Working Supersize 9 Volt Battery Made From Old Lead Acid Cells

Back in the days, when I was still learning about microcontrollers and stuff, I used to make a lot of IC and circuit-based projects. Since all of those projects could be easily powered by single lead-acid batteries or with different variations of those batteries, I used to buy them in bulk. As time passed, I started replacing IC circuits with microcontrollers and lead-acid batteries with better and more efficient Lithium-ion batteries.

A few days back, when I looked at my battery container, I found a huge chunk of those lead-acid batteries, just lying around and getting wasted over time. I thought of putting these old 4v batteries to some good use and make a portable variable power supply with them.

Initially, I planned, just to put the batteries in a group and hook up a voltage regulator module to it, but then I thought I can make this project much better and good looking. I am planning of putting these batteries in a group and cover them in a metallic casing so that they resemble a 9v battery. Therefore having the features of a portable variable power supply enclosed in a package of supersized 9V battery. Wouldn't that be nice and bring back all those memories, when 9V batteries used to be the most prominent ones in the market.

Supplies

Old batteries ( I am using 4V lead acid batteries. If you don't have lead acid batteries, you can salvage Li-ion batteries from old laptops and electronic devices)
Buck convertor (LM2596)
Voltmeter
10K potentiometer (choose a medium size potentiometer and don't forget the knob)
ON/OFF switch
DC power jack
Aluminum sheet
MDF board
some colors( spray paint would work fine)

Step 1: Charging the Old Batteries

My batteries were kept in the cupboard for a very long time and because of this, they had lost some amount of their charge. Generally, lead-acid batteries lose 4% to 5% of their total charge in one year but this percentage may differ according to the life of your battery. So before going any further, I had to make sure that all my batteries were charged to a similar voltage level, that is, around 4V. For the charging, I did not use any balanced charger or any specialized charge. Down below, I have mentioned two methods for charging. Both of them are equally efficient and easy to use.

METHOD 1:

I personally used to method to charge my batteries. I simply hooked up the battery to a variable power supply and turned up its voltage to around 4.2V. Since many of my batteries were on a similar voltage levels, I clubbed them together in a group(connected them in parallel) and charged them from a single power supply. You should not practice this method if voltage gap between the batteries is high, as it may cause unbalanced charging or a sudden shoot up of current and may hinder or damage their internal chemistry.

METHOD 2:

If you don't have a variable supply, you can simply charge the batteries by hooking them up to a mobile phone charger. Today, almost all the smartphone chargers output a steady 5V current (fast charging is neglected). If we hook up a silicone diode in series with the charger, we get 4.3 volts at output. This is because silicon diode has a barrier potential of 0.7V and using it in series will cause a voltage drop. As charging of lead acid batteries with 4.3V goes hand in hand, you can very easily charge them up with this method. Just make sure diode is forward bias else no current will flow through it. To forward bias the diode, connect its cathode to positive of the charger and anode to the positive of the battery. Connect negative of charger to negative of the battery.

Step 2: Making a Battery Pack

When all the batteries were charged up, I began to group them together. While integrating the batteries, I had to keep three aspects in mind, which were:

Dimension of the battery pack. When everything would be done, the whole package should resemble a 9V battery(volumetric ratio of a 9V battery and our battery pack should be similar). Since most of the space is acquired by the batteries, they need to be positioned correctly.
Terminals of the batteries should be align properly so that connecting wire to them is not a hassle and there should be no tension in wires once wiring is done.
It should have a space or void for the electronics, such that the structure also provide support and protection apart from accommodation.

I was using nine of these 4V batteries and decided to break them in group of two. The first group will have six batteries and the second will have three. The smaller group of three batteries will rest on top of the larger group. The larger pack will be in shape of a rectangle and will act as the base of the system and the smaller pack will be in 'L' shape and resting over it. The void or gap of 4th battery will accommodate the electronics and protect them.

To stick the batteries together, I used thick double sided tape. It has a strong grip and also provides cushioning against clashing. Right now, I will only make the two battery packs. I will bind them together once the electronics part is done, as it is easier to work when they are apart.

Step 3: Connecting the Terminals of the Battery Together

The terminals of the lead acid battery are also made from lead. When they are exposed in air for long time, the lead metal gets oxidized and forms a protective coating around itself. This coating prevents further oxidation as well as does not allow the solder to stick on lead. So before connecting any wires to the terminals, we have to get rid of this coating. One good way to do so is by sanding. You can use a fine grit sand paper or a file. Don't sand the entire surface, just do enough that you can connect wires to them. With two three strokes of file on top of terminals, I was able to solder them easily.

As you know, I have 9 batteries in total. Going through various combinations, I found out that putting three batteries in parallel and forming a group, then connecting those three group in series works best for me. This combination outputs 12V at 4.5Ah which is sufficient for my day to day work.

So as mentioned above, I did the same. Connecting 3 batteries in parallel gave me three battery packs of 4V 4.5Ah output and then by connecting those three battery packs in series, I gained a net output of 12V at 4.5Ah.

Step 4: Adding a Voltage Regulator and Power Switch

As of now, our battery pack can be used as it is and it will output a steady 12V current but I want it to be more flexible and cater to different voltage levels also. To achieve this, I added a variable buck convertor to the battery pack. By doing so, I can now get voltages like 5V and 3.3V which are very common in digital electronics and microcontrollers. If you works with voltages higher than 12V, you can hook up a boost convertor instead of buck convertor and get the desired results. The process is almost the same, just make sure your voltmeter is rated for that king of high voltages.

I am using LM2596 buck convertor because they are pretty cheap and can also has a stable voltage with good efficiency. According to the datasheet of the IC, it can output 5Amps of current and can go as low as 1V when being powered from a 12V supply. To this buck convertor, I also added a general purpose ON/OFF switch as it does not have any inbuilt switch or power saving mode. If you notice, the potentiometer(generally blue colored) on buck convertor is very small and need to be adjusted using a screwdriver. To overcome this restriction, I desoldered the stock potentiometer and solder a new 10K medium sized potentiometer. Now we can easily changed the voltage levels. Below are the wiring steps:

Connect negative input of the buck convertor directly to the battery pack
Connect positive input of buck convertor to pin 1 of a switch
Connect pin 2 of switch to +12V of the battery pack
Solder a pair of wires to the output terminal of the buck convertor and leave the other end as it is. We will connect them later

TIP: To desolder the potentiometer, you can use a desoldering wick but if don't posses one, you can remove it through excessive solder method. Melt some soldering wire on the terminals until the solder forms molten tracks. Once the molten solder track is hot enough, gently pull the potentiometer from the bottom. It should come right out. Give a little tap to the module and all the excess solder will fall off.

Step 5: Installing Voltmeter

Our variable power supply is installed and is working perfectly. Now to see how much voltage it is outputting, we'll need a voltmeter. For that, we can use our trusted friendly multimeter, but for such a task, a multimeter would be an overkill. Also, most of us have only one multimeter and if it is engaged in our power supply, we cannot use it for other purposes. So installing a voltmeter which can always give us live output reading seems a good choice.

I personally like this small digital voltmeter that I am currently using. It works on 12V and can operate in voltage levels ranging from 0V to 99V. It has a very compact form and gives fairly accurate readings. To connect your voltmeter, follow these steps:

Connect positive power of voltmeter to input of buck convertor
Connect negative power of voltmeter to negative input of buck convertor
Connect signal of voltmeter to positive output of buck convertor
(Optional) I your voltmeter have a negative signal pin or wire, connect it negative output of buck convertor

Step 6: How to Charge the Battery Pack?

After the project is made and we use it for some time, we'll need some source to recharge the exhausted batteries. Taking out the whole assemble out and recharging each cell individually is really hectic. We need a charger which can recharge the batteries while keeping the whole assembly intact. Since our lead acid batteries are flexible in terms of recharging, I will use a 12V specialized charger for charing.

I used to use this charger for charging my old 12V lead acid battery. It outputs around 14.4V and can very easily charge our battery pack. It automatically detects the charging level and cuts power when the battery is fully charged. Charging the batteries with a specialized charger will give us maximum battery life and efficiency. But if you do not have a specialized charger, you can directly hook them up to a 14.4V constant voltage supply and charge them.

To access the battery terminals from outside, I simple connected a DC power jack to battery pack.

Connect positive of terminal of power jack to +12V of battery
Ground of power jack to negative terminal of the battery

Step 7: Packing the Batteries Together

Electronic part of this project is complete now. As I told you earlier, I will be placing the smaller battery group(of 3 batteries) on top of the larger batter group(of 6 batteries). Directly placing the batteries on top of each other may damage the terminals and hence the entire system. Therefore we need some sort of cushion between the two. For that, I am using some general purpose medication cotton. These cotton are soft in nature and provide excellent cushioning. You can also place a thin spongy instead of cotton but I do not have any of them lying around so had to work my way out with cotton only. Use scissors to cut the cotton in shape of you battery and don't use it in excess. Extra cotton will only flow from sides and acquire space hence increasing the size unnecessarily. To hold this entire assembly together, I used some masking tape. You can use any general purpose tape as long as it has good adhesive power and tensile strength. Try to put hefty amount of tape in there. Also put some tape on cotton as it may try to flow and leak from the sides.

Step 8: Making the Outer Casing

For the outer casing, I initially planned to use MDF board or plywood. Then I switched to acrylic sheets as it was much easier to work with acrylic. Later I rejected all of these options and went with thin aluminum sheets. They were cheap and resembled the body of a 9V battery much better than others.

I bought this sheet from a local hardware store a while back. Though it is not completely rigid and cannot provide great structural strength, it will definitely work in our case as the batteries themselves have good enough structural strength to hold the entire structure together.

I started by making a CAD design of the casing and drew it on the metal sheet using a ruler and a marker. You can do this more easily by printing out a stencil design. Using a metal shear, I removed the required part from metal sheet. I located the points where the sheet was to be folded and removed small equilateral triangles from extremis of those points. These triangular voids will help us in bending the metal easily.

To bend the sheet, I slipped it under a big MDF board and staring apply pressure on the bending edge using my hand. You can also use some piece of wood or hammer to apply pressure. For joining the two ends, I used a double seam joint. If you don't know what a seam joint is and how to make one, I recommend you to go to youtube and watch some videos. It is pretty easily to make and a very common joining process. The three 10mm segments at the extremis of stencil are used for making this joint. Once the joint was made, I secured it with some superglue. Brazing can also be done for securing the joint but I did not have aluminum solder so had to do it with superglue.

Step 9: Making the Terminals and the Base of Enclosure

For the sides, aluminum sheet worked fine but for the base, they could not hold up the wight of the batteries. I needed something sturdy and hard for the base so I used 4mm thick MDF board. It was hard enough to support all the batteries and was not even flexing. I removed two pieces from the MDF board, one for the top and one for the bottom. Dimension of the pieces were same as that of the outer casing ,which is 102mm X 50MM.

On the top MDF board, I drilled holes for the output wires of buck convertor, potentiometer and the switch. I used combination of drill and Dremel to make perfect holes. For voltmeter and DC power jack, I made holes in the the aluminum casing. For the switch, I placed it inside the positive power terminal as it was a perfect fit there.

For making the terminals of the big battery, I used the same aluminum sheet that I used for the outer casing. Aluminum being a conductive metal can pass electricity hence we can use our showcase terminals as actual output terminals and channel power through them.

For making the positive terminal, I simply rolled up a thin strip into a circle and then using some superglue, connected the two ends. I also rolled up the edges of the upper side of terminals so that they get blunt and do not cut our skin.
For the negative terminal, I made two concentric circle on an aluminum sheet with radius of outer being twice as that of inner circle. Then I made three diameters, each being at an angle of 120 degree from the other. From the points where dimeter cuts the inner circle, I projected straight lines on outer circle. Doing this gave me a star like structure. I removed that star structure from the main sheet and bended its arms perpendicular to the base. This is how I made the negative terminal.

Step 10: Painting!

By now, the battery started to take shape but it looked a little dull and unfinished. I decided to give it a few coats of color, to bring out the picture and resemblance. I had an old 9V battery lying around that I used for reference. Using a marker, I drew the necessary partitions on the case and painted the body with spray paints. Since the miniature battery that I have is the most common one used in my country, I used exactly the same color combination of red, white and blue for my design. For the top and bottom MDF pieces, I used only black paint. Once the color was dried, I drew some details and text to make it look more realistic.

Step 11: Summing Up the Project

Everything is done now, we just need to put it together. I started by putting the outer cover on top of the electronics. Then hot glued the voltmeter and the DC power jack to the aluminum casing. I first disconnect the switch from the electronics, hot glued it on the MDF board and reconnected it to the buck convertor.

You remember those output wires that we left unconnected, take them and connect to the terminals that we made few minutes back. Put some hot glue on the terminals and stick them to the MDF board. Put everything together and close the metallic lids of the outer casing.

Hey, the project is complete now. Thanks for staying so long and giving your time to this project. Hope you liked it. Please like and subscribe to my YouTub channel and also subscribe to me on instructables to never miss any project made by me.