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Starter battery isolation switch


Psycloud

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My starter battery has gone belly up (showing 6.5v so guessing 3 dead cells but I'll save that for another thread) and the Bukh DV36 manual recommends an 88ah battery which I have duly ordered.

 

As yet I haven't worked out what killed the battery but it's always bothered me somewhat that both start and leisure banks are isolated by a single 2 pole switch on the negative side, this means I can't isolate just one bank.  Now I know I could swap this out for a 3 position switch but that would be a major ball ache so am going to add a separate isolater switch on the starter battery side which will then allow me to switch off just the starter battery and leave the leisure bank on.  Since the boat rarely moves from the marina there's no reason why the starter battery needs to be permanently connected and by isolating it i can prevent any drain between the 2 banks - not that I have any evidence yet of this happening but since I'm fitting a new battery I may as well take an extra precaution.

 

So the question is, can I put the isolation switch on the positive side or does it have to be on the negative?  

 

Thanks

 

David

 

 

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My DV36 manual mentions some form of split charging system. I think it may have been a diode system and if so it would be better changed to a relay.

 

As long as you have a working split charge system and I expect you do then when the engine is not running   and the ignition switch is off there can be no drain between the two banks because that is what a split charge system does.  This is unlike a three position switch that, depending upon how its wired, can leave both banks working as one large bank.

 

Personally I do not think you need do anything because the split charge system is already doing exactly what you want.

 

There are technical reasons why its probably best t have the master switches in the positive side of a bank but although they could result in expensive electrical damage but on simple boats such an event would be very rare indeed.

 

If it makes you happy by all means put a master switch in both bank's positive leads and I would get rid f the one in the negative at the same time - relocate it if its OK.

 

It is only the charging system froma single aternator that stands any chance of allowing discharge between banks.

 

If you do have split charge diodes then my choice would be to change it for a Voltage Sensitive Relay sensing form the domestic bank but it would be vital to ensure the main charging lead runs to the domestic bank and not the engine one. This has advantages it you have solar or a single output battery charger. In both cases wiring a VSR this way allows solar r the charger to charge both banks when the domestic bank is fairly well charged.

 

 

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Thanks Tony. 

 

I do have a VSR on the alternator side however I also have 520w solar going into a 40a MPPT which goes to the 4xt105 Trojans, and a 30a multistage charger with twin outputs that charges both starter and leisure banks.  The solar tops up the leisure during the day and the charger is on shorepower which I have on a timer to come on in the dark hours (and over winter will be the primary charging source). 

 

My concern was really about whether any current could pass back through the 30a charger.  Just seems odd that the starter battery was fine one day and then down to 6.5v the next without even having started the engine.  For the last month we've been using the solar for the 240v via an inverter (and the timed shorepower charger at night) as an experiment and wondering if that has somehow caused the starter battery to go bell up when drawing high loads - I can't see how really but I don't want to kill my new battery :)

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27 minutes ago, Boater Sam said:

I would also check that the alternator is not discharging the engine battery when its not running, with a neg only isolator it is permanently connected. 

I didn't realise this was even a thing.  :/

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46 minutes ago, Boater Sam said:

I would also check that the alternator is not discharging the engine battery when its not running, with a neg only isolator it is permanently connected. 

Not sure how that works. if you break the negative it should  still isolate the alternator even though the positive charge wire is still connected. I will have to think about that one.

 

1 hour ago, Psycloud said:

Thanks Tony. 

 

I do have a VSR on the alternator side however I also have 520w solar going into a 40a MPPT which goes to the 4xt105 Trojans, and a 30a multistage charger with twin outputs that charges both starter and leisure banks.  The solar tops up the leisure during the day and the charger is on shorepower which I have on a timer to come on in the dark hours (and over winter will be the primary charging source). 

 

My concern was really about whether any current could pass back through the 30a charger.  Just seems odd that the starter battery was fine one day and then down to 6.5v the next without even having started the engine.  For the last month we've been using the solar for the 240v via an inverter (and the timed shorepower charger at night) as an experiment and wondering if that has somehow caused the starter battery to go bell up when drawing high loads - I can't see how really but I don't want to kill my new battery :)

Well f it can pass back through the charger it must be faulty unless its a exceptionally old design.

 

My guess is that one or more of the cells shorted internally due to old age and that caused the failure.

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2 hours ago, Tony Brooks said:

If it makes you happy by all means put a master switch in both bank's positive leads and I would get rid f the one in the negative at the same time - relocate it if its OK.

This would be a better arrangement for sure. 

1 hour ago, Psycloud said:

Just seems odd that the starter battery was fine one day and then down to 6.5v the next without even having started the engine. 

It’s not unknown for batteries to fail suddenly. Was it quite old?

1 hour ago, Boater Sam said:

I would also check that the alternator is not discharging the engine battery when its not running, with a neg only isolator it is permanently connected. 

I can’t see how that can happen. If the battery is isolated then it’s isolated and there’s no circuit. 

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1 hour ago, Psycloud said:

I do have a VSR on the alternator side however I also have 520w solar going into a 40a MPPT which goes to the 4xt105 Trojans...

What do you mean by ‘on the alternator side’?  Ideally you’ll have the alternator wired to the dometics with a VSR connecting the starter battery when the domestics are high enough. That would also allow the Solar to do the same thing, and you could even remove the second output from the charger if you wished and that could also do the same thing. 

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1 minute ago, WotEver said:

This would be a better arrangement for sure. 

It’s not unknown for batteries to fail suddenly. Was it quite old?

I can’t see how that can happen. If the battery is isolated then it’s isolated and there’s no circuit. 

 

Yes the battery was quite old, certainly older than I've owned the boat (5.5 years) so a failure is to expected I'm sure.  I think I'm probably over reacting to the situation and possibly just linking the coincidence of installing the solar equipment and the starter battery failing in a bad way, and down to the same voltage as 1 Trojan.  The battery will accept a charge but instantly drops back to 6.5v, which made me think somehow it's been dragged down to a single Trojan voltage even though the solar doesn't even connect to the starter battery (in my head I'm thinking there is a link somewhere in the system - but it really just is coincidence :) ).

 

All 4 Trojans (albeit 4 years old) hold up at 6.55 after the surface charge has been removed and left to stand for several hours (disconnected from each other).  On close inspection the plates look like they're getting quite buckled, again since the solar installation (and running the 240v off the inverter).  I did used to keep an eye on the cells so know this is a new thing -some of the plates look like they are touching at the top but I guess they can't be or they'd be shorting and not holding at 6.55.  I belive heavy load can cause buckling, and they have been run under load for the past month but never below 80% SOC (BM1 and voltage checking) and always charged back up over night with the mains 3 stage charger. 

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18 minutes ago, WotEver said:

What do you mean by ‘on the alternator side’?  Ideally you’ll have the alternator wired to the dometics with a VSR connecting the starter battery when the domestics are high enough. That would also allow the Solar to do the same thing, and you could even remove the second output from the charger if you wished and that could also do the same thing. 

Hmmm - I didn't fit it and actually need to go have another look to see which way it's wired.  I always just assumed it wired was so that the starter got the charge first then the leisures.  Are you saying that if all the charging sources (alternator, shore charger and solar) connect together at the leisure bank +'ive then branch off to the VSR then all 3 sources will charge leisure first then starter.  When we got the boat it only had the alternator as a source, the shore charger and solar were fitted later.

 

Edit: Perhaps it is wired leisure first then when the relay closes the starter was getting a massive hit from the solar, for a long time?

Edited by Psycloud
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1/ As long as 1 on the VSR is the sense side of the VSR that looks fine and I expect that you did not bother to draw in the negative on the VSR. Plus what the other Tony said.

 

2/ I doubt you can see the plates in the battery, what you can see will be the separators where they stick up above the plates. The part above the plates take no part in battery operation and are there as a sort of half hearted attempt to stop a metal spout shorting the plates. Probably nothing wrong with the actual plates.

 

3/ I think the two voltages are only coincidence. Unless you have a 6V domestic system your 6v batteries are connected in series (and then parallel as there are 4) so the voltage from them for all practical purposes is 13.1 (6.55 x 2).

 

4/ The main reason for feeding the charging outputs to the domestic bank is to minimise the current trying to pass through the VRS and so prolonging its life. As a secondary effect it feed all charge to the domestic bank until the charging voltage reaches around 13.6 to 13.8 and then both banks are charged. The starter battery is only discharged a very few Ah per start with a reasonable engine so the thing about charging it first is pretty much nonsense for marketing purposes.

Edited by Tony Brooks
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3 hours ago, WotEver said:

This would be a better arrangement for sure. 

It’s not unknown for batteries to fail suddenly. Was it quite old?

I can’t see how that can happen. If the battery is isolated then it’s isolated and there’s no circuit. 

Where did it say that the batteries were isolated please? If his leisure battery was on so was his engine battery.

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23 minutes ago, Boater Sam said:

Where did it say that the batteries were isolated please?

You said it...

5 hours ago, Boater Sam said:

I would also check that the alternator is not discharging the engine battery when its not running, with a neg only isolator it is permanently connected. 

How is it connected if it’s isolated?

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I think Sam means when the isolator is closed as it would be if the OP is aboard so he can get domestic electrics. this was the OPs original point.

 

Now we know he has a VSR I can't see how the discharge could happen unless a diode or two has failed short circuit.

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2 hours ago, Tony Brooks said:

I think Sam means when the isolator is closed as it would be if the OP is aboard so he can get domestic electrics. this was the OPs original point.

 

Now we know he has a VSR I can't see how the discharge could happen unless a diode or two has failed short circuit.

Oh I see. I suppose that if the alternator is connected to the engine start battery and it had a fault then that could drag the battery down which I guess is what Sam meant, but if it was that faulty then I wouldn’t expect it to be charging. 

 

Sounds to me like the battery simply died of old age. One day there was sufficient sediment to short out a couple of cells. 

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Yes, correct, thanks. Its a long shot but I've known it happen with cars when the regulator has 1 duff diode out of the 3 that feed it and it discharges the battery, slowly, via the ignition warning light without it getting bright enough to be obvious.

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  • 1 month later...
On 16/06/2018 at 18:04, Tony Brooks said:

4/ The main reason for feeding the charging outputs to the domestic bank is to minimise the current trying to pass through the VRS and so prolonging its life. As a secondary effect it feed all charge to the domestic bank until the charging voltage reaches around 13.6 to 13.8 and then both banks are charged. The starter battery is only discharged a very few Ah per start with a reasonable engine so the thing about charging it first is pretty much nonsense for marketing purposes.

I need to revisit this thread...

 

The VSR was wired Starter first, I am going to change this in the next few days (major cabling tidy-up going on in the engine bay at present).

 

Something that concerns me now though - the Tracer 40a MPPT connected to this setup does a monthly equalise charge at 16.2v so presumably the (new) starter battery gets hit with that too, not just the 4 Trojan T105s?  Is that OK?

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I didn't see one in your doodled wiring diagram but there should be (according to Victron who's VSR I have) a fuse in the line between the leisure batteries,VSR and starter battery. If you know when the equalisation will take place, you could just pull the fuse before this happens.

If you are buying a new starter battery then you have the chance to buy one which will tolerate the equalisation.  I guess this would be FLA.

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12 hours ago, philjw said:

I didn't see one in your doodled wiring diagram but there should be (according to Victron who's VSR I have) a fuse in the line between the leisure batteries,

There should really be two fuses. One near each battery. 

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