Introduction: 120 Volt AC AA Battery Charger
This charger is very unique in that it will charge all different types of AA ,C, D ,9 volt batteries depending on the battery holder that you select for the project.
This project does not require the use of a step down transformer only one diode. This charger is even capable of charging alkaline batteries depending upon the load resistor ( wattage light bulb) in the circuit.
Yes I did say alkaline batteries yes the ones that are not supposed to be recharged due to overheating acid leakage fire and explosion.
Step 1: The Basic Parts You Will Need for the Project
This project can be designed by you anyway you wish. The electronic principles here are basic, but I chose to use these items in order to make it my own. How are you choose to house this project is totally up to you.
A WORD OF CAUTION AND WARNING, this charger has 120 V AC running through the whole thing do not leave it out and open to children or people with no experience. A proper enclosure is important you have been warned!!!
1. You will need an AC power cord of some sort.
2. 1 Diode that can handle several hundred volts. I used 2 x 1N4004 in parallel for safety sake.
3. An on and off switch.
4. And incandescent lightbulb with the wattage suitable for your charge rate. The lightbulb becomes the circuit load resistor. (I will discuss this more later).
5. A battery holder suitable to the type of battery you choose to charge. This charger will charge anything from 4 to probably 12 batteries at a time without an issue. ( *** Just make sure that you can fill your battery holder every time you choose to charge as this is a series circuit, the battery holder will need to be full in order to function. ***)
Step 2: How the Charging Circuit Works
Before we begin building it's a good idea to know how this circuit functions.
For those of you some electronics background imagine if you will the circuit in your head. I have put a schematic picture in this step.
With the use of an AC power cord we are drawing 120 V AC from the wall socket. I diode is added in series with one side of the load and then to the battery holder pack. The diode rectifies the AC current and creates a pulsing DC 60volts DC. (A 50% duty cycle)
A NOTE ABOUT THE POLARITY OF THE DIODE AND HOW IT IS PLACED IN THE CIRCUIT. I placed the negative/cathode side of the diode towards the wall current. The cathode side of the diode is the one with the stripe around the end. The anode/+ side of the diode is connected to the negative pole of the battery power pack.
The pulsing DC current goes through the batteries in the power pack and into a load resistor which will be your light bulb. From the light bulb a switches installed and then the power goes back to the wall socket. A fuse in the circuit is also encouraged.
Ohms law will help us determine the charge current we need to use.
Ohms law tells us that voltage x amperage equals wattage. Since I have chosen to recharge AA alkaline batteries I need to base my charge rate of those. The average AA alkaline battery has a capacity of about 2500 mAH. To safely charger battery like this we don't want to charge current to be more than one 50th of its capacity. So 2500 mAH divided by 50 = 50mAH < = 50 milliamp hours.
So multiplying 120 V times 50 mA equals 6watts, thus the load resistor needs to be 6 W.
It is helpful to bear in mind that AA batteries of different kinds come in different milliamp ratings. I have seen NiCad, nickel metal hydride batteries with capacities ranging from 600 mA hours to 3000 mA hours therefore the charge rate may be slightly different depending of the batteries you choose to charge.
Keep in mind if you are charging rechargeable batteries you may need to base your current more on one 10th of the battery's capacity so that they are able to charge up quicker since the battery could take that amount of current. JUST REMEMBER TO DO THE MATH !
Hi chose to use a 7 1/2 W night light bulbs in order to get the closest to 6 W that I was able to. I could probably use up to a 15 W incandescent and still be on the safe zone.
Just for example if I wanted to charge rate 200 mA I would use a 25 W lightbulb.
***** NOTE the light bulbs in this project MUST to be incandescent, CFL bulbs will not work.
I will try and post visual schematic for this project as well if I failed to do so please someone tell me.
While your batteries are charging it's a good idea to keep track of their temperature at the time that they are on the charge cycle.
Periodically turn off and unplug the charger so that you can monitor voltage of each cell with a multi meter. With alkaline batteries depending on the state of discharge it may take a few hours or longer to bring the batteries back up to a full charge keep your eyes open. REMEMBER DUE TO THE AGE OR STATE OF DISCHARGE SOME BATTERIES MAY NOT RECOVER TO A FULL CHARGE .
Remember you are using a very small amount of current to charge these batteries so that they don't go into thermal runaway and thus overheat, catch fire, or explode.
Step 3: The Enclosure
I chose to use the plastic receptacle box due to its durability and inexpensive cost. This affords me the opportunity to install a standard switch and outlet in order to use the charger more flexibly.
The cord and plug I salvaged from an old computer printer that was out of date. I separated the cord and strip the wires back approximately three quarters of an inch on each side and then inserted the wire into the receptacle box at the back. Receptacle boxes have what are known as knockouts these can be opened with a screwdriver or sometimes a screwdriver and hammer if need be.
You will notice after I inserted the wire inside of the box I tied a knot and then pulled tight and tied another night of the outside of the box to keep the cord from slipping in and out.
I chose to use a plug outlet so that it could be in series with whatever load resistor I chose I'm able to plug in the nightlight very easily this way. You will also see the plug-in bulb adapter this allows me to put in say 15,25, 40,60 or 75 Watt bulbs but according to the charge current I desire to use.
If you look carefully at all of the pictures you will find that everything is in a series circuit, there are zero parallel connections in this project!
I am using a nightlight that has a light sensor switch. I have placed a black plastic sticker over the sensor so that the light remains at all times. There are other models and ways to get 7 W lights into the circuit and you will see those if you go to someplace like Home Depots lighting department.
Step 4: The Battery Holder and Attachment
I purchased a six cell AA battery holder on eBay several years ago and decided to use that in this project. I found it it's physical size would sit very nicely on top of the outlet box.
With the use of a two part epoxy call JB Weld I was able to attach it quite nicely. JB Weld is pretty strong stuff, it is even used to mend cracked engine blocks on cars. Just keep it where you want it until it is dry or it will be stuck where it is :)
You will see the pictures of the epoxy before and after mixing make sure all epoxy's are mixed very well before applied. Some epoxys dry very quickly others do not I chose to use an adjustable clamp to hold everything in place while I worked on the project. When it came time to put the face plate on the project I decided to use rubber bands to hold the battery holder in place until a dried.
I opened another knock out on the back of the box in order to get the positive and negative wire inside from the battery holder.
Step 5: Hooking Everything Up
I think the pictures are fairly self-explanatory both of the joints that come from the battery holder I did use shrink tubing on them in order to give them some strength. I used a larger shrink tubing and encapsulated the diodes as well.
If you search my Instructables I think you will find my soldering help in step number 4 of my Altoid tin light Instructable. If you need some help learning to solder that would be a good place to go. I soldered the diodes the way that I did in order to be able to trim off extra wire so the circuit was not too bulky.
You will notice when I attempted to screw the switch into the blue box that the machine screw was too long and started going into the battery holder if you do this project the way I did it you will need to shorten the screws on top or bottom which ever side you did you choose to use/ or both. A WORD OF CAUTION I CUT ONE OF THE SCREWS TOO SHORT AND COULD NOT REACH THE THREAD AND HAD TO SALVAGE ANOTHER SCREW TO GET IT IN THERE.
Basically this is all there is to assembling the circuit study the pictures well ask questions if you need to.
Step 6: Final Thoughts
After hooking everything up I decided to do the current test with my digital volt amp meter. I found out that with the 7 W light bulb in series the circuit was only drawing 30 mA ( in all reality I could probably go to a 15w bulb if I wanted to. ) this is also dependent on batteries, line voltage, and that the 7 W lightbulb could be 5 W, 9 W, incandescent lightbulbs are kind of an estimated wattage thus your circuit can vary.
OKAY 48 hour update: I put Duracell Pro Cells 6 of them in the charger. All had a start voltage of 1.47 vdc.
I charged at 15 minute intervals at first to make sure the heat was in check. ZERO heating was detected
through the whole charge cycle.
I checked voltage every 30 minutes with an average voltage rise of .4 volts. At the 1.5 hour charge point all cells were
at 1.55 vdc on the multi meter. 1.55 is the full charge voltage for this series battery fresh out of the box.
SO be careful to watch the time and check on the cells until you get a feel for what the charger and batteries are doing and how long they will need to charge to become full again.
Happy Charging, Bryan KC8HPS