Making sense of voltage drop test results

[Ryban]

I recently performed some voltage drop tests on my engine (a Vetus M4.14, 33hp) since at times, even with new, fully charged batteries, it will sometimes hesitate to crank when the key is turned. The batteries are all new and in good shape.

I started to suspect that there was a significant voltage drop somewhere, robbing the starter of the power it needed to start cranking, so I did (most) of the probe tests that Nigel Calder recommends.

I attached a simplified diagram for reference to the points below.

A - B | ~1.3V <-- not good, should be less than < 0.7V according to Calder
A - E | ~ 0.6V
E - B | ~ 0.7V
B - C | ~ 0.05 V <-- good
F - G |~ 0.7V <-- not good, should also be less than 0.3V

for A - B, it's clear that the cumulative drop is occurring as the the sum of the two lengths of wire A - E + E - B most likely. Since I probed on the lugs, not the terminals in these particular tests, it shouldn't be a contact issue (contacts were clean though). The total run between A - B is probably about 15-20 feet, which according to the charts I've seen, the 1 AWG wire I'm using should be producing < 3% of a voltage drop at 100 amps draw for a 20' length (0.03 x 12.9V = 0.387V). But given that I'm seeing 1.3V, there's either 3-4x as much resistance in the length of the two wires, or I'm drawing 300+ amps of current, which seems unlikely?

Given that I have pretty high voltage drops on both the positive and negative side of the circuit, I'm inclined to think that this is indeed the source of my issue, but now it's just a question of how do I resolve it. Before I go to a larger wire gauge size , or re-locate the battery isolation switch panel closer to the battery/engine, should I just run some short lengths from the batteries tot he engine, and see if it solves the problem?

[Wotname]

Presumably these tests were done while cranking the engine with the engine stop engaged (i.e cranking but not starting) and the cranking speed was 'normal' rather than when it was ' hesitating'. It seems you were getting ~10V between D and G which should be good enough although not great. If the cranking hesitates at times, I would suspect an intermittent high resistance 'somewhere'. Perhaps across the contacts in the master switch (E - E1).

What I would like to know is the voltages between D-G and G-A while cranking (with engine stop engaged).

Also be aware of failed cable terminations hiding under insulation and heat shrink or tape etc. The cable lug shown looked OK until the heat shrink and insulation was cut away.

[jpendoley]

I had a similar problem when cranking, turned out to be a faulty neutral safety switch. You might try bypassing the switch (if present) and see if you get full power.

[donradcliffe]

You can pur regular automotive jumper cables in parallel to your wiring to see if it improves things

[jordanbigel]

Originally Posted by Ryban

The total run between A - B is probably about 15-20 feet, which according to the charts I've seen, the 1 AWG wire I'm using should be producing < 3% of a voltage drop at 100 amps draw for a 20' length...

Beware, calculations of voltage drop vs distance are based on round-trip (pos and negative) cable length. So if your run is 20ft, you should be selecting wire gauge based on a 40ft run.

According to my chart, for a 40ft run (erring on the high side since you said 15-20ft - 20' x 2 = 40'), and a <3% voltage drop, carrying 100amps, you need a 2/0 cable.

Note, I am not saying that is the cause of your problem.