reduce cold cranking amps

Thread Starter

Tin.Man

Joined Mar 8, 2020
6
So I'm wondering if there's a way to control the initial cold cranking amps of a car battery. I'm sure you're going to ask why so let me explain. I am using a car battery for spot welding. By simply shorting out the car battery momentarily on a couple of thin strips of metal, the amps are strong enough to weld. In fact they are strong enough to flat out burn the metal strips. I need to control that flow a little but still have upwards of 300 amps flow through. Most car batteries go from about 500 to 1000 amps so it's way over and burns any metal strips i try to weld. I've noticed there even seems to be residual electricity in the heavy gage wire i have connected to the posts. I have a high amp relay connected to one of the posts to allow for instantaneous connect and disconnect of the battery but dispite my pressing and releasing the switch for about a millisecond the metal continues to burn for several fraction of a second longer. An any case, any ideas?
 

MaxHeadRoom

Joined Jul 18, 2013
20,700
Sounds like you are creating a plasma arc, once air is ionized, it becomes a conductor.
You may want to look at the way it is done in projection welding.
A Capacitor bank is initially charged up from a H.V. DC source, once charged, A presure is applied to the material with the object to be welded to, then a Triac is used to discharge the capacitor bank.
Result a projection weld.
Max.
 

Thread Starter

Tin.Man

Joined Mar 8, 2020
6
Sounds like you are creating a plasma arc, once air is ionized, it becomes a conductor.
You may want to look at the way it is done in projection welding.
A Capacitor bank is initially charged up from a H.V. DC source, once charged, A presure is applied to the material with the object to be welded to, then a Triac is used to discharge the capacitor bank.
Result a projection weld.
Max.
That all sounds fine and dandy, but if I'm using a car battery for spot welding, suffice it to say that I can't afford to just buy even the cheapest spot welder. Not to mention this is really for a one time use, no sense in spending hundreds or thousands for a single use project. The car battery really does a good enough job dishing out the amps without the need for capacitors or the HV source. So what about this track and is it the one actually controlling the voltage? I have an AC triac that is supposed to be able to control the speed of a router but it doesn't work all that well for that application. Will it work for DC power?
 

Thread Starter

Tin.Man

Joined Mar 8, 2020
6
You mean just extend the leads? hmmm, well the leads I'm using are 1 guage wire so I'm not sure it would increase the resistance enough. But from what you've saying you think that using maybe a thinner Guage wire would do it? wouldn't that likely make the wire burn instead?
 

Thread Starter

Tin.Man

Joined Mar 8, 2020
6
You mean just extend the leads? hmmm, well the leads I'm using are 1 guage wire so I'm not sure it would increase the resistance enough. But from what you've saying you think that using maybe a thinner Guage wire would do it? wouldn't that likely make the wire burn instead?
But! I happen to spot this scrolling down on the page and I wonder if something like attachig a wire to a ground ground post and somehow diverting some down to ground would be possible?
IMG_20200308_170642.jpg
 

drc_567

Joined Dec 29, 2008
844
...for an economical approach, try using a carbon rod between the battery post and electrode.
The carbon rods are manufactured in standard sizes. Maybe start with 1/4" diameter by 12" length rod ... to see if the weld heat is sufficient for your application. If heat increase is required, saw off a short length of rod and test again.
... The carbon rod is basically a resistor in series with the battery. As you shorten the rod length, you are reducing the resistance.
... The rod may get 'red' hot ... So watch out.
... If it appears that a higher resistance is necessary, there is a 1/8" diameter rod that may work.
 
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crutschow

Joined Mar 14, 2008
24,968
Some car battery testers have a carbon pile resistor to serve as a load for testing the battery cranking amps.
Something like that could work to limit the current.
 

MrAl

Joined Jun 17, 2014
7,592
You mean just extend the leads? hmmm, well the leads I'm using are 1 guage wire so I'm not sure it would increase the resistance enough. But from what you've saying you think that using maybe a thinner Guage wire would do it? wouldn't that likely make the wire burn instead?
Well it is a matter of how much any wire size reduces the current that governs how hot the wire gets.
We could look at some wire resistances, but what is your target current level for welding what you want to weld?
I guess it is possible that it might take a wire too long to be practical though. For example 250 feet of #4 wire to limit current to 200 amps. That's 125 feet per wire lead. That could get expensive. It also depends on how long you need the welding current to exist in one shot. For example, a 1/2 second current will not heat wire as much as a 10 second welding current. This could make smaller wire feasible. Maybe you could get away with 70 feet of #10 wire if the weld only takes 1/2 second to complete before power is cut off. #10 AWG has about 0.001 Ohm per foot. Assuming the voltage falls to 10v while welding, that would mean that 0.050 Ohms would reduce the current to 200 amps. That requires a total of 50 feet of #10 wire, 25 feet per lead. The catch is that this will only work for a short weld cycle like 1/2 second or something like that, and would take significant cool down time between welds possibly as long as 1 minute, but this would have to be tested.

Maybe you could search for some welding circuits. I have a feeling the parts could get expensive there too though.

In the old days we used light bulbs to reduce current for testing things, but that was for 1 or 2 amp circuits.
There are things like resistance cones which work at ratings like 1000 watts, but they are for 120v. To get something like that to work at 12v you'd have to have 10 of them in parallel.
 
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Thread Starter

Tin.Man

Joined Mar 8, 2020
6
If you have several feet of heavy wire you could try that to lower the current.
...for an economical approach, try using a carbon rod between the battery post and electrode.
The carbon rods are manufactured in standard sizes. Maybe start with 1/4" diameter by 12" length rod ... to see if the weld heat is sufficient for your application. If heat increase is required, saw off a short length of rod and test again.
... The carbon rod is basically a resistor in series with the battery. As you shorten the rod length, you are reducing the resistance.
... The rod may get 'red' hot ... So watch out.
... If it appears that a higher resistance is necessary, there is a 1/8" diameter rod that may work.
Both these ideas had merrit, although they did have setbacks when I tried them. I got a carbon core from a cheap zinc battery and it was nice 1/4"-1/2" diameter, the problem with that one is that it started acting like a plasma cutter and burnt and consumed anything I attached to it if there was any movement of the wire It was attached to. For the wire approach, it kinda worked but didn't, I couldn't get the right length to get it to be effecitve, even with math because there was all these little other things I guess that needed to be accounted for.

BUT I feel so dumb not having thought of this before, the simplist way to do what I wanted, was to simply discharge the battery! A not so well charged battery doens't have the cranking amps to turn and engine, then well by all means killl that sucker because for my application the volts really don't even matter. I still had to do a little balancing act and not have the voltage drop too low because A. It damages the battery, and B. In my setup I had a relay that would switch on&off the battery terminal lead quickly and it needed a certain amount of minimum volts, but out side of that, I was good to go.
 

drc_567

Joined Dec 29, 2008
844
It looks like a larger resistance would improve results.
The rod shown here is longer and will have more resistance.:
1/4" carbon rod . ..12" long ,
The price shown sounds a little high.
I purchased several from an industrial supply store for much less ... several years ago.
 

shortbus

Joined Sep 30, 2009
7,688
Why not just do like they do in a real spot welder, control the time? I have a Miller industrial spot welder and that's how it works. This could be done with a push button switch controlling a old style starter relay, one looking like this from a car junk yard -
1584456680766.png
 

Thread Starter

Tin.Man

Joined Mar 8, 2020
6
Why not just do like they do in a real spot welder, control the time? I have a Miller industrial spot welder and that's how it works. This could be done with a push button switch controlling a old style starter relay, one looking like this from a car junk yard -
View attachment 201717
That's exactly the relay I'm talking about and I have it attached to a push button swtich, problem is that even as instantanous as it is, it was still way too much. Instantly burning holes into the thin nickle strips and metal housing of the 18650 battery cells I'm trying to spot weld. Plus, I have a feeling that even that device was getting stuck when tripping it. Like the inside of it was getting welded because several times it not only sparked and burned but it kept burning long after my hand was off the switch.
 

shortbus

Joined Sep 30, 2009
7,688
How much pressure are you putting on the welding electrode? Spot welding takes a very firm pressure on the electrode or it will burn right through thin metal. Most spot welders are AC welders, which now that I think of it my welder is. The battery tab welders are usually made from a rewound MOT(microwave oven transformer) a Google search will get you many hits to look at.
 

jpanhalt

Joined Jan 18, 2008
9,420
A user named tatus1969 essentially does the same thing and uses mosfets to control the current from high-discharge capable batteries. https://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/msg1232857/#msg1232857

EDIT: To emphasize what @shortbus just said, I made a capacitor discharge welder many years ago and used an SCR to control duration, which is quite short. One cause for burnt holes is not the current so much as poor contact. That is, I can be welding nickel strips to batteries without a problem, loose concentration, and cause a burned hole by having poor contact (i.e., not enough pressure). How are you ensuring good and reproducible contact?
 
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