Voltage difference of cells while building packs?

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Dec 20, 2018
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What is the maximum acceptable cell voltage difference while soldering or spot welding packs? What do you think?
 
I'm wondering the same thing and have not come up with a good answer yet. My solution though, I think, is that when I get to building mine I may use a resistor to connect the next cell to the group until the pack balances if it's off by more than 0.1V.
Basically the lowest cell is going to be charged by the others on a "dead short" charge. So how much can the cell take? How fast is it rated to charge/discharge? How long will it take it to equalize? My current guess at 0.1V may still be too much. I think the saying "when in doubt, there is no doubt" comes into play. If you have to ask, then it's too much. Find a controlled way to balance the cell into the pack.

That's my thought on it.
 
I try to stick to 100mV -- same as using a parallel charging board when charging LiPo batteries for RC use.
 
You can easily calculate the current, which flows when paralleling:

U / R = I

U is in this case voltage difference of the cells.
R is internal resistance plus wire/fuse resistance.
I is the current which will flow by paralleling.

First we solve the equation for getting U with maximum current of 2A, when the IR of the cell is 40m? and the R of the fuse is 60m? you'll get a total of 0.1? for R:

0.1? * 2A = 0.2V

So as long as the difference is under 200mV while the resistance (IR + fuse) is over 100m?, the current won't go over 2A.

Hope this helps!
 
There's a few ways to approach this:

1) As DCkiwi states, about 100mV difference, especially if the cells are fully charged. You could potentially go to 200mV, but that's pushing it
2) Only solder the parallel cells together when their voltages are low, around 3.0V. The lower the voltage, the less energy there is to cause an issue.
3) Have a few cell holders that are all connected in parallel. If you have 3 or 4 holders that hold 4 cells each, then you could effectively have 16 cells all at the exact same voltage by paralleling them before you solder them. But, again, this returns back to option 1 & 2. The cells need to be close in voltage to do this. But, they can be slightly further apart as the holder you can solder resistors from the legs to the bus bar to limit the current. Then drop the cells in, let them sit for an hour or 2, then you can solder them together.



Addendum: or what LEDSchlucker said. He posted before I could finish typing :p
 
Different scenerio: if you have 3 cells and two are at 3v can the third cell be 3:2v

Or is the max still .1 V
 
For sure ok. That would also divide the current by 3.

Also I see 0.1V seems to be a good point to try. I like all your ideas about the equalizing of the cells. I got a whole bunch of SkyRC MC3000s wich are so awesome. They have fully adjustable storage program I could use for that. By the whay these are great devices. Loud fans but everything else is just perfect, in my opinion.
 
Maniac_Powerwall said:
[...]
That would also divide the current by 3.
[...]

Not really, those two with 3V would have half the current, but the one with 3.2V would have full current (in this case around 2A).
 
Chablis_m said:
Different scenerio: if you have 3 cells and two are at 3v can the third cell be 3:2v

Or is the max still .1 V

At that low a voltage, I'd maybe do it ... if I had to ... not ideal though. most likely I'd charge the lower two a tad first and pop the third one on after a few minutes ... 0.1V is not a hard and fast rule as much as a helpful guide.


Maniac_Powerwall said:
I got a whole bunch of SkyRC MC3000s wich are so awesome. They have fully adjustable storage program I could use for that. By the whay these are great devices. Loud fans but everything else is just perfect, in my opinion.

a whole bunch?!?!? super jealous ... yes, agreed, perfectly great devices ... excepting the price tag. *sigh*. its on my shopping list for 2019.
 
I got 8 of them doing a great job. Already tested 1000+ Cells with them. Super accurate. Loud and expensive. These are the only problems :)
 
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