Intro: Magnetic Connectors for Batteries
Here is a small tutorial about useful and easy to make battery connectors. I recently started to use 18650 cells batteries from old laptops, and I wanted a quick and easy way to connect them. Connectors using magnets were the best option, but I had to figure out how to use them correctly.
UPDATE:Thank you all for your comments! I have added a new step at the end to answer the most relevant and recurrent questions!
One connector is made of 2 magnet that keep the electric wire between them. Then I used hot glue to protect, glue and insulate the connectors.
- They are really easy to make
- They are really easy to use
- No soldering iron is required
- They avoid short circuits as the connectors repel each other, and if they attract it is on the insulated parts (as both connectors free the same magnetic pole).
- It takes 5 minutes to make (time for the hot glue to harden).
Step 1: Magnets + Wire
In the first step I just added the wire between 2 magnets.
Step 2: Plastic Support
I have put this on a transparent piece of plastic. I used this piece of plastic to melt the hot glue on it and detach it easily. So make sure the plastic is smooth enough (to detach the hot glue) and heat resistant (at least a minimum so it does not deform).
Then I have added a third magnet below the plastic to stick the magnets above to the same place when I am pouring the hot glue.
Step 3: Add the Hot Glue
To stick everything together and insulate the magnets, I proceeded as follow:
-I cut a small part of PVC tube (about 1 cm high, and 2 cm of diameter; it has the same diameter than the 18650 cells)
-Then cut the PCV ring so it is open
-I added the hot glue on the magnets using the PVC ring to maintain the hot glue. (Make sure you are in a well ventilated area)
-Wait few minutes the time for the hot glue to harden;
- If it is not long enough the hot glue will be still liquid
- If it is too long, the hot glue will stick very well to the PVC and plastic piece and it is hard to detach
-Then detach the hot glue
-And remove the PVC ring!
Step 4: Use It!
Now that it is done you can use it to connect easily any electronic device to your battery!
If you make the north pole magnets of all the connector pointing in the same direction, they will repel each other, that is useful if you don't want short circuit. And if you want them to connect, just make other connectors with the magnets upside down!
Step 5: Questions & Answers
In this step I answer some of the frequently asked and most relevant questions:
- What is the resistance of the connectors?
As my multimeter was not accurate enough to measure the resistance of the connectors, I have used the four-terminal sensing technique, and measured the resistance of others connectors to have a comparison:
- One magnetic connector: R=50 milliOhms
- One magnetic connector soldered to the wire (see next question) : R=17milliOhms
- One wire of the same length than the 2 connectors above : R=17milliOhms
To finish, at this range the multimeter's wires resistance might influence the results given above, so the resistance could be even smaller.
As a conclusion I would say that the resistances of the connectors seem quite low to me. The connectors with one wire stuck between 2 magnets without soldering has the higher resistance with 50 milliOhms. Then the connectors soldered to the wire, and the wire alone have the same resistance of about 17 milliOhms.
- Why not soldering the magnets to the wires directly?
I have tried before publishing this instructable, and here are the main reasons I did not solder my connectors:
- First I had some problems to solder the wire to the magnet, the tin did not stick to the wire correctly and flowed on the magnet. I have tried later with another magnet but I had no problems. So I think some magnet might be coated with some kind of product to protect them or whatever.
- The soldering iron is magnetic and it sticks to the magnet. So be prepared if you plan to solder a magnet!
- The heat of the soldering iron might demagnetised the magnets, if the soldering iron is in contact for a long time with the magnet.
- I think it was interesting to publish this article without the use of soldering iron
On the other hand, the resistance of these soldered connectors seems lower, so it is quite interesting as well!
To conclude I would say that both the sandwich technique (wire between the magnets without soldering) and the soldering techniques are useful depending of what you want to make. And to finish the hot glue (no matter which technique you have chose), is something I would recommend because it has 2 main properties:
- It insulates the magnets (and the magnets stick to everything that is ferro magnetic!)
- And it protect the wire end close to the magnet
- Will the magnets drain the battery?
No the magnets will not drain the battery (unless you use them to short-circuit the battery!).