Bow thruster with VSR charging conundrums

[LittleSister]

I have had a bow thruster fitted (wired but not yet connected to batteries) and also intend to install a VSR. I am concerned that the bow thruster (Sleipner SE40, nominal current draw 315A) could potentially fry the VSR, especially if the domestic battery bank to which it is directly connected is depleted.

I currently (arf, arf) have a Paris-Rhone alternator of very modest output (can't find details just now, thought it was just 30A or 40A, but seen some listed as 60A), but likely to be eventually replacing this with something modern giving maybe c75A. Engine battery is c100Ah; Domestic batteries are 2 x c100Ah in parallel; all lead acid. For the purposes of the following lets assume the thruster is connected to the domestic batteries. (Whether it is preferable to connect to the engine battery and alternator is a different question!)

I have, but haven't yet fitted, a VSR and matching battery switches (starting, domestic and 'combine both'), to replace existing diode split-charging and old battery switches, which will all fit very neatly and conveniently in the available space near the existing battery and old battery switches wiring. The VSR, though, has a current capacity of 125A, so would be overloaded if the domestic batteries to which the thruster is wired are depleted, or quite likely even if they aren't.

I have mulled over at length the following thoughts (and now leaning with some trepidation towards Option 3b), and would be grateful for others' views and suggestions on the way forward.

OPTION 1) The (at first) 'obvious' answer would be to fit a high current VSR (e.g Victron i-400A), but this is both (a) very expensive - c£250 (and money is v. tight) - and (b) because of size/layout/style would need to be mounted in a aesthetically and operationally much less than ideal location, also some distance from the batteries, engine and existing & proposed) battery switches, requiring additional wiring and wire lengths and other downsides (when there is already a very long 'to do' list).

OPTION 2) In principle, one could always switch the 'combine both battery banks' closed before using the thruster. In practice, sooner or later it would be forgotten, and in any case I am not sure its operation would necessarily mean that the maximum current capacity of the VSR was never exceeded. (The current draw from engine battery nd alternator would presumably be split approximately in half by the between the two paths - VSR and 'combine both banks' switch.)

OPTION 3a) Fitting a normally closed 12V relay in series with the VSR, which would be operated (opened) by the power supply to the thruster, and therefore disconnect the VSR when the thruster is switched to 'ready to operate'. This (unless/until the really fails - the ones I've seen so far are unbranded and costs only about £5) should prevent any current flowing through the the VSR. If the manual 'combine batteries' switch is not also operated this will prevent the engine battery and alternator contributing to the thruster load, and if it is operated would connect the engine battery and alternator outputs to the thruster draw. The relay current capacity would need to accommodate the maximum charging current to the pair of domestic batteries.

OPTION 3b) I've belatedy thought of this - Fitting a normally open relay, again operated by the thruster power supply as per Option 3a above, in the thin negative wire between the VST and the negative busbar. With this wire open circuit the VSR won't 'see' the 13.7V necessary for it to cut in. (Note this will mean the VSR will have to start from scratch and re-sense whether 12v or 24v supply operation each time the thruster power supply is turned off.)

OPTION 4) Is there an option 4????

What does the team think?

 

[Dunx]

This might help

Bowthruster battery charger

 

[Tranona]

This might help

Bowthruster battery charger

That is about a separate bow thruster battery whereas this question is about using the domestic bank for the thruster.

 

[LittleSister]

This might help

Bowthruster battery charger

Thanks, but sadly not.

 

[Supertramp]

I don't have the answer but you have made me think.

I have a bow thruster and anchor winch connected via long thick cables to my domestic battery bank (2x160ah). I do not have to run the engine to use either but always run the engine with the bow thruster and sometimes not with the anchor winch. I think the anchor winch uses far more current than the thruster. I have a split charging "thing" which charges the engine battery, then domestics, when the engine runs. The alternator is 70A. I have always assumed that the alternator cannot give more than 70A (via charging cables to the batteries) and the majority of the high current drain (via the thick cables from battery to winch/thruster) when anchoring comes from the batteries, and is then replenished by the engine running after the load has gone. I do not knowingly have a VSR unless it is the "thing" next to the engine allocating the alternator charge.

I can see your concern about what is happening to voltages under load but I would have thought the charging and load systems would have different voltages and currents. Charging circuits controlled by the alternator, supply circuits controlled by the load.

Why not just run off the domestic bank with heavy duty cable and switch/relay to activate?

I will retreat back into the undergrowth of ignorance and watch the discussion!

 

[LittleSister]

I don't have the answer but you have made me think.

. . .I think the anchor winch uses far more current than the thruster.

Well, thanks for posting anyway. Perhaps the following will help you understand what you've got and hat the issues area. (I'm no expert, but think I have the following correct.)

My anchor winch uses less current (amps) than thruster, though because the windlass is run for longer each time it may well use more amp-hours.

(My windlass I don't have details to hand, but IIRC it has 100A breaker, and (certainly in accordance with manufacturer's instructions) 10sqmm cables. Thruster has nominal 315A current draw and recommended min 70mm, pref 90mm, cables for my cable run (same length as for windlass).

I have always assumed that the alternator cannot give more than 70A (via charging cables to the batteries) and the majority of the high current drain (via the thick cables from battery to winch/thruster) when anchoring comes from the batteries, and is then replenished by the engine running after the load has gone. I do not knowingly have a VSR unless it is the "thing" next to the engine allocating the alternator charge.

i think unless it actually says 'Voltage Sensitive Relay', 'VSR' or 'DVSR', and/or has an indicator light on it, the 'thing' you have is almost certainly a charge splitting diode array, in which case your assumption is correct - the thruster and windlass current draw is coming from one of your (sets of) your batteries, and both (sets) of batteries are being topped up (during and after load) by the alternator output. With diodes the batteries can't supplement or charge one another, and one battery (set) can't power something connected to the other battery (set). The downside is (a) the diodes' resistance reduces the charge at high currents (=low batteries, i.e. when you most need it!), and (b) it is dividing its charge between two (sets of) batteries, when replenishing the engine battery is likely to be a more urgent priority. Expensive diode sets reduce the resistance penalty compares to the standard ones, but don't eliminate it. This is the same arrangement as on my boat currently, but which I am intending to change .

By contrast, VSR delivers the full charge, and at first only to the engine battery. Then when (and only when) the engine battery is (more or less) fully charged does the alternator start charging the domestic battery (too).

A VSR also allows - and this is a mixed blessing and the root of the challenge in my situation - current to flow between batteries until the voltage of both batteries (or, in some cases, only the engine battery) falls below about 12.7V, when again it disconnects the two and the alternator is then only connected to engine battery.

In my case there is a risk, or even likelihood, that when the thruster is run and also the VSR is open closed because the engine battery is fully charged, it will draw current from both the domestic and engine batteries, and that the current flowing from engine battery and alternator through the VSR towards the domestic batteries and onward to the thruster, will exceed the current capacity of the VSR and quite possibly destroy it. (The current draw of the thruster will rapidly pull the voltage of the batteries to the 12.7 volt level where the VSR would normally open the circuit between the battery banks, and hence stop current flowing from the engine battery and alternator to the domestic battery and thruster, but the VSR has built in delay of 20 seconds (to stop it chattering on and off when near the switching level), and 20 seconds may well be too late.)

Why not just run off the domestic bank with heavy duty cable and switch/relay to activate?

My thruster is already connected via heavy duty cables and a switch/contact breaker reaching to the area of the batteries and alternator, though I haven't yet connected them to a battery as I was intending to upgrade the battery switches and at the same time add the matching VSR and emergency 'combine both batteries' (the latter instead of having to carry jump leads to connect the domestic batteries to assist engine starting if that is needded for any reason).

 

[Sandy]

I have had a bow thruster fitted (wired but not yet connected to batteries) and also intend to install a VSR. I am concerned that the bow thruster (Sleipner SE40, nominal current draw 315A) could potentially fry the VSR, especially if the domestic battery bank to which it is directly connected is depleted.

Why do you think it will fry a VSR?

Bow thrusters are used for short bursts when they draw current from the battery, VSRs sit between the battery and some form of power generation. The result of using your bow thruster would be to drain the available power at the diffrence between the power demand and the power generated.

 

[Supertramp]

Thanks for clarifying. I am surprised the thruster draws so much more than a winch under load.

Yes, I have a split charge diode. It works so I ignore it!

I understand the desire to separate or combine batteries and the function of the VSR. I would want my engine battery reserved solely for engine starting.

Would the same happen in reverse if you combine batteries and start the engine? Isn't the VSR in the charge lines from alternator to batteries and not on the load side? This problem must happen to everyone - why not put a fuse or trip fuse on the engine battery line to check what happens the first time?

 

[LittleSister]

Why do you think it will fry a VSR?

Bow thrusters are used for short bursts when they draw current from the battery, VSRs sit between the battery and some form of power generation. The result of using your bow thruster would be to drain the available power at the diffrence between the power demand and the power generated.

The VSR sits between the the domestic batteries (and in this case they're connected to the thruster) and the engine battery which is connected to the alternator.

When thruster is run it will draw current from (a) the 2 domestic batteries, (b) the engine battery, and (c) the alternator output. The contribution of B and C to the nominal 315A current draw will exceed the 125 capacity of the VSR.

p.s. Manufacturer's circuit diagram showing VSR connects the engine and domestic batteries, and that the alternator is connected only to the engine battery, here - https://www.bepmarine.com/-/media/inriver/710-140A_INSTR_BEP.pdf?modified=20230328095440

 

[LittleSister]

Thanks for clarifying. I am surprised the thruster draws so much more than a winch under load.

Yes, I have a split charge diode. It works so I ignore it!

I understand the desire to separate or combine batteries and the function of the VSR. I would want my engine battery reserved solely for engine starting.

Would the same happen in reverse if you combine batteries and start the engine? Isn't the VSR in the charge lines from alternator to batteries and not on the load side? This problem must happen to everyone - why not put a fuse or trip fuse on the engine battery line to check what happens the first time?

With VSR the engine battery is effectively reserved for engine starting - aside from what is more or less the 'float charge' above 12.7v. As soon as the engine battery falls to this 12.7V level everything else is disconnected and the full available output of the alternator is applied only to the engine battery until it's back to13.2V
.
You have a good point about 'isn't the same happening in reverse', by which I take it you mean when one starts the engine, assuming the batteries are fully charged, a significant part of the current going to the starter motor will initially be coming from the domestic batteries. I don't know how much my Bukh DV36 (1,500cc triple diesel), or even a typical, starter motor draws, but IIRC my engine battery can put out 400A (at least that's its CCA spec, IIRC).

That 'reverse situation'is something I'd like others' advice on. In theory, as best i understand it, the batteries will fall almost immediately to 12.7V (whether its the thruster or starter motor that's the load). The VSR will shut off the connection between the batteries at this voltage, but only after a 20 seconds (deliberate) delay. The question is will the VSR be burnt out (either open or closed circuit) by the excess current in this time, or are there other considerations I'm unaware of.

 

[geem]

A quick look at the technical spec and it says the unit has a 2.2kw motor. The run current at 12v is way less than the quoted current draw. The inrush current will be far higher but only for a very, very short time whilst the motor accelerates from stationary to running speed. You do not normally need to allow for the inrush current in cable sizing and therefore VSR. The run current will be circa 183amps. You probably need a VSR rated for the run current but not the inrush current.
I run my windlass. (1.75kw motor) from the domestic batteries and its surprising how little current it draws. I never see more than 50amp (24v so double it for 12v system) dip on ammeter.

 

[Plum]

I have had a bow thruster fitted (wired but not yet connected to batteries) and also intend to install a VSR. I am concerned that the bow thruster (Sleipner SE40, nominal current draw 315A) could potentially fry the VSR, especially if the domestic battery bank to which it is directly connected is depleted.

I currently (arf, arf) have a Paris-Rhone alternator of very modest output (can't find details just now, thought it was just 30A or 40A, but seen some listed as 60A), but likely to be eventually replacing this with something modern giving maybe c75A. Engine battery is c100Ah; Domestic batteries are 2 x c100Ah in parallel; all lead acid. For the purposes of the following lets assume the thruster is connected to the domestic batteries. (Whether it is preferable to connect to the engine battery and alternator is a different question!)

I have, but haven't yet fitted, a VSR and matching battery switches (starting, domestic and 'combine both'), to replace existing diode split-charging and old battery switches, which will all fit very neatly and conveniently in the available space near the existing battery and old battery switches wiring. The VSR, though, has a current capacity of 125A, so would be overloaded if the domestic batteries to which the thruster is wired are depleted, or quite likely even if they aren't.

I have mulled over at length the following thoughts (and now leaning with some trepidation towards Option 3b), and would be grateful for others' views and suggestions on the way forward.

OPTION 1) The (at first) 'obvious' answer would be to fit a high current VSR (e.g Victron i-400A), but this is both (a) very expensive - c£250 (and money is v. tight) - and (b) because of size/layout/style would need to be mounted in a aesthetically and operationally much less than ideal location, also some distance from the batteries, engine and existing & proposed) battery switches, requiring additional wiring and wire lengths and other downsides (when there is already a very long 'to do' list).

OPTION 2) In principle, one could always switch the 'combine both battery banks' closed before using the thruster. In practice, sooner or later it would be forgotten, and in any case I am not sure its operation would necessarily mean that the maximum current capacity of the VSR was never exceeded. (The current draw from engine battery nd alternator would presumably be split approximately in half by the between the two paths - VSR and 'combine both banks' switch.)

OPTION 3a) Fitting a normally closed 12V relay in series with the VSR, which would be operated (opened) by the power supply to the thruster, and therefore disconnect the VSR when the thruster is switched to 'ready to operate'. This (unless/until the really fails - the ones I've seen so far are unbranded and costs only about £5) should prevent any current flowing through the the VSR. If the manual 'combine batteries' switch is not also operated this will prevent the engine battery and alternator contributing to the thruster load, and if it is operated would connect the engine battery and alternator outputs to the thruster draw. The relay current capacity would need to accommodate the maximum charging current to the pair of domestic batteries.

OPTION 3b) I've belatedy thought of this - Fitting a normally open relay, again operated by the thruster power supply as per Option 3a above, in the thin negative wire between the VST and the negative busbar. With this wire open circuit the VSR won't 'see' the 13.7V necessary for it to cut in. (Note this will mean the VSR will have to start from scratch and re-sense whether 12v or 24v supply operation each time the thruster power supply is turned off.)

OPTION 4) Is there an option 4????

What does the team think?

I agree that in theory, if both batteries are over 12.7 and the VSR contacts are closed (both batteries connected together) and you operate the bow thruster or crank the engine the VSR may be subjected to a burst of amps far exceeding its rated capacity. However, in practice that does not happen. I have a VSR rated at 120A continuous, 180A for 5 seconds. My starter can draw 500A. Not sure of the Bowthruster although it has a 300A fuse. However, next to my VSR I have a 100A fuse to protect it. It has never blown in 6 years, so the theory is not borne out in practice.

www.solocoastalsailing.co.uk

 

[LittleSister]

A quick look at the technical spec and it says the unit has a 2.2kw motor. The run current at 12v is way less than the quoted current draw. The inrush current will be far higher but only for a very, very short time whilst the motor accelerates from stationary to running speed. You do not normally need to allow for the inrush current in cable sizing and therefore VSR. The run current will be circa 183amps. You probably need a VSR rated for the run current but not the inrush current.
I run my windlass. (1.75kw motor) from the domestic batteries and its surprising how little current it draws. I never see more than 50amp (24v so double it for 12v system) dip on ammeter.

Thank you. That's very useful.

I had a vague memory that the start current was very high, and was going to mention it but my post were already rather long. I've never known, though, whether the 'nominal 315A' current draw included or excluded that initial higher current draw.

I'm still a little uncertain where it leaves me, but leaning towards suck it and see with what I've got.

The current version of my VSR (which is actually a slightly earlier version) says max 125A continuous (same as stated on the my VSR itself), and max 140A intermittent (not actually mentioned on mine, and I might or might not have a paper data sheet for it somewhere and can't find one online). If the domestic batteries are at all run down (which they often are, sometimes quite significantly - partly why I hope to improve the switching and charging system) and the engine battery is fully charged (which it typically is), at 315A, if that is the momentary start up current, there would be more than 125A going through the VSR, as there would for the 183A 'running'current' if the domestic batteries are very run down (which would not be very unusual), especially once you add in the current available from the alternator.

 

[LittleSister]

I agree that in theory, if both batteries are over 12.7 and the VSR is closed (both batteries connected together) and you operate the bow thruster or crank the engine the VSR may be subjected to a burst of amps far exceeding its rated capacity. However, in practice that does not happen. I have a VSR rated at 120A continuous, 180A for 5 seconds. My starter can draw 500A. Not sure of the Bowthruster although it has a 300A fuse. However, next to my VSR I have a 100A fuse to protect it. It has never blown in 6 years, so the theory is not borne out in practice.

www.solocoastalsailing.co.uk

That's very reassuring. Thank you!

 

[Tranona]

I think you are making something out of nothing here. The VSR is between the alternator and the batteries and is sized to cover the maximum output from the alternator. A 125A VSR is the basic and will cover alternators up to the now common 115A units fitted to many modern engines (although in reality chage currents are rarely anywhere near the maximum of the alternator with LA batteries). The draw from the bow thruster is from the battery and your cables will be fused to protect the cables from the battery to the thruster. It works whether the engine is running or not. If you are running the engine when using the bow thruster or the windlass the draw will probably be greater than the alternator is putting through the VSR but will be replacing some of the consumption. Sounds like you are using a BEP Marine switch cluster. I have one of these, the third I have fitted. The first one had a Lofrans windlass powered from the house bank and never gave any problems.

I have fitted one to my current project BUT I have a bow battery, just as on my last boat (a Bavaria) that serves the windlass and the thruster. However rather than charge it from a 3 way splitter from the alternator, with the need to run heavy cables through the boat I have used the now common method of a B2B charger from the start battery. This only needs 10mm2 cables. If the thruster is used its battery is immediately charged because the VSR gives priority to the start battery, which itself will already be fully charged.

Not keen on using high draw things like thrusters from the domestic bank. Start, domestic and bow batteries have different requirements and different usage patterns so to my mind should be independent. My start is a high output (950CCA) 28Ah Odyssey AGM, House is 2*95Ah AGMs and bow 110Ah Numax Leisure (came with boat otherwise would be another 95Ah Exide AGM). I don't do long periods away from shorepower so on board charging is from the 70A alternator and shorepower 15A to domestic bank only with trickle charger to start. Not much point in shorepower to bow battery as this will likely be fully charged at the end of a trip and the engine will be running when it is used at the start of the next trip.

 

[PaulRainbow]

I have had a bow thruster fitted (wired but not yet connected to batteries) and also intend to install a VSR. I am concerned that the bow thruster (Sleipner SE40, nominal current draw 315A) could potentially fry the VSR, especially if the domestic battery bank to which it is directly connected is depleted.

I currently (arf, arf) have a Paris-Rhone alternator of very modest output (can't find details just now, thought it was just 30A or 40A, but seen some listed as 60A), but likely to be eventually replacing this with something modern giving maybe c75A. Engine battery is c100Ah; Domestic batteries are 2 x c100Ah in parallel; all lead acid. For the purposes of the following lets assume the thruster is connected to the domestic batteries. (Whether it is preferable to connect to the engine battery and alternator is a different question!)

I have, but haven't yet fitted, a VSR and matching battery switches (starting, domestic and 'combine both') which will all fit very neatly and conveniently in the available space near the existing battery and old battery switches wiring. The VSR, though, has a current capacity of 115A, so would be overloaded if the domestic batteries to which the thruster is wired are depleted, or quite likely even if they aren't.

I have mulled over at length the following thoughts, and would be grateful for others' views and suggestions on the way forward.

OPTION 1) The 'at first obvious' answer would be to fit a high current VSR (e.g Victron i-400A), but this is both (a) very expensive - c£250 (and money is v. tight) - and (b) because of size/layout/style would need to be mounted in a visually and operationally much less than ideal location, also some distance from the batteries, engine and existing & proposed) battery switches, requiring additional wiring and wire lengths and other downsides (when there is already a very long 'to do' list).

OPTION 2) In principle, one could always switch the 'combine both battery banks' closed before using the thruster. In practice, sooner or later it would be forgotten, and in any case I am not sure its operation would necessarily mean that the maximum current capacity of the VSR was never exceeded. (The current draw from engine battery nd alternator would presumably be split approximately in half by the between the two paths - VSR and 'combine both banks' switch.)

OPTION 3) Fitting a normally closed 12V relay in series with the VSR, which would be operated (opened) by the power supply to the thruster, and therefore disconnect the VSR when the thruster is switched to 'ready to operate'. This (unless/until the really fails - the ones I've seen so far are unbranded and costs only about £5) should prevent any current flowing through the the VSR. If the manual 'combine batteries' switch is not also operated this will prevent the engine battery and alternator contributing to the thruster load, and if it is operated would connect the engine battery and alternator outputs to the thruster draw. The relay current capacity would need to accommodate the maximum charging current to the pair of domestic batteries.

OPTION 4) Is there an option 4????

What does the team think?

I'm not sure there would be a problem, but....

Option 4:

fit two normally closed relays, in series, in the negative cable to the VSR. Connect the positive port remote terminal of the thruster to the positive coil wire of one relay and the positive stb remote terminal of the thruster to the positive coil wire of the other relay. When the thruster is operates in either direction the negative supply to the VSR will be broken and the VSR will open.

 

[PaulRainbow]

I think you are making something out of nothing here. The VSR is between the alternator and the batteries and is sized to cover the maximum output from the alternator. A 125A VSR is the basic and will cover alternators up to the now common 115A units fitted to many modern engines (although in reality chage currents are rarely anywhere near the maximum of the alternator with LA batteries).

Sorry, this is incorrect.

The VSR is connected between the two battery banks and closes to parallel the banks when the voltage of one bank or the other reaches the set point of the VSR. It's true that the VSR needs to be rated for the maximum alternator (or other charging source) but it should also be rated to withstand the loads passed through it if the battery that's being drawn from is depleted and the other battery is either fully charged, or is being charged.

 

[Tranona]

Sorry, this is incorrect.

The VSR is connected between the two battery banks and closes to parallel the banks when the voltage of one bank or the other reaches the set point of the VSR. It's true that the VSR needs to be rated for the maximum alternator (or other charging source) but it should also be rated to withstand the loads passed through it if the battery that's being drawn from is depleted and the other battery is either fully charged, or is being charged.

OK so how many VSRs are rated to 315A if that is the current draw of a bow thruster that might go through the VSR?

I must be missing something here as the VSRs I have seen appear to be 140A and must be plenty of boats using them and running bow thrusters off the domestic banks and run the risk you described.

 

[LittleSister]

I'm not even sure there would be a problem, but....

Option 4:

fit two normally closed relays, in series, in the negative cable to the VSR. Connect the positive port remote terminal of the thruster to the positive coil wire of one relay and the positive stb remote terminal of the thruster to the positive coil wire of the other relay. When the thruster is operates in either direction the negative supply to the VSR will be broken and the VSR will open.

Thanks, Paul.

So disarming the VSR by isolating its negative line is not a crazy idea.

I don't think there's a need to connect separate relays to the separate relays for port and starboard. I think a similar thing can be achieved with a single relay. My thruster has a 'master' power supply for the whole thing - both port and starboard relays, and the control panel(s). Even when that's on (which I envisage being on for fairly extended periods for caution - e.g. when approaching any harbour, even though I doubt I'll often actually use the thruster (we'll see! )) the controls then have to be 'armed' (for safety reasons) by pressing a pair of buttons before any of the buttons will actually operate the thruster itself, and they are automatically disarmed again a few minutes after being switched on or last used. I was planning to try to connect any relay disconnecting the VSR so that it is operated only when the thruster controls are armed. This - like your suggested arrangement - would avoid the VSR (and domestic battery charging) being disconnected for relatively extended periods, or even very long periods if I'd forgotten to isolate the thruster circuit.

 

[LittleSister]

OK so how many VSRs are rated to 315A if that is the current draw of a bow thruster that might go through the VSR?

I must be missing something here as the VSRs I have seen appear to be 140A and must be plenty of boats using them and running bow thrusters off the domestic banks and run the risk you described.

Very few VSR are of that sort of capacity - though you can buy them (e.g the Victron i - 400 A), and might need one if you have very high charging capacity for whatever reason.

Like you, I was trying to understand how the typical 140A (or less) VSR could manage the sorts of currents one could imagine being drawn through them by thrusters etc, and that's what this thread was intended to find out.

I still don't know exactly how, but I'm reassured that in practice it is not a problem (or at least not a common one).

Paul is correct that the VSR links the engine battery bank and the domestic battery bank. The alternator is connected only to the engine battery bank, and only charges the domestic battery bank via the VSR when that closes the circuit (when one or other battery reaches , say, 13.2V). The thruster is typically (but not always) connected to the domestic bank unless there is a third dedicated thruster battery bank (other previous threads have addressed that issue - you could have a second VSR between the domestic and thruster battery banks).

You can see that for yourself that the VSR connects the two batteries (or battery banks) on the VSR installation circuit diagram in this manufacturer's data sheet here -
https://www.bepmarine.com/-/media/inriver/710-140A_INSTR_BEP.pdf?modified=20230328095440