We know that 18650 batteries or other Battery packs, including Tesla's battery packs, typically use a spot welder (Spot Welder) during industrial welding. Spot welders, as the name suggests, are electrical welding at a certain point or local area. Spot welders have many principles, such as arc spot welding or resistance spot welding. Of course, there are many products on the Internet, ranging in price from a few hundred yuan to several thousand dollars. The spot welder looks like this:
The welding effect is as follows:
Check the information and find that the principle is quite simple. As an EE essential skill that can make a girl's eyes shine, electric welding must be mastered. His welding principle is summarized in one sentence: a large but very short pulse current at a certain point causes the local metal to melt and join.
(Source: Yokodai.JP Personal Spot Welder PRINCIPLE OF SPOT WELDING)
In the figure, the current is concentrated at two points, so the local part will be very red hot. With the pressure of the external crimping, the metal piece can easily be fused with the battery electrode to form a solder joint at the corresponding part:
The picture above shows that AC welding is more secure than DC soldering, but the principle seems to be less correct (the current reverse current effect should be the same).
Ok, let's start DIY below.
First of all, I have a broken microwave in my house. Disassemble and remove the transformer above:
This is a picture of someone else, because I forgot to take it when I removed it, I used it first. This is a step-up transformer with 220V (or 110V) on the primary side and 2000V on the secondary side. The rated capacity is 1000VA. Use a chainsaw to saw off the secondary winding:
Change the thick wire and wrap it for 3 turns, which is the case below. After one side is connected to 110V, the output of the secondary side is 2V (AC RMS)
The square above is a solid state relay that uses Arduino to control the primary side break. Its circuit diagram is as follows:
The difference is that I replaced the Timer in the figure with Arduino, so I can set the width and number of the pulse current myself.
Complete the map:
There are also welding electrodes on the wire, but it is a pity to sell 10 pieces at a time, a total of 20 knives, I think it is not worthwhile, so I used Dremel to process copper bars to do two:
Then, in order to compare the electrodes, I made some other shapes and different thicknesses:
Then start welding ~~~
Just the old EV brother left a lot of bad batteries, let me practice first:
There are several variables that I need to adjust: welding pressure (by hand control), electrode, number of pulses, pulse width, pulse interval.
Repeated trials found that the double pulse works best, the first pulse is 100 milliseconds, then 300 milliseconds is broken, and the second pulse is 250 milliseconds.
The theoretical maximum output current is 500A (1000VA/2V), but the actual value is definitely less than this value, estimated to be around 200A (actual welding current and electrode material, battery electrode material, welding pressure, electrode shape are related).
If the thickness of the nickel sheet is 0.15mm or more, it is recommended not to use ordinary nickel sheets, but to use such nickel sheets:
Although the nickel piece is thick and has a slit in the middle, it ensures that 90% of the current passes through the lower battery electrode to form a solder joint, otherwise the current will mostly remain over the nickel piece, and the natural welding is not reliable. If you want to solder into a Tesla battery pack shape:
Then you need electrodes of other shapes:
It is to press the wire to the surface of the electrode. Of course, DIY can be, but the industry uses more advanced methods, such as:
This connector comes with an in-line wire that can be smashed like a sewing machine.
I bought a solid state relay for the entire spot welder, $5, and the others were all from the garbage dump, so basically no money...
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