Re: [EVDL] What does it take to build a Muscle EV? - Conductivity

On 18 Nov 2010 at 8:32, Myles Twete wrote:

> Surprising to me is that aluminum alloys (#8000) are now spec'd in the
> building codes.

It was the solid smaller gauge Al (#10-12) which caused problems about 35-40
years ago. The Al would cold flow under terminal screws and oxidize,
leading to poor connections, heat, and fire.

Stranded Al conductors in large gauge are fine in building construction, as
long as you use anti-oxidant on connections.

However, I wouldn't use Al in an EV where there is more exposure to dampness
and corrosive environments.

David Roden - Akron, Ohio, USA
EVDL Administrator

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Myles Twete wrote:
> Thanks guys...I now understand. So the utilities have used aluminum
> for better flexibility, lower weight and lower cost.

If you can master the connection problems, aluminum is lighter for a
given electrical resistance. That's an advantage in an EV.

> Surprising to me is that aluminum alloys (#8000) are now spec'd in
> the building codes. Hopefully they won't regret that in 30 yrs like
> the last aluminum house wiring boom in the 60's...

They're doing it again for the same reason they did it before. Copper is
a lot more expensive than aluminum, so they're doing it just to save
money. Who cares if it burns down a few houses if we get our money
today? :-(

--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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David Dymaxion wrote:

> Lee, I have some small cylindrical Lithium cells I have been
> testing. I just put the cells in a plastic pipe, in a row,
> like in a long flashlight. The electrical connections are
> just pressure contacts.

[...]

> Anyway, my
> limited tests and understanding seem to show it's working
> great -- what am I missing? Should I be tab welding?

I'm sure Bill Dube will chime in, but you may or may not know that this is the sort of arrangement that Bill used in one of the early Killacycle incarnations, when powered by Bolder lead-acid cells.

Perhaps he will provide details of the lessons he learned, but I think the most telling thing is that he has gone to fully-welded connections in the Killacycle pack. ;^>

Cheers,

Roger.

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David Dymaxion <[email protected]>wrote:

> Lee, I have some small cylindrical Lithium cells I have been testing. I
> just put
> the cells in a plastic pipe, in a row, like in a long flashlight. The
> electrical
> connections are just pressure contacts. I have pulled nearly full capacity
> in
> just over 4 minutes (100 and 110 Amps for 10 Ahr cells), and have done 250A
> bursts. The cases get hot eventually, but I haven't noticed the aluminum
> ends
> get any hotter than the case. I saw a paper that said aluminum with a
> freshened
> surface and then allowed to oxidize for 2 months had only microOhms of
> resistance for a 7mm contact circle. I calculated this was less resistance
> than
> busbars alone would have, even if the tab welds had zero resistance.
> Anyway, my
> limited tests and understanding seem to show it's working great -- what am
> I
> missing? Should I be tab welding? Obviously the pressure contacts are much
> easier to do, minimize cable length, and minimize the number of interface
> surfaces.
>
>
>
>
> ________________________________
> From: Lee Hart <[email protected]>
> To: Electric Vehicle Discussion List <[email protected]>
> Sent: Thu, November 18, 2010 12:27:29 PM
> Subject: Re: [EVDL] What does it take to build a Muscle EV? - Conductivity
>
> Myles Twete wrote:
> > Thanks guys...I now understand. So the utilities have used aluminum
> > for better flexibility, lower weight and lower cost.
>
> If you can master the connection problems, aluminum is lighter for a
> given electrical resistance. That's an advantage in an EV.
>
> > Surprising to me is that aluminum alloys (#8000) are now spec'd in
> > the building codes. Hopefully they won't regret that in 30 yrs like
> > the last aluminum house wiring boom in the 60's...
>
> They're doing it again for the same reason they did it before. Copper is
> a lot more expensive than aluminum, so they're doing it just to save
> money. Who cares if it burns down a few houses if we get our money
> today? :-(
>
>
>
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David Dymaxion wrote:
> Lee, I have some small cylindrical Lithium cells I have been testing.
> I just put the cells in a plastic pipe, in a row, like in a long
> flashlight... my limited tests and understanding seem to
> show it's working great -- what am I missing?

I think what you are seeing is best-case contact resistance. That's fine
for a few cells for a short time. I would expect the connection
resistance will go up over time, due to vibration, atmospheric
corrosion, etc.

If you think about it, pressure connections to the ends of cylindrical
cells tends not to be very reliable. As Bill noted, everyone has had
trouble with flashlights etc. where you have to shake it, polish the
ends of the cells etc. to get it to work. It's common to see the light
flickering as the connection resistance changes. And that's at low
voltages and low currents under 1 amp.

If you use simple pressure contacts at higher currents, a bad connection
produces a lot more heat. Higher temperatures greatly speed up the
corrosion reactions. At higher voltages, a momentary open can even
strike an arc, which can lead to very fast contact failures. Since
aluminum burns, an arc between aluminum conductors is very bad!

Study up on how to make good, reliable electrical connections. They
generally require:

- clean surfaces (no oxidation or corrosion)
- sufficient surface area to handle the current
- high contact pressures (hard to achieve with cylindrical cells)
- enough cooling to remove the heat produced by contact resistance
- sliding or wiping action when mated to push aside debris or
corrosion products
- physical arrangements so a momentary arc won't damage or destroy it

Good connectors generally use gold, silver, or tin. Copper and aluminum
are not used. Gold is best (if you can afford it) because it is the only
metal that does not corrode on contact with air and water. So if one
insists on using cylindrical cells, try gold plating the mating surfaces.

The more common approach is to eliminate the connector. This is usually
done by soldering or welding tabs or jumpers between the cells.
--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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