You don't know til you try . . .

DIY Capacitive Discharge Spot Welder, AKA Battery pack maker

, posted: 1-Jan-2010 21:36

Cheap laptops and long battery life don't usually go together.
With reasonably powerful laptops going for under a $100, heck under $50.
p3 1Ghz - P4m 1.6Ghz
Powerful enough for what I want to do with them, run simple VB apps.
1 or 2 hours is not enough.
I want it to run the whole day dammit. 8+ hrs

Got into one of my must have modes.

After a bit of reading I find Lithium batteries don't loose capacity they just loose the ability to deliver it at sufficient amps.
i.e. internal resistance builds up.

I had a few battery packs saved from our last clean up.
You know ones for 5 year old laptops.
Surprisingly most still worked after recharging.
One was sitting at 0.2v.
Once charged, it delivered 1600mAh at 1A, nice.

After taking apart 5 packs I didn't look forward to soldering all those cells.
So I started looking at doing it properly.
The proper way of course is welding.
But the machines aren't cheap, us$2000+

After reading some success stories I though I would give it a go.
Many more, google away "capacitive discharge welder cheap"

Most designs used SRC's as the main switch.
The only down side is they don't turn off until there is no current flowing.
So either all the energy is delivered or you break contact.
Not the most precise control but simple and cheap.

I didn't have any on hand but I did have some high powered MOSFETs.

The numbers looked right for the job
Max Voltage Vds:30V
Pulse Current Idm:960A

The disadvantage with a MOSFET is the on resistance(RDS on).
At 1000A even 0.01 Ohm is significant.

I had a few so I went wild and paralleled 6 of them up.
Could have gotten away with less but better to be safe than sorry.
I am sure I could have done it more elegantly but I wanted to make sure it worked before putting in the effort to pretty it up.

To ensure fast turn on and off, critical at these loads, I used a Driver IC, rather than just a simple transistor.

AVR Tiny13 as the controller.

Drew up a pcb in eagle and milled it out.

Put some code together in Bascom AVR.
Assembled and did some low voltage tests, looking good so far.

I went with the Dual Pulse timing.
Why not, it's only a few more lines of code.
Adjustable via Variable resistor.

Away from any sensitive electronics.
A 1000A pulse generates a 'little' emf.


Power supply is a universal laptop ac adapter. 15-24v 3A
You don't really need anything high powered.
The only advantage is recovery time.
But since we are using a controlled pulse, less energy is wasted so anything with a suitable voltage would do.
What voltage?
From what I have read and gut instinct, as high as the capacitor will go.
Up to 40v. Above this and YOU may start conducting.
Only a light tingle but I would avoid it.
12v will do the job, 24 is better though.
Shorter pulse, better penetration.

The capacitor came from TM
$85, 2.0 Farad Capacitor- American Accessories

Nickel strips/strapping from
I have to give Tony a plug, I was able to collect some on new years eve.
Now that's service.

Wiring from Dick Smith.
4 Gauge Power Wire , 85A 12v
The ends of the cable are not soldered in.
Just held in by pressure of the cable ties.

Probe Tips,
PolyMax 3.5mm Gold Connectors

Misc copper wire.
Spare house wiring I had.

And the shopping list comes to (*I had to buy, rest was on hand)
*85 Cacacitor. Could have saved if I went hunting
30 6x Mosfets. I am sure I could have gotten away with just 2.
50 Laptop AC Adaptor. A used 16v+ one will do the trick.
--controler board
0.5 12v regulator for driving MOSFET
0.5 5v regulator for microcontroller
3 mosfet driver
2.5 Tiny13 a 555 timer would have worked as well if you didn't need the dual pulse
*10 1m 4ga/ Cable. Could have used thinner due to my short run
5 Misc, Button, Switch, Led, VR, Cable tie, Heatshrink, Tips

I am sure I can get it below $100 with a little planning.

Back to the laptop battery.
My test subject will be a IBM R40e.
It will run with 12v via the DC port.
Unfortunately this does not give you a runtime or graceful shutdown when it runs out of power.
And you have to charge them somehow.

The other way is to wire the extra cells into the existing battery controller.
This should work.
It will need to be cycled at least once for the controller to see the extra capacity.

To do, put the switch on the probe.
Maybe use thinner wires if it becomes too difficult to work with.

That was my 2010 new years eve, what did you do?

Photobucket Photobucket Photobucket

Paralleled MOSFETs
Photobucket Photobucket

Control board

Cable ends

Test Setup

It works

Test Battery pack

Other related posts:
Mobility Scooter eCarts conversion
Samsung galaxy cutdown
How big a heatsink do you need to run a green laser pointer continuously?

Comment by Emil Georgiev, on 22-Jul-2010 21:30

I am interested to get urgently spot welding unit for battery pack on cheap price. Can u help me in this matter? Emil Georgiev Sofia, Bulgaria

Comment by Charles, on 28-Aug-2010 18:00

Would you be willing to make and sell me one? I really liked the dual pulse welder. I could buy the capacitor myself but the rest I would prefer to just buy as a complete. Thanks.

Author's note by MrWestie, on 29-Aug-2010 07:47

Hi Charles, my one has been sitting idle since I built it. You are welcome to buy it :) It really needs an extra capacitor for more power though.

Comment by jonathan brophy, on 8-Sep-2010 11:22

id bee keen on one of those i work for danfoss and i have several igbt blocks i was going to do it with but i dont think they are big enough. they were pulled from a 15kw vsd but i think are rated at 160a peak.

Comment by jonathan brophy, on 8-Sep-2010 11:22

id bee keen on one of those i work for danfoss and i have several igbt blocks i was going to do it with but i dont think they are big enough. they were pulled from a 15kw vsd but i think are rated at 160a peak.

Author's note by MrWestie, on 8-Sep-2010 11:41

Hi  Jonathan, you are welcome to this one. Drop me a pm via GZ and I'll see about getting it to you. :)

Comment by Jeremy, on 19-Oct-2010 17:01

Hello, I have been interested in building a spot welder for some time now. I'm very curious as to the quality of the welds and recharge rate between welds it gets? (and do you still ahve this one?) -Jeremy jerry_hayes2(

Comment by james Shriver, on 6-Feb-2011 20:24

Dear Mr. Westie (Charles) If you still have this welder and it can spot weld nickel strips approx. .003" thick to the anode and cathode sides of various rechargeable batteries, I would like to buy it from you, that is if you still have it!?! I been doing quite a bit of research on these type of projects lately and I'm convinced the dual, timed pulse is what makes or breaks it. Please e-mail me considering the price would you be interested in making more? What would you consider a fair asking price. I AM going to buy one from someone eventually. Thank You greatly for your time. James Shriver

Comment by james Shriver, on 6-Feb-2011 20:31

Mr Westie....please see above post: I didn't know my e-mail wasn't going to be posted her it is: Thanks.

Comment by Mario Tovar, on 4-Jul-2011 03:29

Do you have any wiring diagrams or parts list? Are they available for pùrchase?? My e-mail is REGARDS Mario Tovar

Comment by André Héroux, on 3-Oct-2011 06:19

Very interesting projet.. Same for me, do you have any wiring diagrams and parts list? No problem to pay a reasonnable price. Thank you.

Comment by bz30589, on 3-Oct-2011 06:34

Very interresting project...

Same for me, do you have any wiring diagrams or parts list?
No problem to pay a resaonnable price.

Thank you.   

my Email

Author's note by MrWestie, on 11-Oct-2011 13:08

It is pretty straight forward. A one shot 555 would be suitable as a trigger with the correct value components for timing. here Parts? Any suitable N mosfet.

Comment by David Banks, on 1-Apr-2013 04:02

Do you have a circuit diagram showing the componet design? You also mentioned a circuit board, do you have it's design?

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