Basics: Coin Cell Battery Bank

I like to use the CR2032 coin cells in projects. They are small and put out 3 volts. When they no longer have enough of a charge to operate the project, I toss them into a jar in the hopes that someday I'll make a Joule thief or something to make use of the rest of the charge.

There is another way. I found this idea to be an effective way to use those old 3 volt batteries that put out less than 2 volts anymore.

My first effort involved buying a bunch of two-cell holders with a switch. The switch was the best part. But, the wires on those are so thin that they invariably tear off before my interest in the project wears off. That was not the answer.

Then, I got these single cell holders. I tried to solder wire to the leads and make a series of single-cell batteries that way. Those containers would usually fall apart or the wires would pull off or something bad would happen.

Finally, I got this idea, and it seems to work very well. Basically, it's comes down to a quasi-printed circuit board, but with grooves cut into a one-sided copper clad board instead of a printed circuit.

Let's begin.

The tools I use are those that I always use. Some of them may be substituted.

• Dremel tool with a diamond cut-off wheel. The cut-off wheel sounds costly, but they are inexpensive at about $2 for 10 of them on eBay.
• PC and printer
• Drill (Dremel works fine) and small 1/16" drill bit
• Soldering iron and solder
• Long nose pliers and other ordinary tools
• Volt-ohm meter

The materials included the following:

• Avery label - just about any Avery or self-sticking, printable label that measures 2" x 4" or that can be pieced together to make 2" x 4" will do. I used 1" x 3" labels.
• Battery Button Coin Cell Socket Holder Case for CR2032 or CR2025 coin cells. There are a number of styles, For this project, I used the style shown in the picture. I bought these on eBay for about $3 for 20 of them. You need six.
• Single-side copper clad board 70mm x 100mm. I got these on eBay at about $4 for 10 pieces

I made my template in Paint and then copied it into an ApacheOffice Writer document for printing.

This is not rocket science, but I've attached a PDF of the file I used. The critical thing is that you need to make the board close to 2" x 3 3/8" for the most efficient placement of the coin cell holders. Notice that I have depicted the coin cell holders so that the positive and negative ends create a series connection for the batteries.

The small, rectangular end is the positive side and the round end is the negative side. Notice that the pattern is circular. That is, on one side the rectangular ends point to the left and on the other side they point to the right. This is important. If you fail to do this, you will short out the batteries and they will either cause a fire or burn out quickly.

Print the template onto the Avery labels.

I happen to have a very old box that used to contain 5000 labels for a dot-matrix continuous sheet printer and has a lot left. I have been using those for years and years. In my case, I needed to re-position the labels so that there were no gaps between. Then, I printed the template onto the labels.

The objective here is to make grooves into the copper clad side to partition the positive and negative sides of the batteries, so that there is no electrical connection between the partitions.

In my case, it is necessary to paste the template onto the polymer side of the circuit board. The reason being that I am making the grooves with a mini-table saw that I fashioned out of a Dremel tool. So, it's like cutting rabbets or other grooves into wood. They are done from underneath. Hence, I paste the template onto the bottom side; the polymer side of the copper-clad board.

If you approach the groove cutting from the top, then you would paste the template onto the copper-clad side. Then, you could simply hold your groove-cutting tool in your hand and follow the lines.

I made a sled for my mini-table saw and find it very useful for this purpose. I raise the cutting wheel just high enough to make a groove into the copper that would penetrate into the polymer backing.

Using my template, I cut the board to size, 2" x 3 3/8" (or 5 mm x 8.5 mm). Because the cutting wheel depth is hard to set, I set it to make grooves and not to cut off the circuit board. So, I made one cut on one side and then made another cut on the other side to cut the board to size. I could easily snap off the excess with this double-groove.

Then I cut shallow grooves into the the copper-clad side. The grooves are just deep enough to penetrate the polymer backing and sever the various partitions from each other. I cut three grooves cross-wise and one groove almost to the end. Look closely at the picture with the grooves, I put a note at the area where the groove stops.

If you use a top-down method of cutting the grooves, the same basic objective remains. Somehow you want to make grooves into the copper to separate the various sections.

If you marked the holes in the template using the coin cell holder as a pattern and marked the template with them, it is easy to drill the holes in the right place. Just drill the 1/16" holes through the board at each end of the cell. You need 12 holes altogether.

Now, following the pattern on the template, mount the cell holders on the polymer side and push the contacts through the holes so they penetrate through to the copper side. In my case, I didn't like the protruding sharp points of the contacts, so I bent them over with a long-nose pliers and then soldered them to the board.

Notice that ALL of the cells straddle the grooves, thus separating the postive and negative sides of the batteries and allowing the positive and negative poles of the battery series to connect one battery to the next one.

When you finish, it is a good idea to test the connectivity and non-connectivity with an ohm meter, if you have one. I tested the copper side by itself to make sure that there was no connectivity between partitions. You may go one step further, and as you add the batteries, starting at the upper left corner (negative end), connect the negative lead of the voltmeter to the copper board there. Then add the first battery, and see if you get a reading on the voltmeter. If so, proceed to the next battery. You should be getting higher and higher voltage as you go along.

Notice in the copper side picture associated with this step, that the grooves and soldering connections are depicted.

Be sure to check the notes in the copper side picture, they tell you where to solder the postive and negative leads. Heed those instructions. I used a red lead for the positive pole and a black lead for the negative pole.

Make sure that you solder the positive and negative leads to the end of the board where the longitudinal groove is cut all the way to the end.

Most of my batteries had 1.5 to 2.5 volts. If you measure the voltage of each one, sum them up. Then mount the batteries into the cells. Measure the total voltage at the leads. You should get roughly the same voltage sum on your voltmeter as you got when you added the individual voltages together.

You are not limited to using this six-cell battery bank with six cells. If you want to use four cells,you just need to solder a lead across the gap of both of the second cells from the top. If you want to use two cells, you solder a lead across the gap of both of the first cells from the top.

Look at the pictures accompanying this step.

Later, if you want to use more again, you just need to unsolder the leads.

All of this could be easier with a couple of switches. Your choice.

Good Luck!

I should have mentioned a couple of cautions:

1. If you decide to use fewer batteries and you solder a wire across the middle groove, be sure there are no batteries in the lower cells. They will short out and get hot.

2. Remember that the copper side of the board is electrified, and if placed on a metal surface, the probability is very high that there will be a short. I'd suggest covering the whole copper side with an insulator of some kind. Shipping tape, strapping tape, anything that will insulate the whole copper side of the board and prevent an accidental connection or short..