Split charge relays

[PineappleGuy]

I am looking to replace the diode splitter on our narrowboat with a relay. I have read up on the subject including on this forum but opinions seem to vary, leaving me unclear, so let me ask a few questions and see if a consensus emerges.

1.      Does a VSR have any real advantage over an ordinary relay (seems like more to go wrong for the sake of one less wire to connect)?

2.      Should the alternator output go direct to the domestic bank or to the starter battery? I can see the advantages of the former (maximise the volts where they’re most needed, minimise the current through the relay) but then why are VSRs bi-directional?

3.      What size cable should be used to connect in the relay? Firms sell kits using 16 sq.mm cable and 100 amp fuses but I read that a BSS inspector failed someone’s VSR installation on the grounds that it counts as a battery-to-battery connection and so must be done with 25 sq.mm cable.

4.      What happens if I have a stand-alone mains charger connected to the domestic bank and I then start the engine? Is there any risk of blowing up the relay or its associated fuses by drawing starter current through them? Assuming no damage occurs, would the two charging systems then confuse each other?

For reference, the boat has a domestic bank of 3x110Ah, a 90Ah starter battery and an alternator rated at 67A.

[GUMPY]

1 VSR is better, not all VSR are bidirectional mine only triggers from the domestic side

2 Domestic bank, 

3 depends on your BSS inspector 25mm is a push to get into some VSR

4 I have a charger and solar connected to the domestics and no problems when starting the engine. The charging source with the highest voltage wins once the bulk charging phase is done.

For me the big advantage of the VSR is that with the way mine is wired all batteries are charged no matter what the charge source is.

Edited October 25, 2020 by Loddon

[nicknorman]

1/ it has advantages if there is more than one source of charge connected to different batteries. However IMO charging the engine battery from something like solar should not be necessary unless the boat is rarely used. Connect all charge sources to the domestic batteries!

 

2/ Direct to the domestic batteries.

 

3 Questionable whether the intent of the BSS regulations is that split charge relays must use 25mm cable. I’d say not, but of course what is relevant is what your BSS inspector thinks. On the other hand, there are plenty of BSS inspectors to choose from, some competent and sensible, other incompetent and stupid.
 

4/ This is of course the advantage of NOT using fat cable in the split charge relay circuit. The resistance of the interconnect means that not that much current flows through the relay. But not a problem if the relay is a dumb one activated by the alternator. See 1/!
 

When you have multiple charging sources connected together it is no big deal, the one with the higher regulated voltage will tend to do more of the work but it doesn’t matter.

Edited October 25, 2020 by nicknorman

[George and Dragon]

Is this the kind of thing? https://www.midlandchandlers.co.uk/products/victron-cyrix-battery-combiner-relay-12v-24v-120a-vv-044#

[David Mack]

I have one which is an older model of this.

Alternator is connected to the start battery (that's how it was wired when I got the boat), and solar is wired to the domestic. Being bidirectional, either source will charge both batteries.

I wired it with 25mm2 cable, for the reasons given, with a 100A midi fuse in each battery connection. No problem getting 25mm2 ring terminals to bolt on the stud connectors in the back of the VSR.

[Tony Brooks]

9 hours ago, David Mack said:

I have one which is an older model of this.

 

Alternator is connected to the start battery (that's how it was wired when I got the boat), and solar is wired to the domestic. Being bidirectional, either source will charge both batteries.

I wired it with 25mm2 cable, for the reasons given, with a 100A midi fuse in each battery connection. No problem getting 25mm2 ring terminals to bolt on the stud connectors in the back of the VSR.

I am afraid the method by which your boat is wired is suboptimal and risks burning the relay contacts under certain conditions. It was done that way because its easy. The alternator output should run to the battery that is most likely to be the most discharged for the most time.  Your way with well discharged domestic batteries you could get the full alternator output plus more from the well charged start battery passing the contacts.

 

Wiring to the alternator domestic bank with a VSR has  a further advantage in combating contact burning in that charging voltage of a very flat domestic bank will be depressed so it may not be high enough for the VSR to close until the bank has charged  a bit and the maximum charging current the bank will accept has dropped. Some suppliers of charge splitting equipment have a habit of claiming that charging the engine battery without the domestic bank connected is an advantage because you keep the engine battery is kept fully charged. In my view for motor boat, including inland use, this is not a lot more than marketing bullshine  based on the physics that they can't do much about.  Unless the engine battery is faulty it will be fully charged within around half an hour or less of starting because an engine without faults will only discharge the battery by a few Ah when starting. The starting current is high but the duration is exceptionally short.

Edited October 26, 2020 by Tony Brooks

[David Mack]

2 hours ago, Tony Brooks said:

I am afraid the method by which your boat is wired is suboptimal and risks burning the relay contacts under certain conditions. It was done that way because its easy. The alternator output should run to the battery that is most likely to be the most discharged for the most time.  Your way with well discharged domestic batteries you could get the full alternator output plus more from the well charged start battery passing the contacts.

 

 

Agreed its suboptimal. The alternator rated output is unknown, but it's an older basic model, so 70A? 90A at a push? With the 140 A relay contacts protected by 100A fuses what real harm can be done?

[Tony Brooks]

34 minutes ago, David Mack said:

 

Agreed its suboptimal. The alternator rated output is unknown, but it's an older basic model, so 70A? 90A at a push? With the 140 A relay contacts protected by 100A fuses what real harm can be done?

Probably no harm especially as a sensible boater will never intentionally end up with very flat domestic batteries. Remember fuses can, for a while, pass at least 1/3 more current than their continuous rating before blowing so I would not rely upon the fuse. Its not just the contacts than can get harmed, its the plastic case softening when internal parts warm up with high currents. One hopes the internal parts of such a  relay marketed by a main stream marine supplier has allowed for this but the one in the @George and Dragon's link looks too much like a re-badged Durite etc. for my comfort. Have you noticed that the continuous rating of your relay is only 125 amps.

 

I intended my post to use your boat as an example of builders taking the easy option with split charge relays of all types so people thinking about fitting one took a bit of extra time to do the optimum job rather than the easy way.

[PineappleGuy]

Thank you all for the responses, things are a lot clearer. One further question:

5.      With a dumb relay, is it likely in practice that in order to make everything work, the wattage of the ignition warning light would need to be increased (if so, to what value)?

[GUMPY]

Deleted because of brain fade

Edited October 26, 2020 by Loddon

[Tony Brooks]

1 hour ago, PineappleGuy said:

Thank you all for the responses, things are a lot clearer. One further question:

5.      With a dumb relay, is it likely in practice that in order to make everything work, the wattage of the ignition warning light would need to be increased (if so, to what value)?

It all depends upon the warning lamp BULB (LEDs are a no-no for alternator excitation) and the relay. The ones we used on the training engines made little noticeable difference and some would close on just warning lamp current. This is not good because it opens the potential to pass starting current through the relay with a faulty engine battery.

[Sir Nibble]

Just a thought to throw among the technicians. How about running the relay -ve to the starter solenoid terminal?

[Tony Brooks]

Certainly worth considering for a conventional relay, can't see much point for a VSR unless there is a lot of solar keeping the battery voltage high. Even then the voltdrop across the battery under cranking may make it drop out.

 

On a VSR not too sure what the electronics would do if you had a well charged engine battery and well discharged domestic bank, in rare cases there may be  reverse voltage if you are sensing off the domestic bank.

 

Probably confuse the hell out of the average mechanic.

[PineappleGuy]

17 hours ago, Tony Brooks said:

....... some would close on just warning lamp current. This is not good because it opens the potential to pass starting current through the relay with a faulty engine battery.

So if I look at a dumb relay spec, what should I be wary of - low excitation voltage? Any particular make to avoid/choose?

Edited October 27, 2020 by PineappleGuy
Wanted to add a further question

[Tony Brooks]

1 minute ago, PineappleGuy said:

So if I look at a dumb relay spec, what should I be wary of - low excitation voltage?

No idea but it won't be voltage. I think you are worrying about nothing. Fit a decent VSR and stop worrying about it.

 

Its far more important to look at the continuous current rating - get the highest you can. If it does not state maximum current and continuous current then the stated current is likely to be the maximum, the continuous will be lower.

 

Remember that when some of us who have worked as 12/24V electricians discuss things it will be at a deeper level that the average boater needs to understand. If you ask a question  like VSR or dumb relay those with know;ledge are honour bound yo give the best advice and this is so often not clear cut and full of caveats relating to some less common specific conditions so it sounds more complicated that it really is. Certainly ignore the discussion in posts #12 and #13.

 

Having been through all that if you have an advance alternator controller or a battery sensed alternator so the charging voltage is measured at the battery rather than at the alternator output I can see little justification in changing a working split charge diode system to a relay system. Doing so in a proficient way with minimal cable runs often involves terminating 25sq mm + cables and renewing some

[PineappleGuy]

Thanks Tony. We don't have a battery sensed alternator. Even a battery monitor is a relatively recent addition from which we see that we get 13.5V max at the domestic bank resulting in 20A max charge when the batteries are low e.g. after a couple of days moored canal-side in the summer with the fridge on. Running the engine while moored obviously helps a bit but I think it's time we made better use of the cruising hours to replenish the batteries.

[Sir Nibble]

12 hours ago, Tony Brooks said:

Certainly worth considering for a conventional relay, can't see much point for a VSR unless there is a lot of solar keeping the battery voltage high. Even then the voltdrop across the battery under cranking may make it drop out.

 

On a VSR not too sure what the electronics would do if you had a well charged engine battery and well discharged domestic bank, in rare cases there may be  reverse voltage if you are sensing off the domestic bank.

 

Probably confuse the hell out of the average mechanic.

Well reverse voltage is possibly a problem with the conventional relay and a diode would fix that. For a vsr the negatives are common anyway and the relay breaking when cranking is probably the worst of all. I've spent half my life confusing the average mechanic so what's new?

2 minutes ago, PineappleGuy said:

Thanks Tony. We don't have a battery sensed alternator. Even a battery monitor is a relatively recent addition from which we see that we get 13.5V max at the domestic bank resulting in 20A max charge when the batteries are low e.g. after a couple of days moored canal-side in the summer with the fridge on. Running the engine while moored obviously helps a bit but I think it's time we made better use of the cruising hours to replenish the batteries.

That's a cripplingly low voltage!

[Tony Brooks]

1 minute ago, PineappleGuy said:

Thanks Tony. We don't have a battery sensed alternator. Even a battery monitor is a relatively recent addition from which we see that we get 13.5V max at the domestic bank resulting in 20A max charge when the batteries are low e.g. after a couple of days moored canal-side in the summer with the fridge on. Running the engine while moored obviously helps a bit but I think it's time we made better use of the cruising hours to replenish the batteries.

Agreed, fit a VSR with the charge from any source going to the domestic bank and the VSR sensing from the domestic bank if its no a bi-directional one. That way an mains battery charger or solar when/if fitted will charge both banks but not until the domestic bank is well charged enough to get the charging voltage above the VSR cut in voltage.

[1st ade]

On 25/10/2020 at 21:33, PineappleGuy said:

Does a VSR have any real advantage over an ordinary relay (seems like more to go wrong for the sake of one less wire to connect)?

No technical background but quite a bit of real world experience. @magpie patrick has had issues on several boats with ordinary split charge relays. They close on the ignition warning light going out which doesn't mean substantial charge is being generated - Lutine had an alternator on a twin pot Lister where effectively he spent the night using the domestics, in the morning started the engine, split charge closed and the starter battery then topped up the domestics... Repeat until the engine won't start.

 

Ripple was a little better (as a faster revving engine) but had similar issues with a slipping belt; Warning Light out (and batteries linked) although charge was a few amps at best.

[GUMPY]

1 hour ago, PineappleGuy said:

Even a battery monitor is a relatively recent addition from which we see that we get 13.5V max at the domestic bank resulting in 20A max charge

In which case the best things you could do are;

1 change the alternator for one with  a 14.4v regulator  or

2 change the regulator in your present alternator to a 14.4v one if its possible and

3 move the alternator output to the domestics and fit a VSR sensed from the domestics.

 

Having just re read the OP the present alternator is probably 14.1v and 13.5v is due to the diode splitter.

So really only 3 applies .

 

 

Edited October 27, 2020 by Loddon

[Tony Brooks]

6 minutes ago, 1st ade said:

No technical background but quite a bit of real world experience. @magpie patrick has had issues on several boats with ordinary split charge relays. They close on the ignition warning light going out which doesn't mean substantial charge is being generated - Lutine had an alternator on a twin pot Lister where effectively he spent the night using the domestics, in the morning started the engine, split charge closed and the starter battery then topped up the domestics... Repeat until the engine won't start.

 

Ripple was a little better (as a faster revving engine) but had similar issues with a slipping belt; Warning Light out (and batteries linked) although charge was a few amps at best.

Actually the warning lamp has nothing to do with the relay closing although the same electrical flow causes both. The warning lamp goes out when the output of the field diodes is close to battery voltage. The output of the filed diodes also closes the relay.

 

I don't think that you can lay short comings in the charging system at a relay's door because what is the alternative. A split passive split charge diode will make the situation worse in both cases by creating volt drop on the charging lead. A so called zero voltdrop "diode" will be better but it will still draw current to operate its electronics. As we have said so often in the past gizmos do not really compensate for problems in the charging system, be it inheranty low alternator revs, slipping belts, undersized cables, alternators with a failed pahse or two and so on.