Charge Circuit NiCd

This is a simple circuit that rectifies and filters the output of a AC/AC adapter to "trickle" charge a 6.0v NiCd battery pack at a C/10 rate. The circuit is a simple filtered bridge rectifier. DO NOT ATTEMPT TO FAST CHARGE NiCd BATTERIES WITH THIS TYPE OF CIRCUIT" All the parts used in this project were recycled from scrap electronics.

You can use this circuit to charge a 6 volt NiCd battery pack. The charge rate is low enough that continual charging will not damage the batteries although batteries should not be left on charge for more than a few days.

Bridge Rectifier Charge Circuit

The AC/AC wall adapter transforms the 110vAC to 6.5vAC which is connected to one side of the bridge rectifier. Each positive and negative cycle of the AC power results in a positive DC cycle on the other side of the bridge. As the parallel capacitor becomes charged it provides current during the transition phase of one DC cycle to the next. This results in a rippled DC current with an average voltage slightly lower than the AC voltage supplied to the bridge. The NiCd battery pack is charged by the DC current which is limited by the resistor in series with the battery pack.

Circuit Design

We will start with the AC adapter which has a rated output of 6.5vAC at 250mA. We calculate the power output of the AC adapter 6.5 x .250 = 1.625 watts therfore we want our rectifier to have a power rating of at least 5 watts. You can build the bridge using four rectifier diodes but packeged rectifiers, such as the RBV-404 we are using, are easy to find on used power supply circuit boards. The RBV-404 is rated around 5 watts.

We want to limit the current going to the batteries to charge at a c/10 rate. Our batteries are rated at 1300mAh so our c/10 rate is 130mA. NiCd batteries have a very low internal resistance so we will just assume it is zero we will add a small value 3.9ohm 5 watt resistor (easy to locate used) in series with the battery pack. If we assume the bridge output to be 6.5v we subtract the battery voltage 6.0v leaving .5v across the 3.9 ohm resistor which gives us 128mA of current.

NiCd Batteries

The 6 volt NiCd battery pack is made from five NiCd cells in series. NiCd cells are rated at 1.2 volts giving us our 6 volts (5 x 1.2). NiCd batteries are rated by milliamp hours. Size "C" NiCd batteries are rated at 1300mAh. Simply put a fully charged "C" NiCd battery can provide a constant current of 1300 milliamps for 1 hour at which time it would be fully discharged.

Assembly

Assemble the battery pack by connecting the batteries in series. Use stranded wire which can be recycled from PC power supplies. When soldering leads onto NiCd batteries the surface must be sanded clean with fine grit sand paper. Do not apply the soldering iron (20watt recommended) for longer than 5 seconds. If the surface is clean a small bead of solder will bond to the battery surface. After it cools check for a good bond by trying to remopve the solder bead with your thumbnail, sand the surface again and resolder if required.

Connect the AC power pack to the two center leads of the RBV-404. Connect the capacitor across the two outside leads making sure to connect the negative side of the capacitor to the negative side of the bridge. Connect the resistor in series with the positive battery lead. Finally connect the battery leads.

Testing the Circuit

Plug the adapter into a power bar that has a on/off switch. Quickly turn the power bar switch on then off watching the circuit. If nothing sparks or starts to smoke turn the power switch back on. Check the components for exsessive heat and disconnect the circuit if overheating is detected. With this circuit all of the components including the batteries and AC adapter should remain cool. Measure the voltage accross the 3.9 ohm resistor and calculate the current (Voltage / Resistance = Current). Depending on the charge of the battery pack the current should be around 70mA to 100mA.

Let the batteries charge over night then disconnet the circuit and measure the open circuit voltage accross the batteries.