Introduction: DIY Lead Acid Battery Charger
Actually this could be used to charge any sort of battery where you want a constant current and a constant voltage.
In this instructable I will take you through the whole process to producing a final boxed system.
It will take an input from any AC/DC adapter with a jack. You simply have to make sure that the adapter is rated for the voltage and current you want to create. This system will allow up to 36V and 2Amps.
Step 1: The Components & Tools Needed
Components needed are :
selection of resistors between 1and 5 Ohms,
820 Ohm resistor
Wiring - some jump leads ( suitable for connecting parts of circuit board together), some cable with two power carrying leads inside ( pos + neg )
2.1mm or 2.5mm input jack ( depending on your power source )
The whole circuit revolves around the L200C current/voltage regulator ( the circuit diagram we will be sticking with is shown below ). You can download the datasheet from HERE
Tools needed are
Screwdriver ( Philips ) and a very small flatbladed screwdriver
Step 2: The Box
The project box is made from ABS plastic, if you plan to use the chip to its full potential you may need a metal box. This will be explained a little later. It should be of suffient size to allow insertion of your copper stripboard and also have some headroom for the L200C chip - this chip can generate some heat and unless the box is metal you dont want it pressing against the box.
You can see that a hole has been drilled into the box to accomodate the DC Input jack. If you look at the DC input you will see that it has 3 tabs. The one attached to the centre is the positive, the next one out is the negative - these are the only two we are interested in.
Please be aware that jack plugs have polarity too - usually the polarity is as shown in the 2nd picture - always check. ( I even ringed the important info around in red )
Step 3: First Things First
Check that the copper stripboard fits into your box, you may need to trim it - I have designed the circuit so it will fit a board with 23 holes and 9 strips. One hole either end is not used to allow it to slide into the slots provided by the project box. Best to make sure of a fit now before you start any soldering.
You will also need to drill a 2nd hole in the other end of the box. The black wire containing your main two output power lines should fit through the plastic grommit. Drill the hole, install the grommit and check out the cable runs through - it should be a tightish fit so your cable wont pull out and strain the curcuit board.
Step 4: What Voltage/Current Should I Use?
You should charge your lead acid battery according to the specification of the manufacturer. Below you can see the one I was charging - 6.5volts at .7Amps. Build you circuit around the typical batteries you need to charge.
Step 5: The Circuit
I include two versions of the circuit board, You have the traditional circuit diagram and a graphical representation of the copper stripboard.
C1 is a 220nF capacitor
C2 is a 100nF capacitor
The two capacitors help smooth and filter the input and the output voltages.
R2 is a 820 Ohm Resisitor.
W1 through to W6 are all jumper wires of various lengths. Most electronics shops have them available.
The X marks you see on the tracks are breaks in the copper strips. You can break them using a stripboard track breaking tool - a supplier I use for them can be found at Electronic Projects Online
R1 is the 5K or 10K potentiometer.
The 3 x R3 resistors make up the value of Ohms you need to supply the correct current. Notice that they are set up in parallel. This is using 0.25W capable resistors making a total of 0.75W. The current passes directly through these resistors so it need to be rated correctly. We will talk about the equations for calculating correct values shortly.
Finally you can see the L200C. It has the pins numbered which you can match up from the datasheet. You will have to do a small amount of gentle bending to get the pins lines up as I have them - sadly the pins are just a little too close together to fit perfectly into the strip board.
Pin 1 accepts accepts the positive lead from the powersupply. Pin 3 is ground ( negative ). Pin 5 is the output. Pin 2 and Pin 4 are used to determine the correct voltage and current.
R3 = 0.45 / Amps
So in my case I wanted it to limit the current to 700mA
R3 = 0.45 / 0.7 = 0.64 Ohms
In my case I used 3 different resistors to get close to that value - 1,2.5 and 5 Ohms. The way to calculate resistors in parallel is
1 / (( 1/R1)+(1/R2)+(1/R3))
in my case that is
1 / (( 1/1) + (1/2.5) + (1/5))
= 1 / ( 1 + 0.4 + 0.2 ) = 1 / 1.6 = 0.625 Ohms
Which is close enough! To work out the current you get from a set Ohm value you can go backwards - its useful to find out how your approximations with resistors gets you.
Current = 0.45 / 0.625 Ohms = 0.72Amps
The power going through R3 is 0.45*0.45 / R3 in Ohms
In my case this is 0.45*0.45 / 0.625 = 0.324W, considering the 3 resistors allow a total of 0.75W we are well within the tolerance.
Working out the value of R1 is easy.
R1 = (Vout/2.77 - 1) * R2
We know what R2 is 820 Ohms and we know what we want out VOut to be so ( in my case )
R1 = ((6.5V/2.77) - 1) * 820 = 1104 Ohms
The simplest way is to attach your multimeter to Vout and then adjust the potentimeter.
1) your Volts IN needs to be about 2Volts higher than your required Volts out.
2) The chip burns of the excess voltage/current as heat. To keep the heat down try not to have VIN much greater than VOut - taking into account point 1.
To work out the Watts being dissipated by the chip you need to do (Vin-Vout) * current selected. Mine version is 12V-6.5V * 0.7 = 3.85W. I have also clipped a heatsink to my chip and the box DOES get quite warm - though it seems quite capable of dealing with it. Things might get very tricky if Vin was 24V and Vout was 6V and you were at the full 2A current.... pretty hot at 36W .. FAN PLEASE lol
Step 6: Building the Circuit - Step One
Make sure you have your soldering area setup and your components near to hand. I use a sponge to help keep my components in the board when I turn it over to solder... hmmm it just occurred to me.. would blue-tack or some kind of putty help to hold them in place... I will try that next and and let you know..
Print out the strip board diagram and have it where you can see it. Remember that as you set your components onto the board you need to leave that one hole border left and right so you can slide it into the box.
If you have had little experience soldering - do not worry - there are plenty of links on the internet and a strip board is one of the easiest ways to get some practice in.
Step 7: Building the Circuit - Step Two
Once you have built the circuit minus the final power leads, its a good idea just to tie on some temporary leads ( so that they touch the correct copper row ) so you can test the circuit. First measure the current with your multi-meter and then the voltage. Adjust the potentiometer until you get the required voltage. Then you can solder in the final power leads and then insert the circuit.
You will then need to attach the input power leads to the DC input jack ( shown in picture 3 and 4 ). You should also add the headsink to the L200C - you can see it in picture 4. You can see that the spades/crocodile clips have been connected too in Picture 4.
One final tip - if you circuit board is loose fitting, you can add a few dabs of glue where the board is slotted into the box, ie on the runners. This will stop the board moving up and down. You can also see from the images that I have the board situated so that the chip is as close to the centre as possible - as far away from the plastic as I could manage. Saying that, in the configuration I choose the box doesnt get hot.
Step 8: Finishing Up
The first picture shows the box with all the connections made. The 2nd with the lid on and 3rd and 4th charging the battery.
If anyone is interested in purchasing a kit to build yourself I have a few for sale in my ebay shop
There are actually two kits, a basic and an advanced kit. The basic kit provides you with a much more detailed explanation that found here but with pretty much the same outcome. It gives you all the components you need to build it apart from the tools. The advanced kit comes with two knobs and larger potentiometers so you can adjust both the current and the voltage. There are also metal box versions.
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