Soldering cable

I would use the paste flux and solid core solder. acid core might leave some residual acid that could cause corosion of the copper.

If your cable and connector are adequately sized for the intended use, there is no danger of overheating the connection except in a shorted condition. Even then your breakers of fuses should eliminte the short before it becomes a problem.

If you do decide to use comression lugs, make sure that you use a full compression lug and die, which is what I believe Sunour is refering to. Partial compression (which is what I think of when I hear the word crimp) is a connection looking to cause trouble.

I stand corrected. Oso is correct, I was refering to a full compression lug and die.
If I may defend myself?
At work "crimp" has become a slang of sorts for us. Its easier to say than "full compression lug and die." The other guys I work with would know this considering the type of work we were doing at the time it was said. This slang just "spilled over" into my writing. I will try to be more precise and use less slang in future postings.
Also where one might think I was refering only to the THHN wire going to the motors. the motor leads are fine stranded wire cables and to date I have yet to see any soldered. I wish they were because they are always all spread apart and I have to spend time "smoothing" them out so that they will go into the lug. We use full compression lugs and die on those as well. Using lugs with single hole tangs on each, and then bolting them together in thier appropreate groups with the THHN at the "cathead." Then each splice is wrap in; VC tape, rubber tape, and then finished off with super 88.
I should have been more clear as well about not using solder only as a means to bond the wire to the lug. If the lug gets hot for any reason, hot enough to losen the solder. This will cause the bond to get loser and hotter and loser and in time, fail. This is why I suggested using a full compression lug and die along with the soldered wire ends.
Here are some web sites with links that might be of interest to you. Although some of them deal with harsh conditions that a stationary RE system will never see, the techniques used should insure a very reliable stationary RE system for years to come.
http://ebusiness.ilsco.com/webapp/wcs/stores/servlet/CategoryDisplay?storeId=10001&langId=-1&catalogId=1&categoryId=106
http://www.evparts.com/firstpage.php
http://www.nedra.com/
http://www.keison.co.uk/furse/compression_tools.htm

When I here solder, it brings to my mind the technology used in Square-D heater strips that are used on motor starter overload blocks. If the amperage exceeds the motors rated amps the heater strip gets hot which losens a solder rated for a preset temprature and releases a spring loaded contact which in turn causes the coil to drop out and opens the circuit.

So I geuss we are in agreement that a clean and tight connection is needed when terminating wires. There are several ways availible to accomplish this. So which way is the best? I geuss it all depends on who one trust.

I asked a third party for thier opinion.

Hi, there.  You're right, a poorly made termination will generate a lot
of heat, as well as cut down on system efficiency, when carrying a high
current.  Unfortunately, there's not a lot of agreement on what the
"right" way to make them is.  Jerry Halstead's conversion diary at
http://jerryrig.com/convert/ details how he made his own out of copper
tubing.  Electro automotive (www.electroauto.com) advocates using the
great big (and very expensive) magnalugs and hex crimper, and filling
the magnalugs with noalox before crimping.  However, ther are a lot of
opinions.

On the EV list, Bill Dube' has a reputation for excellent perfoming
racing conversions.  He has advocated making your own crimping die by
splitting a hex socket in two, and using an arbor press to perform the
crimp.

The only thing everyone seems to agree on is that crimping is better
than soldering.  My own experience has borne this out.  When I first
got megavolt, my old electric escort, the connections were original and
some got hot.  A friend of mine who has a motor shop soldered the worst
ones, which helped the conductivity, but it made the cables very stiff
because the solder wicked up the cable.

Several advocate the electro automotive practice of using noalox, an
anti-oxidation compound, but several have actually cut open crimped
joints that were made without adding this stuff years later, to find
absolutely no degradation or corrosion.  A proper crimp actually fuses
the cable and lug into a single mass of metal at the crimp point.  A
good layer of heat shrink between the insulation on the cable and the
lug end seals out any corrosion.  On the other hand, the presence of
the noalox does not hurt anything.  The conductivity is still the same.

People have split open crimped joints made using several methods
(crimper and lug, hammer crimper and lug, split socket and lug, and
hammered copper tubing), to find this exact same thing, so it seems
that a crimped joint, as long as it is crimped well, is the way to go.
They are also mechanically very strong, and if they do get hot, they
still conduct, while solder gets soft and melts.

My own opinion is kind of at odds with EVparts, who I'm representing in
this FAQ, since I get my wiring supplies elsewhere.  I use real cast
copper lugs and a hammer crimper (it works best if you set it on
concrete), and put a small amount of noalox in the lugs before crimping
(after all, it doesn't hurt).  If there is too much, it splatters all
over everything when you crimp it.  I buy the "extra heavy duty"
electrolytic copper lugs from www.waytekwire.com.  They are the same as
magnalugs, just a different brand.  they are thick enough to
effectively carry high amperages and to effectively crimp to large gage
cable.

Since the hammer crimper deforms the lug a bit differently than a real
hex crimper, I try to do two locations along the length.  It has worked
well for me so far.  Both when I rebuilt megavolt, and on this go-kart
I'm doing now.

You may also check out Ken Norwick's conversion diary at
http://advanceguard.dhs.org/conversion/ConversionStart.htm to see how
he did his connections.

One other note, I found surplus heavy gage cable assemblies (4/0 gage)
with silicon based insulation and lugs already assembled at a surplus
place, and I used those as the motor cables on megavolt, since motor
current can be higher than battery current.  I think I got them at
www.surplussales.com, but I'm not sure.

Hope I haven't muddied the waters too much.  Basically, most have had
good experiences with crimped joints, made with a variety of methods.
It is necessary to ensure the crimp is well done, though.