Starter cable continuity to earth.. Strange

[Stephen Oliver]

HI Guys

Weird one here.. and probably something I wouldnt have noticed if I had not redone my wiring / battery charging etc. circuits.

I have recently redone all of my battery system as per the attached. Mainly chasing a low voltage issue and resolving a woefully undersize cable install (not to mention many meters too much of cable)

As part of the install I have two ACR/VSR’s one from each crank to the house bank, I also have had to make a mains sensing relay box that turns off the ACR/VSR’s when shore charging so the 3 banks of the charger dont get paralleled up and go bank. This box uses a relay to break the ACR/VSR negative and prevents it from operating. Works great too.. except…. and this is where is gets wierd.

The ACR/VSR have digital displays and show status / voltage. When the crank battery switches are off and the house bank is on – the starboard crank to house ACR/VSR’s display comes on even when the negative is removed. Having done some investigation it appears that there is some continuity from the engine main +ve cable to negative – enough to allow the ACR/VSR to power itself up when a positive feed is fed from the house – the engine cable provides the negative – albeit faintly.. but enough to shove about 7v through the system.

If I switch the engine battery switch to on – all behaves normally – ACR/VSR goes off when the negative is removed – switch the battery off and the ACR/VSR comes back on.

So has anyone got any ideas where the feed to -ve could be coming from or do I have to start stripping wires off here and there until I find it – to make it worse the STB engine starter is almost impossible to get to so removing cables is done by feel and cursing!

Any ideas where to look would be greatly appreciated – or other testing I could do?

Steve O

Topic

[Tony Athens]

After everything has been cold for at least 12 hours, feel both alternators all around.. If either feels warm compared to the engine, there is your .7 amp draw.. The field is staying on..

Tony

[Stephen Oliver]

Hahaha

Only on this boat.. no circuit breakers on either engine. Location is there just no breakers. Any idea if this was factory done or just bodgied by the builder?

Any ideas where else to look ?

Steve

[Stephen Oliver]

Hi Rob

Thanks for that I’ll check it out and see if it makes a difference. I thought it was right as we were having starting issues on the port side – but now after the rewire – it starts well but voltage is still down!

Now trying to isolate a drain on the house side – 0.7amps drain with everything off! The joys of owning a boat that someone tinkered with – very badly!

Steve

[Rob Schepis]

“Cranks are only monitored via the standard cummins gauge (which I think may be a bit dodgy on the port side as it reads very low)”

12V at the helm is fed by the engine mounted 10amp circuit breaker. If you’re engine package voltmeter is doing as you say check the connections under the breaker. Here’s a prior thread about it.

https://www.sbmar.com/community/topic/engine-mounted-circuit-breaker/

[Stephen Oliver]

Hi Tony

Thanks for your reply – seems to be a duplicate post as it disappeared when I posted it so I posted it again. Either way I appreciate your reply – as I too keep reading it and have been doing some research.

The reason I chose these ACR’s is that they are one way only (similar to the isolator so crank to house only) and readily available here in Perth. I have also used them in the past on my other boat and never had an issue – so I have nothing to bias me either way.

I chose to go both alternators being linked to the house via ACR as I will potentially have faster charge initially with it being a 600ah bank. I was originally only going to go one acr and run the house charge of one of the alternators, but after reading it seemed ok to go the 2.. In this instance also its very easy for me to disable the ACR as I have a relay that cuts the negative feed and in theory should switch the ACR’s off therefore I can then test what alternator is doing what via the start batterys.

The house bank is also fully monitored by a digital battery monitor / shunt on the negative side of the house only – this is a dual direction monitor so reads charging too. Cranks are only monitored via the standard cummins gauge (which I think may be a bit dodgy on the port side as it reads very low)

From what i have found there is a very faint negative feed coming through the main starter cable when all power is off and a multimeter is used to measure between house +ve and the starter cable +ve – it reads something like 7 or 9v’s which as the ACR only requires a tiny amount of power for the display and can work at low voltages it seems like this causes it to ‘operate’ for want of a better description.

From what I have found starters can fill with carbon and this could cause some continuity back to the starter case – which may be the cause and explains why when the crank battery is switched on all operates as it should. Short of me actually removing the starter – which is a crap job as its not accessible easily due to the tank – I will have to live with it. I do however intend to remove the HX’s later this month so that may give me better access to all things so I can find out what is going on – I also note there are additional wires added between the alternator and the starter (possibly by a previous owner) but these have been checked and not causing it. I certainly thing the standard loom has been added to for what ever reason though! My other reasoning on this is that the Port starter looks like it has been replaced (not white) so this may explain if newer why the port engine doesnt do the same.

If I find out what it is i’ll let you know.

Steve

[Tony Athens]

Steve,

I have read this a couple of times over the part 2 weeks.. I have not answered because I do not have a solid answer……….Once you have more than one VSR’s, then to me, they become impossible to troubleshoot as to what one is working right or not, and that is one of the two main reasons I do not use them..

The other reason is once the the DC power distribution systems positive side “talk to each other”, it basically impossible to figure out what alternator is working, what your AC battering changing system is really doing, or the actual condition of each individual battery bank when actually operating the vessel..

Lots of politics on this subject, but I need to be able to do this “over the phone” with 100’s of different vessels at sea, so things need to “separate” when trying to isolate an issue.

Using properly set-up diode isolators and DVM’s to each battery bank reading battery terminal voltage is what we have been doing for 30+ years and I’d never consider changing even after working with various set-ups using VSR systems over the years

Tony

[Author]

Replies