Help: ideas wanted

A project log for Coin Cell Jump Starter

Starting a car with a CR2477 coin cell

Ted YapoTed Yapo 12/12/2017 at 17:0916 Comments

UPDATE 2017-12-21

Thanks to everyone for the very helpful suggestions!  I've settled on a solution, shown here:

The assembly consists of two threaded hex spacers, four screws, four washers, and four custom 1.5mm aluminum bars I punched and cut to size:

It should provide a low-resistance connection, and might not melt :-)

I need to come up with a good way to connect the capacitors in series with a low-resistance connection.  I've gone round and round in my head, and haven't come up with the right way yet, so I figured I'd ask you, dear readers, for your input before I just slap something together.

Here is a picture of two of the capacitors as they will be wired.  There are three positive terminals and one negative on each capacitor.  The "outside" positive terminals (smaller pins) seem to be there just for mounting reinforcement, so I'm going to ignore those and only connect the rectangular tabs.  The tabs are 5mm wide by 6mm long by 1mm thick.  Coincidentally, 0.2" Faston connectors seem to fit, but they aren't suitable for the large currents involved.

So, I need to come up with a way to connect those two terminals.  I have thought of machining a copper bar with slots to fit the tabs then soldering, or cutting short pieces of solid copper ground wire to fit in-between them, but no ideas so far feel right.

Ideally I'd like a secure mechanical connection first, then add solder to that.



griffe1154 wrote 12/13/2017 at 19:01 point

Some searching turned up an ebay listing with a photo that shows what the bus bars look like. They are the same kind that go into a household breaker box. I have purchased them at ACE hardware but i am sure there are many other sources. All 7090 aluminum already drilled and taped with matching screws and you can cut it to whatever length you need. I believe they are rated for 250 Amps continuous duty so probably several times that peak. Here is a link with photo.

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griffe1154 wrote 12/13/2017 at 18:51 point

My approach to connecting the capacitors would be to use a piece of aluminum buss bar and you can drill and thread the connections you need. This is an off the shelf item and is designed for this sort of thing. We used this material for the starter circuit on a competition race car built for the formula SAE. The aluminum they use is compatible with both copper and aluminum wires and provides solid mechanical connection. You can add solder to it but we didn't and I don't think it is necessary or advisable. The likelihood of damaging the capacitors is high and you might end up with higher resistance. I will do some googling to see if I can find a source with a photo. 

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Ted Yapo wrote 12/13/2017 at 18:59 point

I see things like this:

pre-drilled and tapped that might work with the caps turned 90 degrees as suggested below.

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EricH wrote 12/13/2017 at 07:20 point
I vaguely recall some sort of clamp connector with a bolt and a flat piece that nests in, for lack of a better word, a [ shaped piece... was pretty heavy/thick metal... otherwise, alternate caps upright and upside-down? Mount with hose clamps...

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Ted Yapo wrote 12/13/2017 at 18:58 point

Yes, the alternating upside down idea would let me solder the terminals directly together.  That's interesting.

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jaromir.sukuba wrote 12/12/2017 at 22:42 point

You may also employ M3 or M2,5 brass hexagonal spacer of appropriate length like this

and bolt it from both sides to enlarged holes in capacitor lugs, between capacitors. You may want hexagonal head or Allen key bolts for simpler operation.

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Ted Yapo wrote 12/13/2017 at 01:22 point

Yes, I was playing around with some aluminum spacers like that on the bench earlier today.  I might put one on each side of the lug to avoid drilling the holes.  And I found some Allen-head screws, which is a good idea.

To my surprise, I found that aluminum is a better conductor than brass.

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Alastair Young wrote 12/12/2017 at 21:31 point

copper, brass or aluminum blocks and small machine screws or self tappers. Lots of amps for this application.

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Dr. Cockroach wrote 12/12/2017 at 22:19 point

You got that right. Initial 500-700 amps then down to about 200 amps will need some good solid connections for those three seconds.

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deʃhipu wrote 12/12/2017 at 18:03 point

If you rotated them 90°, you could just use a simple strip of copper with holes and maybe some brass bolts?

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Ted Yapo wrote 12/12/2017 at 19:21 point

Hmmm...I naturally turned them this way to minimize the distance, but the 90-degree idea opens up some possibilities...thanks!

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Jarrett wrote 12/12/2017 at 17:28 point

Those terminals aren't really designed for soldering. Get yourself some thick gauge wire and some appropriate lugs and bolt those suckers on.

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ActualDragon wrote 12/12/2017 at 17:39 point

i dunno, i'd solder, seems easier

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davedarko wrote 12/12/2017 at 17:47 point


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ActualDragon wrote 12/12/2017 at 18:33 point

it really depends on what gauge wire you use

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Ted Yapo wrote 12/12/2017 at 19:24 point

The hole in the lugs is too small to fit any reasonable screw.  I thought about drilling them out to fit maybe an M3 or #4-40. Then bolting becomes easier.  In general, bolting seems like a better way to go - less chance of damaging the caps with heat, too.

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