A123 broken spot welds savable?

[Electricdirtbiker]

Upon disassembling Dewalt packs, I find some of the spot welds holding the tabs to the cell have broken off. It can be 1, 2, or 3 spots. I don't have ascess to a spot welder to repair them. I will be drawing 30-40amps through the tabs. How much current can the remaining spot welds carry? If I try to add some solder to the tab, I may as well just solder a wire straight to the battery. Heat is heat. Any ideas?

Thanks

 

[Jeff]

Wow, that pack must have been exposed to some serious vibration or dropped a few dozen times in order to completely fracture those welds.

If there are breaks on the positive side, and welds on the foil circle have broken, then you're pretty much done.
That foil thingy is the safety vent. And the depth of the weld, if broken, will often expose the cell to oxygen.

You might take the pack into a local batteries plus or similar business if one is available. They'll have to crank the current up to around 300ws to reattach those factory tabs.
Please don't solder the tabs to the cell. I know of far too many perfectly good M1 cells destroyed by folks soldering directly to them..

Jeff

 

[Electricdirtbiker]

Jeff,
Thanks for your response.
Dewalt or whoever assembles the packs seem to have only fair quality control on spot welding. I notice some cells have nice deep welds and some that the welds are shallow. The shallow ones pop off with the lightest amount of handling. The weld spots don't appare to leave a hole. This is in about 15% of the cells.
Then the best solution is find a spot welder.
I have a few cells that accidently went up to 4.2v, for a short time, while charging. Are they damaged?
Thanks

Wes

 

[Doctorbass]

I dissassembled and manipulated more than 1000 A123 cells from now and i can say that bad spot weld happened more in the 2006 years.. then later they seems that they solved the problem.

this is VERY RARE that i see some... I think i see that 10x more on the makita...

If you have a good skill with welder, you can solder directly o the tab.. but that must me done very fast!

to do that you must prepare the surface with alcool and sand both surface with something around 220 grind or lower..

Then apply stai on one surface.. then on the other.. and finally join them.. between these steps you can let them to cool down a bit...

But if you only have one bad, the 3 left spotweld is still for current under 30A

Doc

Doc

 

[Jeff]

I have a few cells that accidently went up to 4.2v, for a short time, while charging. Are they damaged?
Thanks Wes

Wes, any damage from overvoltage can be measured using an AC impedance test at a known SOC (State Of Charge).
Often the same folks who can perform resistance welding would also have an AC impedance measurement tool on hand. Above 9mohms at 50% SOC is a strong indication there's been some metallic Lithium plating on the Anode. This affect is not recoverable. Impedance testing through Discharge Analysis can get you a close value, although temperature change weights the impedance over the cycle. And if the cell were allowed to get hot, ie: left in an overvoltage condition too long, then the plating will be more pronounced. This is also a *big* problem when charging the M1 cells at or near freezing temps. The chemistry has a rather sharp knee, and since the impedance is so low, they do not tend to self heat during charge.

Here's something other members of the E:S may be interested in too :
The temperature derated charging spec's for the A123 M1 cell.
Regards, Jeff


___________________________________________________________________________________________
-Private email from A123 engineer in 2006-
Here are our guidelines for max charge rates as a function of temperature. This has not been published.
continuous charge to 3.6 V

max charge rate (C rate)
Temperature=celsius

0.01C
-20celsius -stop if lower

0.05C
-10 celsius

0.2C
0 celsius

0.8C
10 celsius

2C
23 celsius

30 celsius
4C
This is for continuous charging, and does not guarantee that the cell will actually reach full state of charge by the time it hits 3.6 V, it just sets a limit for the maximum charge rate to prevent degradation of cell capacity (plating).
_________________________________________________________________________________________

 

[Electricdirtbiker]

Thanks for all the information.

Doc, most of my batteries are 2006, so having some poor welds is not so surprising then. The good welds appare round and slightly bowled up in the middle. The poor welds are not so round and roughly flat. Just my observation.
"Stai" is this a solder flux? Where is it available? Is it used with regular electronics solder?
Theres a lot of caution about soldering directly to a123 cells. Would it be any less harmful to the cell to solder the + side of the can only. Very quickly of course. This would save about 1/2 of my cells with poor welds. I could save the remainder of the cells for low power uses or locate a welder.

Jeff, the charge derating vs temp is interesting. This is likely the same for most rechargable batteries.
My overcharged batteries didn't have a chance to get warm, maybe only a few minutes. But maybe I've already lost a few mahs?

How are pictures attached and reduced to fit?

Thanks Wes

 

[fechter]

I've soldered directly to a bunch of cells, but I have not tested them afterward much, and they were abused to start with, so I have no way to test degradation other than the voltages look OK. The solder joints look excellent with good flow.

Like Doc says, you have to go fast to avoid heat spread to the sensitive parts. I'd try to stay under 5 seconds at a time. Leave a long cooling time between passes.

I think what he was trying to tell you was to tin both sides first and allow them to cool, then hold them together with some pressure and heat again until they reflow. Sandpaper, alcohol and soldering flux are all good too.