Battery Spot Welder

You do not solder batteries. More precisely, the people who do are the ones who later wonder why the pack runs hot and loses capacity: soldering means prolonged heating, and an 18650 does not take that well — both its internal resistance and the safety mechanisms inside suffer.

The right way is to weld nickel strip on: a short pulse of current melts metal at two points in milliseconds, and the cell itself has no time to heat up.

The welder runs off its own battery, so there is no mains cable anywhere near the cells. The unit has no name: there is no model and no serial number on it, so everything it does is described here.

What is adjustable

Three parameters on the screen:

• The pre-pulse — a short pulse before the main one. It breaks through the oxide and presses the strip onto the cell, so that the main pulse goes through a proper contact rather than an air gap.
• The main pulse — the one that actually makes the weld. The longer it is, the stronger the spot and the more heat goes into the cell.
• The delay between them.

In automatic mode the welder fires by itself as soon as both electrodes touch the strip. There is no separate trigger, and that is one reason to be careful: contact is already a weld.

Practise first

Do not start on real cells. There is a stock of nickel strip in the container — take a piece and weld it to something you do not need, an offcut of the same strip, a dead cell.

You need a feel for two things: how the parameters change the spot, and what a good weld looks like. A weak spot peels off by hand and leaves the strip smooth. A strong one tears the strip but does not leave the cell — pulling it off leaves a hole in the strip and a trace of metal on the can. That is what you tune the pulse length for.

How to use

1. Take the welder and the strip from the container.
2. Practise on offcuts until the spot comes out consistent.
3. Cut the strip to length, lay it on the cell, press the electrodes down.
4. Place the electrodes close together but not touching: too close and the current goes along the strip instead of through the cell, too far apart and the spots come out uneven.
5. Make two spots per contact, not one.
6. Check by hand: the strip should not lift.
7. Put the welder and the leftover strip back in the container.

Care with lithium

An 18650 is not a toy. A 2–3 Ah cell can deliver tens of amps into a short circuit, and that ends exactly one way: a fire that water does not put out.

• Watch that an electrode, the strip or a tool does not bridge plus and minus. In a pack that is especially easy: the strip is long and the next cell is right there.
• Do not weld to the side of a cell's can — ends only.
• Do not weld damaged, swollen or suspiciously warm cells.
• Do not leave a half-built pack with bare strips sticking out.
• Work somewhere you could throw a cell if something goes wrong.

If you are unsure, ask us before welding, not after.

Renting it

The welder goes home with you. The strip is a consumable: take what you need from the container, and if you have used up a noticeable amount, tell us so that more gets ordered. The daily rate and the term are in the rental block on this page.

Access

The welder is in a container on the shelf in the hackspace room, so a plan that includes hackspace access is required. No induction is needed, but read the lithium section before your first weld.