Spring pressure connection

[Buk]

I'm looking to buy (or make) an enclosure for a pair of 4200mAh 21700 cells (parallel). The cells are (spec'd as being) capable of delivering 30A output.

I'd like to be able to swap depleted cells rapidly (for external charging) and that means pressure rather than soldered connections.

Can I get away with spring-based connectors, or do I need screw up connections?

And if the latter, what torque is safe to apply to the cells without damage?

Maybe I am overthinking this, and I am pretty sure that I will never be drawing anywhere near 30A, but I'd rather overthink it now, than discover I underthought it later.

Are the any standards or specifications for this?

Thoughts, pointers, wisdom?

 

[augustinetez]

To my mind, spring connectors would not be a good idea at 30A (I know you said you won't be drawing that but...), the connection resistance between a spring contact and the battery would be high enough to be a possible overheating/fire problem, especially with a little bit of contamination/dirt.

 

[Buk]

To my mind, spring connectors would not be a good idea at 30A (I know you said you won't be drawing that but...), the connection resistance between a spring contact and the battery would be high enough to be a possible overheating/fire problem, especially with a little bit of contamination/dirt.

That's the point of the post. Doubt!

Of course, the other side of the equation is the risk of overtorquing a screw connection and damaging the cell internally, which is far more pernicious.

Of course, there are springs and springs and springs.

Someone, somewhere must have researched this?

 

[Tony Stewart]

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[rjenkinsgb]

The standard is to use a plug-in battery pack with appropriate connectors rated for the current, and swap the whole pack.
(The "xxC" rating is the peak discharge current rating, as a multiple of the capacity).
eg.

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Ones with lower peak current ratings are a bit cheaper; these for transmitters are rated possibly ~5C from other listings, so possibly 25A & 2A charge:

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[Buk]

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Interesting. I don't need/want the sled, as the case will hold the cells + other stuff, but you're effectively voting for spring clips.

Those here are "keystone 228 & 209", which can bebought separately on mouser, and even have data sheets. Just a shame there's not a single mention of their safe carry capacity. Nor even physical layout requirements.

By way of example, here are 3 of those clips. The centre one as supplied (as best as can be derived from the rubbish dimensioning in the spec sheets!).
The other two under tension from (as yet) invisible cells:

The difference between the two outer springs is 1° of rotation of the front face about the lower radius centre, which translates to 0.1332mm of axial displacement.

That difference is critical. If the insertion of the cell causes the face of the clip to rotate exactly 24°, then the interface between them looks like this:

So if the face of the 9 dimples, and the face of the cell are exactly flat and parallel, you get a gnat's over 7mm², which is a little more than 5.2mm² cross-section of 10 AWG cable recommended for carrying 30A DC.

But if the alignment is such that the face of the spring clip rotates 1° less (or more) then the interface between them looks like this:

And that really is "best case".

Those clips may be adequate for the levels of current deliverable by AA or even C or D size dry cells. but for Li-ion cells, not even close.

 

[Buk]

The standard is to use a plug-in battery pack with appropriate connectors rated for the current,

I'm not working on a toy car/plane, and I'm not interested in LiPo.

 

[rjenkinsgb]

OK, if you must stick to cylindrical cells - spring electrodes looped with a wide copper braid band, so the spring does not conduct current but maintains a good area contact pressure between the braid and battery electrode.

Or you could use a spring plus a strip of eg. beryllium copper as the power electrode.

If the cells and container are consistent enough, even pads of silicone rubber or dense foam could be used for the backing pressure behind the braid or flexible strip contacts.

[A battery is a battery - it's irrelevant what the makers expect it to be used for, if it works in some other application. I use power tool batteries for all sorts of stuff, totally unrelated to power tools, just as it's a convenient format for some applications].

 

[Nigel Goodwin]

I'm not working on a toy car/plane, and I'm not interested in LiPo.

A battery pack doesn't have to be LiPo, in fact most probably aren't - just spot weld tabs across 18650's or whatever, to make your own battery pack - like everyone else does.

 

[Buk]

OK, if you must stick to cylindrical cells - spring electrodes looped with a wide copper braid band, so the spring does not conduct current but maintains a good area contact pressure between the braid and battery electrode.

Or you could use a spring plus a strip of eg. beryllium copper as the power electrode.

If the cells and container are consistent enough, even pads of silicone rubber or dense foam could be used for the backing pressure behind the braid or flexible strip contacts.

Some good ideas there. Thanks. I especially like the idea of using silicone rubber to provide the spring pressure.

[A battery is a battery - it's irrelevant what the makers expect it to be used for, if it works in some other application. I use power tool batteries for all sorts of stuff, totally unrelated to power tools, just as it's a convenient format for some applications].

I appreciate LiPo can be used for my application, I'm just not enamoured with LiPo. Soft cased cells are too fragile for my liking.