Split Chargers - B2B & Relays: Which one - or Both?

[wildebus]

A little post about the a feature I noticed/discovered about the Victron B2B Charger which is kind of linked to this discussion ...

I am using a Remote Input Control feature on the Victron to disable it when the Cyrix VSR is active instead.
It tells you when that is the case when you look at the unit using Victron Connect

However, that is not the full story! Most of the time, the Charger will be off for a different reason ...

This is telling you the Charger is off as the Engine is off - or more accurately, the input voltage is under the turn-on threshold value.

Same Difference though? well, actually no! The Charger being off due to the voltage being too low is the first condition that is checked for and the Remote Input is secondary.
Does it matter? Well, it can do and this is something that can have an unusual effect.
If the Charger sees a voltage above the set threshold, it will try to go on. If it then sees it can't go on due to the remote input disabling it, it will not, of course, go on but ... it will remember it wanted to! And then, no matter and regardless of what the input voltage is at the time, it WILL turn on as soon as the Remote Input stop is no longer present.
If the Starter Battery is at a low voltage, the Charger will still turn on. It will not stay on for long as the turn-off voltage will be hit immediately, but the charger will still run for a minimum run time (around 2 or 3 minutes) regardless.

I found this was happening for the last few days as I have my van setup so the VSR is enabled for an hour each morning to let the solar do some Starter Battery boosting. This raises the Starter Battery voltage enough that the B2B turn-on threshold (currently 13.2V) is reached, and even though the B2B could not go on (as the VSR is on and it is one OR the other), it remembered it 'wanted' to. So as soon as the VSR went off, the B2B came on, and the Starter Battery voltage instantly dropped as it was suddenly supplying 30A to the Leisure Battery.
You see from the graph below at 11:00 the VSR goes off (the VSR is on from 10:00 to 11:00 each day), The B2B is no longer disabled and kicks on due to sometime in the last hour the starter battery voltage must have hit 13.2V even though it is below that at 11:00.

But does it really matter? Well, not really at the moment here, but depending on the voltage parameters you set on the Victron B2B, you could end up with a situation where you are putting a trickle charge into a Starter Battery as a maintenance thing and then wiping out that good work by taking a big slug of juice via the B2B, so it is something to be aware of I think, especially if you have an 'unintelligent' Starter Battery Maintainer such as a Battery Master, a Solar Charger that trickles a current as a secondary output or a basic connection.

I chose the thresholds on my Orion Smart-tr to be suitable to go on when the Engine starts, but I didn't really take account of the effect of the Solar on the Starter Battery - because I knew the B2B could not go on when the Solar was boosting the Starter - but I didn't realise about the Victrons apparent Memory.

An easy fix - just change the parameters (These are my current ones that I need to alter)

But if you are not monitoring the voltages, you would not realise this is happening and may wonder why your Starter Battery is apparently not being maintained despite having something in place to do just that!

 

[xsilvergs]

A little post about the a feature I noticed/discovered about the Victron B2B Charger which is kind of linked to this discussion ...

I am using a Remote Input Control feature on the Victron to disable it when the Cyrix VSR is active instead.
It tells you when that is the case when you look at the unit using Victron Connect
View attachment 2700

However, that is not the full story! Most of the time, the Charger will be off for a different reason ...
View attachment 2701
This is telling you the Charger is off as the Engine is off - or more accurately, the input voltage is under the turn-on threshold value.

Same Difference though? well, actually no! The Charger being off due to the voltage being too low is the first condition that is checked for and the Remote Input is secondary.
Does it matter? Well, it can do and this is something that can have an unusual effect.
If the Charger sees a voltage above the set threshold, it will try to go on. If it then sees it can't go on due to the remote input disabling it, it will not, of course, go on but ... it will remember it wanted to! And then, no matter and regardless of what the input voltage is at the time, it WILL turn on as soon as the Remote Input stop is no longer present.
If the Starter Battery is at a low voltage, the Charger will still turn on. It will not stay on for long as the turn-off voltage will be hit immediately, but the charger will still run for a minimum run time (around 2 or 3 minutes) regardless.

I found this was happening for the last few days as I have my van setup so the VSR is enabled for an hour each morning to let the solar do some Starter Battery boosting. This raises the Starter Battery voltage enough that the B2B turn-on threshold (currently 13.2V) is reached, and even though the B2B could not go on (as the VSR is on and it is one OR the other), it remembered it 'wanted' to. So as soon as the VSR went off, the B2B came on, and the Starter Battery voltage instantly dropped as it was suddenly supplying 30A to the Leisure Battery.
You see from the graph below at 11:00 the VSR goes off (the VSR is on from 10:00 to 11:00 each day), The B2B is no longer disabled and kicks on due to sometime in the last hour the starter battery voltage must have hit 13.2V even though it is below that at 11:00.
View attachment 2702

But does it really matter? Well, not really at the moment here, but depending on the voltage parameters you set on the Victron B2B, you could end up with a situation where you are putting a trickle charge into a Starter Battery as a maintenance thing and then wiping out that good work by taking a big slug of juice via the B2B, so it is something to be aware of I think, especially if you have an 'unintelligent' Starter Battery Maintainer such as a Battery Master, a Solar Charger that trickles a current as a secondary output or a basic connection.

I chose the thresholds on my Orion Smart-tr to be suitable to go on when the Engine starts, but I didn't really take account of the effect of the Solar on the Starter Battery - because I knew the B2B could not go on when the Solar was boosting the Starter - but I didn't realise about the Victrons apparent Memory.

An easy fix - just change the parameters (These are my current ones that I need to alter)
View attachment 2703

But if you are not monitoring the voltages, you would not realise this is happening and may wonder why your Starter Battery is apparently not being maintained despite having something in place to do just that!

When the BMV's relay is grounding the Cyrix (BMV relay energised) the NC contact could be floating, would a pull-up resistor on the on the NC help? 10K is always a starting point.

A bit "suck it and see".

 

[wildebus]

When the BMV's relay is grounding the Cyrix (BMV relay energised) the NC contact could be floating, would a pull-up resistor on the on the NC help? 10K is always a starting point.

A bit "suck it and see".

Not sure where this is going? I don't think there are any issues with spurious floats.

I know the B2B is not coming on when the VSR is on (I think that is maybe the thought?) as the timing is very clear that it comes on as soon as the relay clicks off.

 

[xsilvergs]

Not sure where this is going? I don't think there are any issues with spurious floats.

I know the B2B is not coming on when the VSR is on (I think that is maybe the thought?) as the timing is very clear that it comes on as soon as the relay clicks off.

Just wondering why the Orion seemed to remember a previous.

 

[wildebus]

Just wondering why the Orion seemed to remember a previous.

I think it is just the way it is programmed. If it were a problem with the remote input not working, then it would have come on at that point I am sure.

Ref those screenshots of the Victron App, what I recall seeing when I was first checking it out was it would say "disabled due to Engine Shutdown" always (regardless of remote input state) and if I started the engine and had not brought the "L" low, it would then change to "disabled due to Remote Input" so it is always 'awake' and listening.
Logically, it should actually be the other way round I think and prioritise the remote input. I might send a little note to Victron on this as it is both totally consistent and repeatable, but also not really what should happen

I replicated this yesterday specifically by turning on the Relay for a few minutes (to disable the B2B while enabling the VSR), and then whilst that relay was on, turned on the engine for a few seconds and then off again. I also had the headlights on full beam (so around 220W as my dipped stay on with the beam) to avoid the starter battery maintaining any high residual voltage after turning off the engine.
This graph shows the time from just before the regular 11:00 B2B switch-on (not changed the voltage threshold yet) to my little test.

(note that the timeline on the relay graph is a little offset so it doesn't line up when scannng up and down - you need to look at the X-Axis to compare the times - usually I adjust to line it up for better visual viewing)

The info bubble for 11:37 shows the highest voltage that the Venus polled since the last switch on - just 12.84V (the on-threshold for the B2B is 13.2V). The first dip before that time (at 11:36) is when I turned the ignition on and the glowplugs come on (there is always a significant voltage drop with that I see from the cab voltmeter - actually drops to 10.5V for a few seconds!); then the engine start and so voltage will have shot up to the high 13Vs with the alternator - only for some seconds whilst the engine was running, so not long enough for the Venus system to have logged it, but the Victron B2B will have detected it and tried to start - if only the remote input hadn't disabled it!
Then at 11:38 the Control Relay went off, sending the Remote Input "L" down to ground and removing the Remote Disabling line. Victron B2B now remembers it wanted to start and without checking the current Voltage threshold, just starts, crashing the Starter Battery Voltage down to 11.70, and then turns off again as the off-Threshold Voltage was met. Now bear in mind the Input Voltage at 12.84V was actually BELOW even the off-Threshold at the time the B2B started up, let alone being below the ON-Threshold and there is a definate bit of coding that should be fixed IMO.
The chance of it having a significant impact on something is pretty slight, but it really is not working precisely as it should IMO.


(as an aside, you can see that at 11:36/11:37 in the second graph the current goes to around -20A. The Cyrix-ct VSR Relay is not just enabled but actually on as the Leisure Battery voltage is high enough (from solar charging) so what you see there when the engine is cranking, the Leisure Battery Bank is providing assistance to the Starter Battery to start it. It doesn't really need it and 20A is nothing really compared to what is needed to start an engine, but it shows how the Leisure Battery Bank could help if needed and the Victron Cyrix-ct does feature a potentially very useful "start assist" feature that can force it on regardless of voltage level (as long over 4V))

Update: Changed to show the Leisure Voltage on the graph as well as it illustrated the 'Start Assist' bit nicely

 

[xsilvergs]

I think it is just the way it is programmed. If it were a problem with the remote input not working, then it would have come on at that point I am sure.

Ref those screenshots of the Victron App, what I recall seeing when I was first checking it out was it would say "disabled due to Engine Shutdown" always (regardless of remote input state) and if I started the engine and had not brought the "L" low, it would then change to "disabled due to Remote Input" so it is always 'awake' and listening.
Logically, it should actually be the other way round I think and prioritise the remote input. I might send a little note to Victron on this as it is both totally consistent and repeatable, but also not really what should happen

I replicated this yesterday specifically by turning on the Relay for a few minutes (to disable the B2B while enabling the VSR), and then whilst that relay was on, turned on the engine for a few seconds and then off again. I also had the headlights on full beam (so around 220W as my dipped stay on with the beam) to avoid the starter battery maintaining any high residual voltage after turning off the engine.
This graph shows the time from just before the regular 11:00 B2B switch-on (not changed the voltage threshold yet) to my little test.
View attachment 2709
The info bubble for 11:37 shows the highest voltage that the Venus polled since the last switch on - just 12.84V (the on-threshold for the B2B is 13.2V). The first dip before that time (at 11:36) is when I turned the ignition on and the glowplugs come on (there is always a significant voltage drop with that I see from the cab voltmeter - actually drops to 10.5V for a few seconds!); then the engine start and so voltage will have shot up to the high 13Vs with the alternator - only for some seconds whilst the engine was running, so not long enough for the Venus system to have logged it, but the Victron B2B will have detected it and tried to start - if only the remote input hadn't disabled it!
Then at 11:38 the Control Relay went off, sending the Remote Input "L" down to ground and removing the Remote Disabling line. Victron B2B now remembers it wanted to start and without checking the current Voltage threshold, just starts, crashing the Starter Battery Voltage down to 11.70, and then turns off again as the off-Threshold Voltage was met. Now bear in mind the Input Voltage at 12.84V was actually BELOW even the off-Threshold at the time the B2B started up, let alone being below the ON-Threshold and there is a definate bit of coding that should be fixed IMO.
The chance of it having a significant impact on something is pretty slight, but it really is not working precisely as it should IMO.


(as an aside, you can see that at 11:36/11:37 in the second graph the current goes to around -20A. The Cyrix-ct VSR Relay is not just enabled but actually on as the Leisure Battery voltage is high enough (from solar charging) so what you see there when the engine is cranking, the Leisure Battery Bank is providing assistance to the Starter Battery to start it. It doesn't really need it and 20A is nothing really compared to what is needed to start an engine, but it shows how the Leisure Battery Bank could help if needed and the Victron Cyrix-ct does feature a potentially very useful "start assist" feature that can force it on regardless of voltage level (as long over 4V))

To me I just thought, when the BMV's relay is de-energised the Orion's "L" pin is tied to ground, when the relay is energised what state is the "L" pin in now? It isn't tied to ground and it isn't pulled high. Now I don't know what is inside the Orion but what state does the Orion read the "L" pin as? Is it still low, it hasn't seen "L" change state.

Just my mind wondering why it does what it does.

I guess monitoring the app would give some idea what state it thinks its in.

 

[wildebus]

To me I just thought, when the BMV's relay is de-energised the Orion's "L" pin is tied to ground, when the relay is energised what state is the "L" pin in now? It isn't tied to ground and it isn't pulled high. Now I don't know what is inside the Orion but what state does the Orion read the "L" pin as? Is it still low, it hasn't seen "L" change state.

Just my mind wondering why it does what it does.

I guess monitoring the app would give some idea what state it thinks its in.

If it is floating then it isn't ground and so is disabled (as using "L" input needs 0V to be active)
When it comes to the relay, NC is NC - Not connected

It might seem strange, but a float can ba legitimate state and not some kind of is it/isn't it confused situation. For example, the Redarc BCDC B2B Controller uses a single wire as a input to specify three different charging profiles. If you ground it, it is one profile; you take it to 12V, it is another profile; and leave it unconnected (float) it is a third profile.

 

[xsilvergs]

If it is floating then it isn't ground and so is disabled (as using "L" input needs 0V to be active)
When it comes to the relay, NC is NC - Not connected

It might seem strange, but a float can ba legitimate state and not some kind of is it/isn't it confused situation. For example, the Redarc BCDC B2B Controller uses a single wire as a input to specify three different charging profiles. If you ground it, it is one profile; you take it to 12V, it is another profile; and leave it unconnected (float) it is a third profile.

So that would be tri-state logic

Three-state logic - Wikipedia

en.wikipedia.org

 

[xsilvergs]

For me there is some clarification from Margreet Leeftink of Victron into the L and H pin, I read this as a change of state detected at > or < 7V.

https://community.victronenergy.com/questions/49423/orion-smart-engine-switch.html

 

[wildebus]

For me there is some clarification from Margreet Leeftink of Victron into the L and H pin, I read this as a change of state detected at > or < 7V.

https://community.victronenergy.com/questions/49423/orion-smart-engine-switch.html

yup, that sounds right. It requires a Low Voltage to be enabled when using the L input. Similairly the H needs a High Voltage to be enabled.
However .... if you connect L and H together the device is also enabled, so the internal logic is not quite as simple as just checking for either of those as options to enable are L=Low or H=High, but also include L=H, which is valid as a on-state (which would seem to be contrary to the first two as <7V cannot = >7V )

 

[xsilvergs]

<7V cannot = >7V

What < 7 means is that if the signal level is less than 7 volts it is read as a logic zero or False in Boolean
What > 7 means is that if the signal level is greater than 7 volts it is read as a logic one or True in Boolean

You can find on the internet many tutorial which will explain and give examples of "General Operators"

 

[wildebus]

What < 7 means is that if the signal level is less than 7 volts it is read as a logic zero or False in Boolean
What > 7 means is that if the signal level is greater than 7 volts it is read as a logic one or True in Boolean

You can find on the internet many tutorial which will explain and give examples of "General Operators"

Exactly my point. Logic 0 cannot equal Logic 1; or False cannot equal True.
That is why the remote input is not a simple Yes/No signal level control.

 

[linkshouse]

A little post about the a feature I noticed/discovered about the Victron B2B Charger which is kind of linked to this discussion ...

I am using a Remote Input Control feature on the Victron to disable it when the Cyrix VSR is active instead.
It tells you when that is the case when you look at the unit using Victron Connect
View attachment 2700

However, that is not the full story! Most of the time, the Charger will be off for a different reason ...
View attachment 2701
This is telling you the Charger is off as the Engine is off - or more accurately, the input voltage is under the turn-on threshold value.

Same Difference though? well, actually no! The Charger being off due to the voltage being too low is the first condition that is checked for and the Remote Input is secondary.
Does it matter? Well, it can do and this is something that can have an unusual effect.
If the Charger sees a voltage above the set threshold, it will try to go on. If it then sees it can't go on due to the remote input disabling it, it will not, of course, go on but ... it will remember it wanted to! And then, no matter and regardless of what the input voltage is at the time, it WILL turn on as soon as the Remote Input stop is no longer present.
If the Starter Battery is at a low voltage, the Charger will still turn on. It will not stay on for long as the turn-off voltage will be hit immediately, but the charger will still run for a minimum run time (around 2 or 3 minutes) regardless.

I found this was happening for the last few days as I have my van setup so the VSR is enabled for an hour each morning to let the solar do some Starter Battery boosting. This raises the Starter Battery voltage enough that the B2B turn-on threshold (currently 13.2V) is reached, and even though the B2B could not go on (as the VSR is on and it is one OR the other), it remembered it 'wanted' to. So as soon as the VSR went off, the B2B came on, and the Starter Battery voltage instantly dropped as it was suddenly supplying 30A to the Leisure Battery.
You see from the graph below at 11:00 the VSR goes off (the VSR is on from 10:00 to 11:00 each day), The B2B is no longer disabled and kicks on due to sometime in the last hour the starter battery voltage must have hit 13.2V even though it is below that at 11:00.
View attachment 2702

But does it really matter? Well, not really at the moment here, but depending on the voltage parameters you set on the Victron B2B, you could end up with a situation where you are putting a trickle charge into a Starter Battery as a maintenance thing and then wiping out that good work by taking a big slug of juice via the B2B, so it is something to be aware of I think, especially if you have an 'unintelligent' Starter Battery Maintainer such as a Battery Master, a Solar Charger that trickles a current as a secondary output or a basic connection.

I chose the thresholds on my Orion Smart-tr to be suitable to go on when the Engine starts, but I didn't really take account of the effect of the Solar on the Starter Battery - because I knew the B2B could not go on when the Solar was boosting the Starter - but I didn't realise about the Victrons apparent Memory.

An easy fix - just change the parameters (These are my current ones that I need to alter)
View attachment 2703

But if you are not monitoring the voltages, you would not realise this is happening and may wonder why your Starter Battery is apparently not being maintained despite having something in place to do just that!

Sorry for being dense, but what are the shutdown and restart values now that you have adjusted them?

Thanks

Phill

 

[wildebus]

Sorry for being dense, but what are the shutdown and restart values now that you have adjusted them?

Thanks

Phill

I actually never got round to adjusting them If I had got round to it, it would have been to chose a On Threshold that is a little below what the Alternator puts out in terms of Voltage, so I would get a meter on the starter battery and see what the numbers are when the engine is running.
Shutoff a bit below that.

 

[linkshouse]

Okey dokey, thanks for getting back to me.

I have a tr-smart on order with the intention of replacing my VSR with it and leaving the van battery to its own devices so to speak.

But seeing this I’m thinking I could go for the same set up as you, and keep my vehicle battery topped up.

I have the BMV700 which only has a single contact relay so would have to insert a secondary relay to get the change over contacts, but at least I have one to hand.

Regards

Phill

 

[linkshouse]

I've been thinking about this some more, oh my have I ever, talk about sleepless night! It's quite worrying me actually. I'm a retired electrical engineer and I had a good reputation for my technical knowledge, now I seem to be tying myself up in knots with this. I genuinely worry that I'm losing the plot!

Anyway, enough of my personal trauma

Sorry, this may ramble on a bit, please bear with me...

I originally just went for a VSR on my van install as this was cheaper than a B2B. I installed this along with 3 x 110Ah batteries and a 200W solar system.

This has worked well for us all through the summer. Of course like everyone else we haven't travelled much this summer so the van has sat largely on the drive with the solar keeping both the leisure and vehicle batteries at just over 14V.

On our last trip it was very dull, therefore limited solar harvesting and although we were travelling quite a bit the battery bank was just not getting charged back up as it should. A quick bit of investigation revealed that it didn't look as though it was getting anything from the alternator.

Hmm, maybe I was wrong about my vehicle not having a smart alternator and working with the VSR. Of course I didn't have my meter with me to check further on our travels.

Further checking when we got home reveals that the alternator is charging but doesn't seem to get above around 13.6V which isn't enough to energise the VSR.

With the above in mind I have already ordered a Victron TR-Smart B2B and had in mind to follow Wildebus' example above for the set up.

So here are the "problems" that have been going round and round in circle with in my head...

If the SOC is below 80% the B2B will be off and the VSR on, but if it is a dull day there will be nothing from the solar and although the VSR is enabled neither the solar or the alternator will be generating a high enough voltage to switch it on so no charging of the leisure batteries will take place.

If the van is parked up at home for a long period with no significant load on the leisure batteries and the SOC is higher than 80% (possibly fully charged) then the VSR will be disabled and the B2B enabled. This means that the vehicle battery will not be receiving any maintenance charge from the solar. Whilst this is not the primary purpose of the setup it is one of the intended benefits of a VSR.

Or, as per my opening statement, have I lost the plot and got this wrapped round my head?

Any advice, greatly appreciated.

Phill

 

[xsilvergs]

I've been thinking about this some more, oh my have I ever, talk about sleepless night! It's quite worrying me actually. I'm a retired electrical engineer and I had a good reputation for my technical knowledge, now I seem to be tying myself up in knots with this. I genuinely worry that I'm losing the plot!

Anyway, enough of my personal trauma

Sorry, this may ramble on a bit, please bear with me...

I originally just went for a VSR on my van install as this was cheaper than a B2B. I installed this along with 3 x 110Ah batteries and a 200W solar system.

This has worked well for us all through the summer. Of course like everyone else we haven't travelled much this summer so the van has sat largely on the drive with the solar keeping both the leisure and vehicle batteries at just over 14V.

On our last trip it was very dull, therefore limited solar harvesting and although we were travelling quite a bit the battery bank was just not getting charged back up as it should. A quick bit of investigation revealed that it didn't look as though it was getting anything from the alternator.

Hmm, maybe I was wrong about my vehicle not having a smart alternator and working with the VSR. Of course I didn't have my meter with me to check further on our travels.

Further checking when we got home reveals that the alternator is charging but doesn't seem to get above around 13.6V which isn't enough to energise the VSR.

With the above in mind I have already ordered a Victron TR-Smart B2B and had in mind to follow Wildebus' example above for the set up.

So here are the "problems" that have been going round and round in circle with in my head...

If the SOC is below 80% the B2B will be off and the VSR on, but if it is a dull day there will be nothing from the solar and although the VSR is enabled neither the solar or the alternator will be generating a high enough voltage to switch it on so no charging of the leisure batteries will take place.

If the van is parked up at home for a long period with no significant load on the leisure batteries and the SOC is higher than 80% (possibly fully charged) then the VSR will be disabled and the B2B enabled. This means that the vehicle battery will not be receiving any maintenance charge from the solar. Whilst this is not the primary purpose of the setup it is one of the intended benefits of a VSR.

Or, as per my opening statement, have I lost the plot and got this wrapped round my head?

Any advice, greatly appreciated.

Phill

We have an Nordelettronica controller in our motorhome and an AGM Leisure battery. The LB is slow to charge, the alternator puts out a little over 14 volts, it ISN'T a smart alternator in my view.

I am preparing to fit a Victron Orion TR-Smart 30A at this moment.

I will NOT be fitting a VSR, it just adds complication and is something else to go wrong. In fact I have the added difficulty of disabling the Split charge Relay which is part of the Nord's PCB.

To charge the SB I use one of these from David

Store – Wildebus

wildebus.com

After the B2B install I intend to go LiFePO4, I'll just parallel another TR-Smart.

Hope that helps.

 

[wildebus]

Just a comment about the concept of B2B AND VSR in the one install. It works well and and be worth doing IF you have the setup that would benefit. i.e. a big bank that you take down to a fairly low SOC on a regular basis.
As an example, in my camper, sometimes when I plug in, I could be feeding the 645Ah of batteries in excess of 100A for 2 or more hours. In THAT case, a mains charger that only delivers 30A would obviously be over 3 times slower. Transfer that to when driving and if you had an alternator that could deliver 100A+ to the battery it would be much faster than if it went through a 30A B2B instead.
But few alternators can deliver close to 100A in reality (in my camper, it seems to be around 50A max). Plus few batteries (talking Lead Acid now) will take a big charge.

Phil, you talk about the 80% SOC switchover from VSR to B2B. This is something I did using a relay and control from a battery monitor. It is not something that is built into any charger. I just want to make that point as you post some fairly detailed info but didn't mention a battery monitor.
The way you would typically determine the switch over point from a VSR to a B2B is at what SOC the charge current lowers to the max output of the B2B. So you have a 30A B2B installed and a VSR. You find that when using the VSR, the current gradually drops as the batteries fill, and the current hits 30A when the SOC is say 75% (the value will vary from install to install due to different battery setups), then that 75% is around your changeover sweet spot to the B2B. If you had a 60A B2B, the changeover SOC would be lower. The sooner you switch to B2B the better the quality (voltage) of the charge. Before that point you are getting quantity (higher current). You need the charge to be at the right voltage level to complete the charging. This is why VSRs are not the best option to charge a battery as the alternator does not put out a high enough voltage to properly charge a battery (a VSR is not a battery charger, it is just an automated passthrough for the alternator).
The switchover point does also depend on the voltage output of the alternator. If it is pretty low you may need to switch to B2B earlier to get the boost. If the Voltage is around what the batteries like, then switching later is fine.

13.6V sounds remarkably low output? If it is that, then your starter battery will not be getting a decent charge to start with anyway (which could be a different issue you need to manage?) A B2B will work fine setup with the right thresholds but as you say, the VSR will not click on. A battery maintainer such as the AMT would probably be the best match for the B2B.

 

[linkshouse]

I have a BMV700 and would have to insert a relay between it and the rest of the circuit as the BMV700 has only a single contact.

I’m not a massively heavy leisure battery user, albeit we have an inverter powered fridge.

I’m thinking my simplest solution may be to just go with the tr-smart on its own and then see how things work out.

I can always come back to the combined setup if I find I need to.

The only downside I see to this is the loss of the maintenance charge for the vehicle battery. Why I liked this was because the van is our only vehicle and between jaunts away it tends to only get used once a week or so to go to the shop and then that is only a round trip of about 2 miles.

it may be that the alternator voltage is so low because both the leisure and vehicle batteries were already fully charged through the solar.

 

[SquirrellCook]

Store – Wildebus

wildebus.com

Forgot about this little devil, it could be the answer to holding up the starter batteries when the multiplus is turned off.