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Constant Current AA Battery Charger


I wanted the ability to charge AA NiMH batteries from my 12 volt solar power system, and so built this simple constant current charger. The circuit principle is not new. An LM317T regulator provides a fixed voltage across a resistor (Rc), which due to ohms law (I = V/R) produces a fixed current through that resistor. It is this current that is passed through the batteries to charge them. The LM317T is programmable by using two external resistors to set the output anywhere between 1.25V and 37V, but here one resistor is omitted, so the output remains fixed at the minimum level of 1.25V. The formula for this can be found in the data sheet. Because the voltage across Rc is low, the power dissipated in Rc will also be low (P=V x I) so it doesn't get hot. With the component values shown, the circuit produces a charge current of 1.25V divided by 6.8R = 0.184A. 10mA (approx) must also be added to this figure due to the inclusion of the LED, which makes a total of 0.194A (194mA). This is about right for a lot of AA batteries available at the moment. To increase or decrease the current for other types of battery, the value of Rc must be changed accordingly. The schematic diagram is shown below





The LED provides a charge indicator when the batteries are connnected. Here it's across the input of the regulator rather than the output because there's not enough voltage on the output to run an LED, but regardless of where the LED is placed, it will still add a few extra milliamps to the charge current which is not a problem. The charger is simply placed in series with the batteries, the number of which is irrelevant as long as there's enough voltage supply available. With 12V the circuit can charge up to six batteries.
The best way to connect batteries to the charger is to use a battery box with a PP3 clip. These are readily available in various sizes depending on the number of cells you want to charge, and are quickly and easily swopped between each other. Although the batteries are shown connected in the negative supply, they can also be placed in the positive. It's all the same in a series circuit. A veroboard layout is shown below





Circuit notes: Although this is a constant current charger, the charge current will vary slightly depending on the number of cells that are connected. This is not caused by the current in Rc varying as this is always constant, it's caused by the LED series resistor. When a large number of batteries are connected to the circuit, the adjustment pin (pin 1) sits higher above ground, effectively decreasing the input voltage of the regulator. This will also decrease the voltage across the LED series resistor, and so the current through it will drop slightly due to ohms law. Conversely, if fewer batteries are connected, there will be a higher voltage across the LED series resistor causing the current through it to increase. In practice though, this variation is small and does not affect performance, but I mention it only for information

When less batteries are being charged, the regulator gets quite hot. As mentioned, pin 1 sits higher above ground the more cells that are connected, so with only a couple, the regulator sees a higher input voltage, which means there will be a higher voltage drop from input to output, and as Power = Voltage x Current, the power dissipation is increased. When the maximum six cells are used, everything runs much cooler, but a heat sink is still required. All this could be alleviated by increasing the output voltage of the regulator, but then Rc would get hot instead, and the circuit would not be able to charge as many cells. Take your choice! The resistor Rc is a 3 watt wirewound which was used only because I had one to hand, and although it doesn't need to be this large, it still gets warm. So yes it is over engineered, but I would rather have something run warm than sit there cooking for several hours!




The finished charger