Massive voltage drop when attempting to start; new batteries

[DaveBen]

You will have to use Soldering Flux Paste to get the solder to flow into the bucket.

 

[BeastMaster]

You will have to use Soldering Flux Paste to get the solder to flow into the bucket.

Use rosin flux paste. You can get it at shops that deal in electrical work. Stay clear of anything marked "acid" or for plumbing. The Rosin based stuff is usually dark brown, very sticky, and often smells of pine, and comes in cans that look like shoe polish cans. Ive also seen it in dropper and brush bottles - much thinner ( usually for printed circuit board work - and much more expensive). I'd go for the stuff in the can. It can be diluted or cleaned off with ethanol lacquer thinner denatured alcohol. You can always make thin stuff from the thick stuff that way ( I do it a lot - so I don't end up buying expensive dispensers...I just refill the ones I have, the flux will get too gummy with age as the ethanol evaporates. Just add more ethanol ( denatured alcohol ) to suit, shake, and keep going .

The problem with the acid flux is that it will wick up into the copper strands and relentlessly eat away at your connection, and it's damn near impossible to clean out once it wicks in. What you will have in a few years is a few inches of green powder under the wire insulation where the wire used to be.

Admittedly, the plumbing solder flux is a lot more aggressive, I think it's based on Zinc Chloride. It's water soluble, easy to use with torch as it doesn't burn, and when the copper sweat joint is done, excess solder and flux are typically wiped off with a rag before the solder cools, leaving a clean job.

Just a heads up from someone else who took two lessons before it sank in - one from someone else, and one self-taught reminder because I forgot.

 

[divemaster5734]

The problem with testing just looking at resistance is most ohm meters we can get our hands on just are not sensitive enough. When you are talking several hundred amps starter draw it does not take much in the way of resistance to create large voltage drops.

I have even tested for large voltage drops as it sounds like the OP has just using my hands. Turn on every electrical load you can for about a minute and then start feeling the connections and cables from the batteries to the starter looking for hot spots. Those hot spots are your high resistance spots.

As to the suggestion of disconnecting the batteries first, in most cases not a bad suggestion. In this case I would not as you are looking for poor connections and the less items you disturb until you locate the issue the better off you are. Find the problem then fix it. If you are disconnecting many things you may make the problem go away but is it really fixed?

Agreed.
Once I finally decided to inspect the cables more closely I used a DC voltmeter and resistance at each battery, with the engine off and at idle.
I discovered that while the resistance indicated a continuous conductor, (making a connection), the voltage at each battery, using the terminals at each battery for the measurement, showed a 1.25VDC difference between the left and right battery at idle.
That meant the drivers side battery cable was losing voltage through the cable across the width of the engine bay.
Replacing the terminals eliminated the voltage difference and therefore the charging imbalance.
As far as the solder vs crimp replacement, if you are crimping remember to pretreat the cables with a oxidation protection, De-OX or Noalox is what's used throughout the NEC, and will ensure your new terminals will never rust. Unfortunately it will boil out if you solder.
A crimper is cheap, and you don't have to worry about damaging the insulation with the torch.
If you are worried about something drawing power you can go through the wires on the external start solenoid one at a time, or get a cheap DC clamp ammeter.