Constant Current Battery Charging Jeff Damm Battery charging systems can be constructed from very simple to extremely complex circuits. Many people starting out to learn how to make electronics work for them with charging systems only need a few components to get the job done. The general rule is that charge controllers will increase their user friendliness as they get more complex. In this article we will learn about some basic techniques for accomplishing the task of battery charging. Figure 1 is a block diagram of a typical charging system for use with NiCads (Nickel Cadmium), Gel cells (Gelled Sulfuric Acid electrolyte, lead acid type, mechanically sealed) and standard Lead Acid (Sulfuric Acid liquid electrolyte) batteries. The voltage source represents any one of many different energy sources: an alternator in a hydro system, PV (photovoltaic) panels, an electronic bench power supply, or even another battery that is fully charged. The fundamental assumption behind all of the diagrams in this article is that the voltage source terminal voltage must be greater than the terminal voltage of the battery to be charged. The "extra" voltage difference can be anywhere from about 5 to 15 volts. Typical applications will require a voltage difference of around 5 or 6 volts. This means that charging a single 1.2 volt AA NiCad would only require a source of 6 to 7 volts. Charging a 12 volt battery will require between 17 and 20 volts. The constant current block represents whatever circuitry is used for the purpose of charging the battery of interest. The constant current block will force a predetermined current through the battery, independent of the terminal voltage at the voltage source. A simple constant current source is shown in figure 2. The components are non critical in nature. The devices inside the dashed line box represent the essence of the constant current source. Transistor Q2 can be any small signal NPN of the 2N3904 or 2N2222A variety. Almost any junkbox NPN will work as Q2. R1 can be any value from 220 ohms to 1000 ohms. A series diode is included to prevent any possibility of battery discharge if the input power fails or if the input voltage drops below the battery voltage. The series diode must have a current rating greater than what will flow through it. A good generic diode for 1 Amp or less is the 1N4000 series devices. These are listed as 1N4004, 1N4005, 1N4006 etc. Silicon rectifiers are a better choice than the smaller "signal" diodes due to their higher current ratings. Transistor Q1 can be almost any silicon NPN device that has a power dissipation rating greater than the power it must dissipate. Two choices to start with would be the TIP31 or the 2N3055. Currents beyond a few amps with the 2N3055 may require a Darlington connection to replace the Q1 function. This value is calculated by multiplying the voltage difference between the source and "load" battery and the charging current. Remember that a heat sink will usually be necessary for helping to keep Q1 cool enough to operate safely. A good rule is that if you cannot keep your finger on the metal case of Q1, then it needs a heat sink. Remember that Q1 can get very hot very quick if it needs a heat sink and does not have one. I usually perform this test with a wet finger so that the moisture will be vaporized instead of getting a burnt finger! Resistor R2 is the real magic element in this circuit. The value of R2 is what determines the constant current value. It is calculated as the ratio of the base-emitter voltage drop of Q2 divided by the current that we want to flow through the circuit. This can be simplified by assuming that the diode drop is about .68 volts. We can express this equation as R=.68/I where I is in amps. For example, to establish a constant current of 50 mA to charge AA NiCads at C/10 we would use R=.68/.05=13.6 ohms. Since this is not a standard resistor value, we would want to use either 12 ohms or 15 ohms. A 1/4 watt (or greater) power dissipation resistor would work well. Remember that the R2 dissipation must be greater than it's actual power dissipation. Figure 3 is a simple variant of figure 2 using a power FET (Field Effect Transistor) as the "pass transistor" element. Circuit operation is essentially the same as that of figure 2. A very good choice for the FET would be the IRF511 (International Rectifier) or IRF530. Almost any N channel Enhancement FET will work in figure 3. Another constant current source that can be used instead of the circuits in this article was presented in Home Power #21 on page 82. The real benefit of using constant current charge controllers is that you can get more use out of your PV panels. A classic example is the ability to charge different voltage batteries at different currents simultaneously. The only restriction is that the total current drawn from the panel must be such that the panel terminal voltage stays about 5 volts (or more) above the voltage of the battery we want to charge. Access: Jeff Damm 6565 S.W. Imperial Dr., Beaverton, OR 97005 ù 503- 646-4217.