Aluminum vs copper in pack building

[John in CR]

I'm in the final stages of some of my Konion pack building. The series strings have the tabs soldered together, but since I'm going to all this effort I want to make permanent sub packs to parallel my 5kwh+ pack, I want to tie all the cells at each level together in parallel too. At the very least this will enable me to check the packs for balance, and balance them if they get out of whack sometime in the future.

I haven't been able to find copper in any form down here other than wire, but I already have some aluminum sheet that I can cut into strips to work kinda like Doc Bass's copper bus bars. I looked into low temp soldering of aluminum, and the only issue seems to be the same as welding aluminum, and that is making sure you clean off the oxidation very well first.

I'll be connecting to the existing tabs between cells at the negative end to eliminate the chance of creating a short if some of the green plastic cover melts like is possible at the positive end.

I looked at the coefficients of thermal expansion and copper is much closer to that of steel and nickle, which make up the tabs. However, I'm not sure if that's even a factor in these low temperature solder connections, especially since aluminum more closely matches the thermal expansion of the tin and lead making up typical solder.

Is there any reason not to use aluminum cut into strips with holes drilled for each solder connection, like a cheap version of the good doctor's approach, but with materials I already possess?

John

 

[recumpence]

You will need special flux to solder aluminum. I do my own LiPo pack building. Many LiPo cells use aluminum tabs. I got some of this special flux from Brian at Tanic. It smells like amonia. :?

Matt

 

[GGoodrum]

Over time, aluminum will oxidize and could cause connection problem. Soldering is a bitch, even with the special flux. Can you find any brass down there? it conducts as well as copper, but doesn't oxidize at all.

-- Gary

 

[Mr. Mik]

Can you get copper pipes from plumbing supplies? They can be cut in half or hammered flat.

 

[John in CR]

Thanks guys, I thought about using copper tubing, but with 17 strips for each pack that's a lot of work. Regarding the difficulty of soldering AL, I'll have to test it first, and I think some of these Makita tabs may be aluminum anyway. Regarding oxidation, copper does too, but I plan on a thin coat of epoxy over all of the tabs and connectors. I don't want my packs going bad like some of the Makitas do, so I want none of my tabs exposed or able to move and my end result should be almost waterproof.

John

 

[TylerDurden]

There's also the galvanic reaction of the alu and your non-alu hardware.

 

[Drunkskunk]

there's always plumber's strap. its a copper strapping used for hanging pipes, and sold in most of the Box hardware stores. (lowes, Home depot, Ace, ect..) its thin-ish, but can be doubled or trippled up, and it has holes already drilled in it.

Aluminum just doesn't conduct very will. Copper does. 17 connecting bars of aluminum would increase your pack's resistance.

 

[rkosiorek]

The main problem with aluminum wiring is a phenomenon known as
"cold creep". When aluminum wiring warms up, it expands. When
it cools down, it contracts. Unlike copper, when aluminum goes
through a number of warm/cool cycles it loses a bit of tightness each
time. To make the problem worse, aluminum oxidises, or corrodes
when in contact with certain types of metal, so the resistance
of the connection goes up. Which causes it to heat up and corrode/
oxidize still more. Eventually the wire may start getting very hot,
melt the insulation or fixture it's attached to, and possibly even
cause a fire.
quote from - http://www.faqs.org/faqs/electrical-wiring/part2/section-16.html

also there is a problem of connecting from aluminum to copper. AFAIK you can't just solder them together reliably. so i would stick to copper if i could.

sources for copper:
- check machinists supply outlets. they sell copper and brass for use as shim stock. they also call it shim stock.

- check hobby shops that cater the railroad buffs. the sell small sheets of copper 4" X 8" in various thicknesses up to 0.025"

- large art supply/craft stores sell copper sheet for use in various embossed copper art projects and sculptures.

- roofing contractors ( the industrial ones, not the "shingle gypsies") that do metal roofs will have copper sheet. if there is one locally it may pay to just visit and see if they have any small scraps that will part with. with the high cost of copper they try to recycle everything.

rick

 

[Ypedal]

Daaaaamn.. copper roofs.. never thought of that one before..

* puts on ninja suit and goes copper roof hunting..

 

[GGoodrum]

Ypedal has really gone green now...

 

[Patriot]

I used 3/4"x1/16" thick aluminum flat stock on mine, with SS lock washers for the screw terminals. The resulting conductivity is equivalent to a 4awg wire. It's resistance is so low, that even at 50a constant current, it heats up almost nothing compared to the other parts of the wiring going into the motor itself, which is 10awg. I've ran my motor at max 50a quite a few times, and the battery doesn't even get warm. The only thing that even got remotely warm was my Kelly controller.

I went to work and talked to an electrican there who does commercial work. Even he said it is overkill, and should be just fine. They even use alot of those aluminum blocks with huge allen screws for connecting copper wire to bus bars and breaker panels.

Hopefully, I won't have any serious problems with it

 

[Lapwing]

Tips on packbuilding for durability.

When Aluminum based tabs/studs or strips are combined with anything in the Copper/brass bronze or steels including stainless then use a paste like Noalox

http://www.idealindustries.com/products/wire_installation/accessories/noalox.jsp Get the stuff pretty cheaply at Home Despot or Lowes, or buy online. I generally use it an all screwed connections in the pack. Only a little is required to stop dissimilar metal corrosion.

Wherever possible I use brass or better yet bronze fastners with copper.


I use stainless steel fastners with aluminum. Keep in mind that stainless steel screws and washers are much less conductive than copper or aluminum, so in high current applications pay attention to placement of lugs, washers and nuts. I am careful not to use a stainless washer in the conducting path. They get hot!

I try not to put aluminum and copper containing metals (brass/bronze) in direct contact. Aluminum is great for connecting bars if the battery has aluminum terminals, but if they are brass then using Noalox is a must, for good conduction longterm. Also, while there is no problem using good quallity tinned/copper electrical connectors and lugs with aluminum connecting bars, again Noalox is important to limit metal/metal interactions, and maintain conductivity.