|Here is a constant current supply circuit schematic diagram using the LM317 variable voltage regulator.
It is a very simple circuit for charging alkaline batteries. It will provide a stable constant current which is adjustable by switching different values of resistors. The input voltage must be at least 6v higher than the battery(s) you wish to charge. The LED, BC548 and 470Ω resistor provide an indication of current flow to show that your battery connections are good. They can be omitted if you wish to make the circuit simpler. I used a 12 way rotary switch set to 5 way to select different resistors to give output currents of around 5, 10, 20, 30 and 40mA. The idea being for 9v PP3 types I would use 5mA. For AAA's 10mA. AA's 20mA, C's 30mA and D's 40mA. This is just my guideline, you can try what you like! Just remember more current is not good for charging alkaline non-rechargeable batteries.
You can use no switch and fix the current, or use a simple toggle switch to toggle between 2 or 3 different currents or whatever you prefer!
The constant current may be set by choosing the appropriate resistor. R = 1.25 ÷ I Where R is the resistor value in ohms, 1.25 the regulator's reference drop voltage in Volts and I is the constant current in Amps.
For example, if you want a 100mA constant current, the R value will be: 1.25 ÷ 0.1 = 12.5ohm. The dissipated power on the resistor R in this example is: P = V x I = 1.25 x 0.05 = 0.125W or 125mW.
The dissipated power on the LM317 IC is:(Vin - Vout) x Charging Current. A heatsink is not required for the LM317 (TO220) in this low power circuit. If you design one with more than 40mA output current, you should heatsink it. Notice that the IC's metal package or tab also carries the Vout, so it's necessary to use isolating washers if you attach the heatsink to a metal case.
How this works. The LM317 keeps a constant 1.25v over the resistor regardless of input voltage or output load. This means when the load current increases or decreases, the regulator adjusts its output to keep a constant voltage over the resistor of 1.25V at all times and therefore a current of 1.25÷R.
One of the reasons this circuit is so simple is that most of the circuitry is inside the LM317 itself. It's complicated circuit can be seen in its internal schematic diagram below:
Yes that whole circuit is packaged inside the LM317. The three pins in, out & adjust can be seen on the left of the schematic. Inside are 26 transistors, 26 resistors, 3 capacitors and 4 zener diodes.
More info on my alkaline charging experiments and photo's of my charger will be here soon!
Page last updated:
April 20, 2010
Disclaimer: Battery manufacturers clearly state that alkaline batteries should not be recharged. There is a possibility of leaking of chemicals / gasses and/or an explosion. Some alkaline batteries contain small amounts of mercury and/or cadmium. Always wear safety equipment such as gloves and goggles when experimenting with batteries, and clean up any spills of battery fluid immediately. Do not leave charging unattended indoors. Use this circuit at your own risk!