The interface washers that is place between aluminum and copper conductors that
we use have a AL-Cu stamp on them, meaning these bi-metal devices are approved
for this type of work.
Aluminum electrical fittings actual are plated with a bi-metal compound that is
the same color as the aluminum. They look like a aluminum fitting with this
bi-metal plating.
Copper fittings are also plated with this bi-metal compound which may be zinc
or cadmium or sometimes call Chrome-Cad. We use Chrome-Cad. plated steel
channels and electrical suspension structures which is use in expose switch
gears and overhead electrical structures.
The Chrome-Cad. plating over steel is less resistance than steel where the
electrical current flows over the surface of the structure. The plating has a
golden color to it.
You can purchase electrical fittings that has these bi-metal separators install
in them or you can purchase bi-metal washers. We use Thomas and Betts
electrical fittings that uses bi-metal devices or are bi-metal surface plated.
Before I install the battery connection and/or links, I coat the positive end
with a Red rubber and the negative end with Black rubber compound which is
electrical, acid and oil resistance which I purchase from a electrical supply
house. It sometimes call liquid electrical tape.
Latter I found that the liquid tool handle compound which is use to insulated
tools also work. In purchasing this type of compound, make sure you read the
label that it listed for electrical resistance, acid, and oil proof. I use
Performix Plastic Dip Rubber Coating.
To apply this coating, I clean the battery connections by submersion it into
lacquer thinner and cover the electrical matting surfaces with electrical tape.
Do not touch these surfaces with bare hands. Used latex throw away gloves
when working with any surfaces that is to be coated or painted.
If you are using a battery clamp for a taper battery post, press in a small
cork or use a roll up paper towel. Dip the battery connection end in to the
rubber coating overlapping the heat shrink of the cable or the entire link of a
buss bar.
Hang vertical until one end of the link is dry and than do the other end. I
used black on the negative end and red on the positive end.
Roland
----- Original Message -----
From: David Nelson<mailto:[email protected]>
To: Electric Vehicle Discussion List<mailto:[email protected]>
Sent: Monday, January 13, 2014 10:41 PM
Subject: Re: [EVDL] Dissimilar Metal Contact on LiFePO4 Battery Posts
Thank you for posting that, Manfred. Very enlightening and helps
understanding what is going on. Maybe I should move the brass flat
washers between the Al posts and Cu straps on my pack. If I ever get a
chance I'll see if I can test it out, especially if I ever have to
reconfigure my pack. Maybe some voltage drops across some joints would
be in order.
I knew about the differences in the coefficient of expansion but
didn't think about the shear forces at the mating surface. It was also
interesting to note that petroleum jelly improved on the joint too.
On Mon, Jan 13, 2014 at 7:03 PM, Manfred Bartz
<[email protected]<mailto:[email protected]>> wrote:
> I found a relevant paper on IEEE Xplore: "Electrical performance of
> aluminium/copper bolted joints".
>
http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=04643511<http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=04643511>
>
> After an initial read, the points I noted are:
> - Surface preparation is very important.
> - Petroleum jelly is useful.
> - Avoid load-spreading washers -- small-area high-pressure is better
> than large-area low-pressure.
> - A brass interface washer between the Cu and Al can be beneficial.
>
> Note that the paper discusses bus-bar joints using a steel bolt. The
> situation where the bolt sits in an Al thread is not covered.
>
> For those who do not have access to IEEE Xplore, below are some extracts.
> -
> Manfred
>
>
> *Electrical performance of aluminium/copper bolted joints*
> R.L. Jackson, B.Sc. {Eng.), C.Eng., F.I.E.E.
>
> Indexing terms: Power transmission and distribution, Cables and
> overhead lines, Bolted joints, Aluminium I copper contacts
>
> Abstract: The 1350 aluminium/copper bolted joint is fundamentally less
> secure than the all-aluminium
> connection. Differential expansion shearing force at the interface is
> considered to be a significant degrading influence. As a consequence,
> substantial clamping pressures are needed in addition to having regard
> to surface preparation and bolting torques. Although inert greases are
> used at the interface to restrict corrosion outdoors, they do not
> improve joint stability. A hard plating, such as nickel, on the copper
> surface is advantageous, but softer deposits (e.g. thick electrotin or
> silver) cause rapid deterioration and are best avoided. An interface
> washer is described which will improve the stability of the bimetal
> joint and eliminate the need for interface cleaning.
>
> Introduction
> Although there has been a considerable increase in the use of
> aluminium for busbars and cables, much of the terminal plant
> (i.e. transformers, circuit breakers etc.) retains copper
> conductors. As a result there are many instances where
> aluminium cables have to be connected up to copper tails, thus
> creating a dissimilar metal contact. It is well known [1,2] that
> special attention has to be paid to the preparation and
> assembly of aluminium surfaces to obtain stable low-resistance
> aluminium/aluminium joints, whereas copper/copper joints are
> much less sensitive in this regard. It is natural, therefore, to
> assume the aluminium/copper interface lies between these two
> extremes in terms of need for care during assembly, or at most
> requiring no more diligence than the all-aluminium
> connection. However, experience in service has not alway
> supported this view and aluminium/copper joints, particularly
> those operating at high temperature, have, on occasion, been a
> source of trouble. Indeed many manufacturers of terminals for
> aluminium cables consider it desirable to offer a special transition
> lug in which the aluminium/copper interface (hereafter
> referred to as Al/Cu) is friction- or cold-welded, and the
> installer is simply left with making a copper/copper joint. ...
>
> 4 Degradation of aluminium/copper joints
> It is evident that because the degradation of this joint is more
> severe and rapid than the all-aluminium connection, despite
> good surface preparation, then one or more of the mechanisms
> proposed for contact degradation [1] is enhanced and/or there
> is another degrading influence which occurs with the dissimilar
> metal joint. Of the existing mechanisms, we need to consider
> if direct oxidation can cause erosion of contact spots. Of the
> remaining factors, both the creep relaxation and the differential
> expansion which occurs with steel-bolt clamping are less
> with the bimetallic joint and, as a result, the residual clamping
> load will be higher. By implication, the resistance to
> external mechanical influences will also be improved so that
> there is a strong inference of another factor affecting performance.
> This has already been suggested by Bond [3], where
> evidence is presented to show that interfacial shear forces
> caused by differences in thermal expansion of aluminium and
> copper cause significant degradation.
>
> 4.1 Direct oxidation
> Whereas the natural oxidation of aluminium is independent of
> temperature up to 450°C, the growth of copper-oxide films
> is very much affected by temperature [4]. ...
> Provided, however, we limit our attention to
> contact spot temperatures of around 100°C or less, the oxide
> film on copper will grow to only about 15-20nm thickness
> which is too thin to significantly erode contact spots between
> well prepared surfaces. ... For the normal conditions,
> however, the evidence suggests that copper contacts operating
> at or below 100°C are not susceptible to progressive oxidation
> attack since the film formed is self-limiting in much the same
> way as aluminium. On this basis it seems unlikely that the
> Al/Cu joint is any more affected by direct erosion of the contact
> spots than the all-aluminium or all-copper joint.
>
> 4.2 Interfacial shear forces
> The thermal expansion coefficient of aluminium is 1.36 times
> that of copper. Thus, when an assembly such as that shown in
> Fig. 3 is clamped at ambient temperature and then heated by
> the passage of current, there will be a tendency for the
> aluminium to expand relative to the copper causing movement
> at the interface (as shown by the arrows). If the frictionresisting
> force at the interface is sufficiently high this movement
> will be slight. Movement causing a significant number
> of contact spot ruptures will thus depend on the degree of
> clamping, the temperature rise and the contact spot adhesion.
> ... there is the evidence for degradation of both
> dry and greased aluminium/copper contacts at high cycling
> temperature and low clamping load. The latter is an important
> proviso as interfacial movement, altering the geometry of spots
> in contact, is necessary to sustain the degradation. The use of
> inert grease in the aluminium/copper joint is seen as helpful
> in an outdoor environment where galvanic corrosion can
> occur, but it does nothing to reduce the inherent sensitivity of
> this interface to low clamping load and high operating temperature.
> This is in contrast to the effect of grease in the all-aluminium
> connection [1].
>
> 5 The role of clamping pressure
> We have seen that, despite good surface-preparation
> techniques, the 1350-grade aluminium/copper joint is less
> stable than the all-aluminium connection. ...
> it was found that it was important to ensure adequate
> clamping load and, apart from a miniumum-pressure requirement,
> the only effect of the pressure was in influencing the
> creep relaxation of the assembly. ... What was proposed in the
> paper of Reference 1 was the use of thick large-diameter
> washers to ensure that excessive pressures on the bulk material
> were avoided. ... The large-area low-pressure sample exhibits
> more instability than the small-area high-pressure sample. This
> result suggests that clamping pressure is important when
> dissimilar metal joints are involved. ... There is little doubt however that
> this bimetallic interface is potentially more unstable than the
> autogenous joint, and this has led the writer to consider
> further ideas for improvement — Section 7.
>
> 6 Aluminium in contact with plated copper
> ... It can be concluded that platings such as tin and silver on a
> substrate of copper are not an aid to good electrical joints
> when in contact with 1350 aluminium busbar. The initial
> resistances are variable owing to the inability of the soft
> platings to crack the aluminium oxide, and the poor
> mechanical properties of the resulting contacts cause
> interfacial rupture due to differential thermal expansion. In
> contrast, the nickel-plated copper is a significant improvement
> on the plain copper surface.
>
> 7 Improving the performance of the Al/Cu joint
> It has been shown that plating a copper surface with nickel is a
> way of improving the performance of Al/Cu joints. This, allied
> to diligent surface preparation of the aluminium and adequate
> clamping load and pressure on the interface, should ensure
> low-resistance, stable contact between these metals. An
> alternative approach is to consider the use of a suitable
> transition material inserted between the mating faces, and
> which is electrically compatible to both the parent metals. ...
> Since various reports [9, 10] have shown that brass is more
> stable than copper when in contact with aluminium, it is conceivable
> that a washer or plate made of brass could be used as
> a transition material. ... A 60/40 leaded-brass busbar (BS
> 2974, Czl21) was used, bolted up to 1350-grade aluminium,
> both surfaces being well abraded prior to assembly. The results
> of current cycling these joints are illustrated in Table 3, which
> lists two typical results from each batch tested. Results on the
> Al/Cu joint are included for comparison. There is clearly an
> improvement in performance and further experiments of the
> type shown in Fig. 4 show none of the erratic resistance
> changes ascribed to differential expansion forces. The performance
> with petroleum jelly is again superior to the results
> obtained with the aluminium/copper interface, which could
> imply that the poor performance of the latter is associated
> with mechanical effects (e.g. reduced friction) due to the
> differential shear forces. It is important to note that, although
> the aluminium to 60/40 brass contact, tested here, is superior
> to aluminium/copper contact, the results were obtained with
> good abrasion of the mating surfaces, and contact loads and
> pressures consistent with what is now known to be desirable.
> Several factors could contribute to the superior performance
> of the aluminium/brass contact. Perhaps, most importantly,
> the thermal expansion coefficient of the 60/40 brass is nearer
> to aluminium (Al = 23 x 10"6/degC, 60/40 brass = 20 x 10~6/
> degC, copper = 17 x 10 "*/degC) so that the differential
> shearing force at the interface of the aluminium/brass contact
> is only half that for the Al/Cu contact. Improved creep resistance
> of the brass asperites my also be helpful and there is the
> possibility that the changed nature of the oxide film, from
> Cu2O for copper to ZnO for zinc rich brass, could render the
> assembly less susceptible to oxidation attack following slight
> interfacial movement. In contrast to the very satisfactory
> results obtained with plain brass, tests using electrontinned
> (l\ nm deposit) brass snowed high and variable initial contact
> resistances similar to electrotin on copper. In consequence,
> despite reduced thermal expansion forces, high temperatures
> were produced and continuous degradation due to the soft
> tin. ...
>
> 8 Conclusions
> The 1350-grade aluminium/copper joint operating at
> temperatures approaching 100°C is even more sensitive to surface
> preparation and clamping force than the all-aluminium
> joint. An important contribution to the enhanced degradation
> of the bimetal interface is attributable to the shearing forces of
> differential thermal expansion which are shown to depend on
> area. Load-spreading plates are thus disadvantageous, but the
> washers proposed for the aluminium-aluminium joint are
> effective provided the overlap area is comparable to the washer
> area. The improved performance of 6101 aluminium alloy in
> contact with copper is ascribed to the superior mechanical properties
> of the alloy asperities at temperature. Normal thickness
> deposits of tin and silver on a copper substrate are helpful for
> use in contact with 1350-aluminium.High initial resistances are
> produced which increase further under current cycling. These
> deposits are therefore to be avoided unless sufficient test data
> is available on a particular arrangement to confirm suitability.
> Instability of bimetal joints is not always accompanied by a
> rise in the joint resistance measured off-load at ambient temperature.
> This observation should be borne in mind when
> routine checks are made of suspect joints. Nickel-plated
> copper is more satisfactory and good stability was obtained
> under load-current cycling. A brass insert with profiled surface
> offers the possibility of avoiding the need for surface preparation
> of 1350-grade aluminium, while at the same time
> ensuring that this material can be successfully jointed to
> copper and tinned-copper parts.
> _______________________________________________
> UNSUBSCRIBE:
http://www.evdl.org/help/index.html#usub<http://www.evdl.org/help/index.html#usub>
>
http://lists.evdl.org/listinfo.cgi/ev-evdl.org<http://lists.evdl.org/listinfo.cgi/ev-evdl.org>
> For EV drag racing discussion, please use NEDRA
(http://groups.yahoo.com/group/NEDRA<http://groups.yahoo.com/group/NEDRA>)
>
--
David D. Nelson
http://evalbum.com/1328<http://evalbum.com/1328>
http://www.levforum.com<http://www.levforum.com/>
_______________________________________________
UNSUBSCRIBE:
http://www.evdl.org/help/index.html#usub<http://www.evdl.org/help/index.html#usub>
http://lists.evdl.org/listinfo.cgi/ev-evdl.org<http://lists.evdl.org/listinfo.cgi/ev-evdl.org>
For EV drag racing discussion, please use NEDRA
(http://groups.yahoo.com/group/NEDRA<http://groups.yahoo.com/group/NEDRA>)
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20140114/37ae6e6a/attachment.htm>
_______________________________________________
UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
http://lists.evdl.org/listinfo.cgi/ev-evdl.org
For EV drag racing discussion, please use NEDRA
(http://groups.yahoo.com/group/NEDRA)
