In no case can not be confused "+" and "-". I have a "+" power plug connected to the central contact with a black wire with a white stripe. A “-” power supply goes through the black (without a strip) wire to the outer contact of the plug. On the USB cord, the “+” goes to the red wire and the “-” to black. We solder plus with plus and minus with minus. Soldering places are carefully insulated. Next, we check the cord for a short circuit by connecting the tester in the resistance measurement mode to the plug terminals. The tester should show infinite resistance. Everything must be carefully rechecked, whatever the USB port. If everything is fine, connect our cable to the USB port and check the voltage on the plug.The tester should show 5 volts.
check
The last step in the setup is to set the charging current. To do this, break the circuit of the diode VD1 and "+" battery. In the break we connect the tester in the current measurement mode on the limit of 200 mA. Plus a tester for a diode, and a minus for the battery.
 scheme
Insert the battery in place, respecting the polarity, and supply power. This should light up the LED. It indicates that the battery is connected. Next, changing the resistance R1, set the required charge current. In our case, it is about 100 mA. When the resistance of the resistor R1 decreases, the charging current increases and decreases as the increase.
 charging current
The same is done for the second battery. After that, we twist our case and the charger is ready for use. Because different finger batteries have different capacity, it takes a different time to charge these batteries. Batteries with a capacity of 1400 mA / h with a voltage of 1.2 V will need to be charged using this circuit for approximately 14 hours,and the 700 mA / h batteries would take only 7 hours. I have 2700 mA / h batteries. But I did not want to charge them for 27 hours from the USB port. That's why I made a power socket for an external power supply of 5 volts 1A, which I had to do without work.
 charger
Here are some more photos of the finished device.
charger
 indicators
 nest
 label
 do-it-yourself charger
Labels drew the program FrontDesigner 3.0. Then printed out on a laser printer. Cut out with scissors, stuck his face on a thin adhesive tape with a width of 20 mm. Extra scotch cut off. I used glue stick as glue, after having smeared it with a sticker and the place where it is glued. How reliable it is, I don’t know yet. Now the pros and cons of this scheme. Plus, the scheme does not contain scarce and expensive parts and is literally going to the knee. It is also possible to power from the USB-port, which is important for novice radio amateurs.No need to puzzle where to power the circuit. Despite the fact that the scheme is very simple, this charging method is used in many industrial chargers. You can also make the charging current switching a bit more complicated.
 scheme
By selection R1, R3 and R4 can set the charging current for batteries of different capacity, thereby providing the recommended charging current for a given battery, which is usually 0.1C (C-capacity of the battery). Now the minuses. The biggest is the lack of stabilization of the charging current. That is When the input voltage changes, the charging current will change. Also, if there is an error in the installation or short circuit of the circuit, there is a high probability of burning the USB port.