Did you like this article? Please 

According to Afroman's article the schematic shows a constant current source that can be found in any text book. The circuit works and will in fact recharge (non-rechargeable type) alkaline batteries and if you go to his site you can read the description to understand how the circuit works. During testing of his circuit I successfully recharged and reused several sets of batteries.

The problem I was having with his circuit is that the transistor is not thermally stabilized and drifts a lot during charging.  What that means is the transistor heats up which causes its internal resistance to decrease and causes the current to increase. To handle the heat and stabilize things I used a fan to keep it cool.  But I needed a better solution.

I thought about redesigning his circuit to compensate for thermal drift but then I remembered that an adjustable voltage regulator (LM317T) could be setup as a constant current source. (See Picture 1.) It would be capable of carrying more current than what is needed for recharging batteries which definitely helps when trying to prevent overheating. Most important to me is that the LM317T also has built in protection against over-voltage, over-current and over-heating.  Picture 2 shows the complete setup from the wall wart through the circuit and out to the battery pack.

Picture 2:   Complete setup including wall wart, battery pack and completed circuit board.

Self promotion:

I ripped the circuit in Drawing 1 from the specification sheet of the LM317T, page 16 I think, and then made a couple of modifications. I'm happy to say I've had very good results with this new circuit. It also uses fewer components than Afroman's little circuit and doesn't need a fan to keep the thing alive. The new circuit generates a little bit of heat which is easily handled by a small heat sink. But this is really just overkill.

Drawing 1:     50mA Constant Current Battery Charger

As for safety the first thing you will hear is that during charging these batteries can generate hydrogen gas. This is true but the amounts are so small that you would need many batteries to achieve a satisfactory explosion. I normally only charge the batteries to around 10% over their rated voltage so two 1.5 volt batteries in series get charged to 3.3 volts which does wonders for reusability. I tried overcharging one set of batteries but after about 6 hours I got bored and gave up. I'm thinking it would take several Amps to actually get a battery to blow up. :-)

Parts list:

First you need to get your hands on an LM317T variable voltage regulator, a 1N4004 diode (or similar generic diode), a resistor of the proper value (see Calculating R3 resistor value), a small circuit board and a battery holder. I actually have three different battery holders for AA, AAA and 9V batteries but I usually just put the AAA's in my AA holder. To simplify swapping the battery holders I've added a two terminal lug. Finally you will need a power source. One of those wall warts will work just fine. The one used for this project is a 12VDC unit capable of providing 200mA of current. I could have used one with a lot higher output as the voltage regulator input can accept voltages up to 40 volts higher than the output voltage.

You can try getting everything from Radio Shack but it seems that they have less parts every time I go in there. Usually I prefer buying all my parts on-line because it saves me a 45 minute trip, one way, to my nearest “real” electronics store. In most cases it costs more in gas than it does in shipping. If you would like to order parts on-line then understand that I'm just a customer of All Electronics.  I like them because they always deliver good junk for a good price.  If you don't have this stuff in your junk pile go to their web site and copy the catalog numbers on the left from the table below into their search box to get everything you need.

Assembly:

When you start to assemble the board there are only a couple of things to watch for. First, make sure you get the diode pointing in the right direction. On the diode symbol, in the schematic (Drawing 1), the line on the tip of the arrow corresponds to the line on the actual diode. Second, the diode is optional but it's there to prevent the LM317T from “running backwards” should you kill the power before disconnecting the batteries.

Drawing 2:   Printed circuit board created in Eagle CAD

Drawing 3:   Component layout also done with Eagle CAD

To assemble the board you need to follow the blue lines in Drawing 2 when connecting the pins (represented by green pads.)  Drawing 3 shows the component layout and both pictures are oriented in the correct direction. For example, the LM317T is shown with A, O, and I which correspond to the Adjust, Out, and In pins respectively.  Those relate directly to the three oblong green pads in the top left corner of Drawing 2.

Circuit Description:

In the schematic in Drawing 1 it shows that R3 is a 24 ohm resistor and the caption says that this is a 50mA constant current supply. Resistor R3 is adjustable and should be chosen to set a current that you would like to use. Obviously, the higher the current the faster the batteries will charge. The trick is to keep the current low enough so the batteries don't heat up. The heat damages the batteries during charging and will make them useless. The test board I have is set to charge at 83mA using a 15 ohm resistor for R3.

Calculating R3 resistor value:

To calculate the value for R3 you divide 1.25 volts by the charging current and that will give you the resistor value. For example if you wanted to charge your batteries at 75mA then 1.25/.075 would tell you that you needed a 16.67 Ohm resistor. This is not a standard size so pick the next closest resistor value and you should be OK. You could also divide 1.25 volts by the value of some resistors you have laying around. Then just choose the resistor that gives the closest value to the charging current that you want. That's how I chose the 15 ohm resistor.  It gives me a charging current of 83mA.

At 83mA the test board does not seem to cause any heating in the batteries at all. That is based on the finger test so your mileage may vary. I suspect the charging rate can be set higher but I would monitor the temperature of the batteries to find the safest charging rate.

Conclusion:

There are definite benefits to using this device. The most obvious is the savings you will see. AA batteries can cost as much as $4.00 a pack in the U.S. so if you can reuse the batteries even once then you cut that price in half. You will also benefit the environment by reducing, by half or more, the amount of hazardous battery waste that you dump each year.

The safety factors should not be ignored and I can't promise there aren't any dangers. Take the appropriate safety precautions when using this device. However, in my limited experience I have yet to do anything worse than ruin an already dead battery.

Afroman's guide to recharging Alkaline batteries:
http://www.afrotechmods.com/reallycheap/batteries/batts.htm

National Semiconductor LM317 spec sheet:
http://www.national.com/ds/LM/LM117.pdf

All Electronics:
http://www.allelectronics.com

If you use Eagle CAD, or want to, then you can download the Gerber files Alkaline Batt. Charger.zip then make any changes and even create your own printed circuit boards.

Did you like this article? Please