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Small Boat BMS/LFP with backup

I got the cerbo (and the gx50) because it ties the whole system together. It speaks VE.Direct to my MPPT, VE.Bus to the inverter/charger, and CAN Bus to my sensor network, BMS, and alternator. It’s what allows the BMS to drive the whole charging/discharging process so that I will never have a disconnect. It’s also a very nice piece of kit that allows me to monitor the boat remotely, and adjust my electrical system remotely.

While the boat is in what amounts to “storage mode” I set the DVCC function to 13.2v which limits things to 50% SoC or so. Setting that one setting on the Cerbo adjusts all my charging sources (other than the alternator I suppose), and works very well. Then, the day before I’m going out for an overnight sailing I’ll log in, remove the limit, turn on the inverter/charger, and let my batteries charge up to 100%.

I also got it for all the other automation and monitoring capabilities. We have it watching bilge levels, the fuel tank, the water tank, blackwater… plus it can control the bilge pumps. Lastly, we’re using it as a SignalK server, and starting to play with automation in NodeRed.

It’s a very flexible, very cool piece of kit, that’s under constant development.

As far as the fuse sizing for the alternator, remember fuses are there to protect the wire. I have a 160A fuse on my alternator lead, which is a 2awg wire. That fuse is undersized for the wire, but my alternator is only an 85A unit. I probably won’t swap it out though when we repower and switch to a 120A alternator.
 
I got the cerbo (and the gx50) because it ties the whole system together. It speaks VE.Direct to my MPPT, VE.Bus to the inverter/charger, and CAN Bus to my sensor network, BMS, and alternator. It’s what allows the BMS to drive the whole charging/discharging process so that I will never have a disconnect. It’s also a very nice piece of kit that allows me to monitor the boat remotely, and adjust my electrical system remotely.

While the boat is in what amounts to “storage mode” I set the DVCC function to 13.2v which limits things to 50% SoC or so. Setting that one setting on the Cerbo adjusts all my charging sources (other than the alternator I suppose), and works very well. Then, the day before I’m going out for an overnight sailing I’ll log in, remove the limit, turn on the inverter/charger, and let my batteries charge up to 100%.

I also got it for all the other automation and monitoring capabilities. We have it watching bilge levels, the fuel tank, the water tank, blackwater… plus it can control the bilge pumps. Lastly, we’re using it as a SignalK server, and starting to play with automation in NodeRed.

It’s a very flexible, very cool piece of kit, that’s under constant development.

As far as the fuse sizing for the alternator, remember fuses are there to protect the wire. I have a 160A fuse on my alternator lead, which is a 2awg wire. That fuse is undersized for the wire, but my alternator is only an 85A unit. I probably won’t swap it out though when we repower and switch to a 120A alternator.
Thanks svagres
 
@svsagres @wholybee

Referring to the most current diagram in post #56

Regarding the alternator, I don't show a Mega fuse for the Alternator. Where should it be and roughly what size for 160A field limited to 100A?

Also do you have a SmartBatteryProtect 65 device between the DC Panel and the Battery to protect the battery from discharge? Or do you consider the BMS disconnect switch adequate? I thought that normal it is best to have the BMS disconnect as the last protection.

The diagram made by svagres in his thread, does show the smartbatteryProtect 65...
 
I consider the SmartBatteryProtect unnecessary for a FET based BMS. If I understand svagres's drawing, it is being used as an Solid State Relay because the REC BMS does not otherwise have the ability to stop discharge.

The BMS is the last line of defense. The BMS should never need to disconnect. SmartBatteryProtect or not doesn't change that.
 
I consider the SmartBatteryProtect unnecessary for a FET based BMS. If I understand svagres's drawing, it is being used as an Solid State Relay because the REC BMS does not otherwise have the ability to stop discharge.

The BMS is the last line of defense. The BMS should never need to disconnect. SmartBatteryProtect or not doesn't change that.
That’s it exactly, and why I used the Victron as an SSR. Since these contactors/relays should never disengage, I want them using as little power as possible. The Victron units are reasonably priced, and use very little power. Unfortunately they’re unidirectional, so I used the BlueSea 7713 on the high side.
 
SSR= Solid State Relay (I think). The Smart Battery Protect is a FET based product ,similar to an ArgoFet (but different purpose)? It does not draw much current. So the Smart Battery Protect 65 is used to turn off the DC Panel use of LFP before the BMS has to activate the disconnect. It is a belt and suspenders solution, which uses the BMS as last line of defense for LFP over discharge protection or load bus.

Svsagres, uses "high side" for the "charging bus".

I think the Victron Smart Battery Protect 65 is a good thing to have, but it could be called (optional) because the BMS will do the same job (if it has FETs like the DalyBMS common bus). Smart Battery Protect 65 specs include
Power consumption max 1.5 mA, Load disconnect delay when triggered by BMS is instanteneous,
Load disconnect delay when triggered by low voltage of 10.5 or 21 Volts is 90 Seconds, Load reconnect delay 30 seconds when voltage gets to 12 or 24 Volts, Built-in Bluetooth makes programming easy and allows incremental settings.

I don't believe the Daly BMS single common port will have the ability to trigger the Smart Protect in advance of its disconnection of the LFP, (DalyBMS has FETS to disconnect all built in) so a REC, TAO, or Overkill BMS might be better. However the Smart Protect will also disconnect on low Voltage of 10.5v.

Thank you both wholybee and svsagres for your explanations.
 
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Now I still have the question of where the fuse goes for the alternator and how big a fuse, for a 160A alternator field limited to <100A.

I will need to make some adjustments to the diagram. Then I will try to work out how to configure it for my boat, perhaps using Victron Lynx to simplify as svsagres did.
 
The fuse is there to protect the wiring. This means that it normally goes closest to the largest source of current, which would be your battery. Based on the ABYC rules, your alternator would be a current limited device, so as long as the wiring is sized to carry all the current the alternator is rated to produce, it doesn’t need to be fused at the alternator end.
 
I consider the SmartBatteryProtect unnecessary for a FET based BMS. If I understand svagres's drawing, it is being used as an Solid State Relay because the REC BMS does not otherwise have the ability to stop discharge.

The BMS is the last line of defense. The BMS should never need to disconnect. SmartBatteryProtect or not doesn't change that.
"because the REC BMS does not otherwise have the ability to stop discharge" Yes, I understand what you mean, but actually the REC BMS depends on having relay or fet devices which react to a signal (Blue Seas #7713 ML-RBS for the Charge bus and Smart BatteryProtect for the Discharge Bus, see diagram at post #42) . The REC BMS has multiple control circuits that are adjustible.

FET Based BMS like the DalyBMS common bus, have the FET disconnects built in. Some people don't like the use of FETS built into BMS because they don't have control about their quality however some BMS have very robust FETS.
 
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Yes, in that drawing the alt sense would need to go to the common terminal of the switch.
Mainesail (Rod) explains and shows why here https://marinehowto.com/alternators-voltage-sensing/
Also he has a good diagram for a "safe" Alternator Service Switch installation and also shows fuse locations, although one is not necessary at the Alternator if the wiring is sized right. (svsagres) However you do need a fuse at the battery end to protect the wiring as I understand it.

svsagres wrote about fusing for Alternator:

As far as the fuse sizing for the alternator, remember fuses are there to protect the wire. I have a 160A fuse on my alternator lead, which is a 2awg wire. That fuse is undersized for the wire, but my alternator is only an 85A unit. I probably won’t swap it out though when we repower and switch to a 120A alternator.
 
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Back to post #47 and ABYC Wire Size Calculator to size Alternator wires from the LFP to the 160A Alternator using the full capacity of the alternator (even though it will be field current limited to 100A)
1. Use 3% max voltage drop.
2. Partly goes through engine compartment, so insulation should be for engine temps or 75Cdegrees
3. Will measure distance to LFP and double it for all the cable run, for example use 26 feet (add pos and neg cable lengths together)
4. What amps should I use? using 12vdc at 160amps, 2 conductors (but not in a bundle!) 00 (2/0) AWG cable cross section.
5. Going to the calculator's ampacity section, ampacity of the conductor is 198.8 ampere and the fuse should not exceed that. So we could use a 160A fuse on the battery side.

If the 26' length of wire turns out to be 22' (or positive 11' + negative 11') then I can use 0 (1/0) AWG wire.
Also, I can change from 3% max voltage drop, to 10% since I am not going to be charging at that rate, but at 100A, and that will drop the wire size required down to 0 (1/0) AWG wire for from 1' - 60' of wire. Also the ampacity stays the same 198.8.

Then I can size the 100A Alternator charging intended as 3% drop, so for 26' total, in an engine compartment, 1 AWG would be ok, but I need to use 0 (1/0) AWG and protect the wire with a 160A fuse which is less than the 198.8 ampere ampacity calculated.
 
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Revised today 8/9/2022 Revision 4
Simple Common Bus BMS Rev 8-9-2022-4.jpg
 
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Revised today 8/9/2022 Revision 4
View attachment 106183
This scheme is not ISO/TS 23625:2021 compatible and in fact is against the spirt of ABYC TE-13 and therefore is not suitable for boats , ISO does not allow different chemistries to be connected together and doesn't allow any device to be upstream of the battery disconnect ( except the BMS) , in this case the Orion TR1212 / MPPT controllers , alternator should be downstream of the cutoff and to be ISO compliant the 3 way switch should be removed ( or no common connection option allowed ) . I notice no remote BMS shutdown of the WS500 is shown . Note also that ISO and ABYC require advance warning of any intended Li disconnect event, this is not shown either

for proper charging tail current measurements should be fed to the MPPT controllers or better yet allow the BMS to dictate all charger termination commands rather then individual charge controllers deciding themselves

Note also as I see it , if the 3 way switch was switched while the alternator was running the disconnect break before make action would cause the alternator to blow its diodes and the spike would likely kill anything connected to the alternator. note that an APD can protect the alternator but it cannot protect downstream electronics as typically the APD TVS diodes will be 28V or 32V clamping ( for a 12V system ) and this can take out 12V equipment not so rated( the MPPT controllers would be very vulnerable here ). where the alternator is subject to high load abrupt disconnects , no other 12v device should be in circuit even with an APD fitted , in a good system any alternator disconnect would be via a field circuit shutdown

This diagram is not a good template for such a system , ether for a van and especially for a boat

my own view is that Li disconnects should be high side mechanical based , FETS have significant weaknesses and fail shorted , most MOSFETS have about 80V DS max limits and these systems can easily spike above that and FETS are also sensitive to rapid di/dt currents and can auto turn in extremis. Low side disconnects are not a good idea as the risk of an inadvertent path to DC negative can bypass the disconnect
 
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@Goboatingnow You are being a super alarmist. This is under development and I have asked for review and suggestions.

Furthermore the Locking 3 way switch is there for a purpose, to shutdown the engine, unlock the switch and use the start battery in lieu of the LFP, the purpose is to use the start bank in the event of LFP/BMS issues.

Additionally, the DalyBMS common port is shown as a simple inexpensive solution with internal FETS, with the option of using the more expensive REC BMS or TAO BMS interlinked with the Wakespeed WS500 as shown in earlier diagrams.

I don't understand some of what you are talking about. Strongly held opinions are fine, but absolutes simply asserted are not appropriate and do not contribute to learning. Perhaps others can advise.

I think you just don't like the DalyBMS common bus FETS and the possible connection of locked 3-way to the SLA. Perhaps you can make some positive suggestions to achieve the goal to continue to power essential equipment with the SLA? While avoiding excessive expense and being cognizant of the issues you've raised.

Also, the way I intend to use this system does not join different chemistries, so it would comply. The 3-way switch would not have a "BOTH"!!!
 
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@Goboatingnow You are being a super alarmist. This is under development and I have asked for review and suggestions.

Furthermore the Locking 3 way switch is there for a purpose, to shutdown the engine, unlock the switch and use the start battery in lieu of the LFP, the purpose is to use the start bank in the event of LFP/BMS issues.

Additionally, the DalyBMS common port is shown as a simple inexpensive solution with internal FETS, with the option of using the more expensive REC BMS or TAO BMS interlinked with the Wakespeed WS500 as shown in earlier diagrams.

I don't understand some of what you are talking about. Strongly held opinions are fine, but absolutes simply asserted are not appropriate and do not contribute to learning. Perhaps others can advise.

I think you just don't like the DalyBMS common bus FETS and the possible connection of locked 3-way to the SLA. Perhaps you can make some positive suggestions to achieve the goal to continue to power essential equipment with the SLA? While avoiding excessive expense and being cognizant of the issues you've raised.

Also, the way I intend to use this system does not join different chemistries, so it would comply. The 3-way switch would not have a "BOTH"!!!
I accept ( and I mention this ) that of the 3 way switch has no common then the system would be acceptable. Personally I would provide that backup switching using jump leads , the reality is disconnected Li should be an extremely rare condition and one that virtually never happens. I carry a set of jump leads for that very purpose in my case.

Secondly alternator protection is unnecessary if proper field coil disconnect is present and in An Li system it should always be. Alternator protection can protect the alternator but can fry attached 12v electronics

I see no point to the panel associated smart switch. The bms is doing this function anyway. A low voltage Li situation should never realistically occur , if it does regularly the system is sized incorrectly.

A better situation is to break the dc panel into high and low priority loads and to only disconnect low priority loads first but it’s an unnecessary complication

Also my own view ( based on tests ) is cell balancing is unnecessary in pre balanced Li batteries. It’s something that may be needed once or twice a year. As all it results in is a slight loss of capacity, the evidence is with fractional C charging and discharging , LFp actually stays remarkably in balance over long periods. Hence your bms should be focused on LVC , HVC and temp . A better name would be a cell monitor

Preferentially , the Bms should centrally control all Li charge sources including alternator , mains and solar rather then each device deciding on its own. Solutions like Cerbo Gx can integrate these solutions. , Victron mppt also has a remote off solution to aid this type of control.

As I said LI disconnect should be viewed as a major fault. It should never ever happen in the normal course of events, charging should stop well before HVC and loads should be preferentially disconnected well before LVC. In my view also overcurrent protection should be handled by fuses not the BMS. Li disconnect should be mechanical with extremely high interrupt ability and arguably be “fail safe “ , ie opens automatically if the monitoring input fails.

Your diagram contains many correct and appropriate solutions just missing a few key points

I’m a professional retired EE with a background in industrial battery systems amongst many other fields. I have read the relevant ABYC and ISO lithium standards.
 
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"because the REC BMS does not otherwise have the ability to stop discharge" Yes, I understand what you mean, but actually the REC BMS depends on having relay or fet devices which react to a signal (Blue Seas #7713 ML-RBS for the Charge bus and Smart BatteryProtect for the Discharge Bus, see diagram at post #42) . The REC BMS has multiple control circuits that are adjustible.

FET Based BMS like the DalyBMS common bus, have the FET disconnects built in. Some people don't like the use of FETS built into BMS because they don't have control about their quality however some BMS have very robust FETS.
Correct. But looking at the drawing, the REC is not connected to a relay or FET device, even though it could be. The Battery Protect is doing that. The battery protect also can be controlled by the REC, although it isn't shown but that is how I would configure it.
 
I accept ( and I mention this ) that of the 3 way switch has no common then the system would be acceptable. Personally I would provide that backup switching using jump leads , the reality is disconnected Li should be an extremely rare condition and one that virtually never happens. I carry a set of jump leads for that very purpose in my case.

Secondly alternator protection is unnecessary if proper field coil disconnect is present and in An Li system it should always be. Alternator protection can protect the alternator but can fry attached 12v electronics

I see no point to the panel associated smart switch. The bms is doing this function anyway. A low voltage Li situation should never realistically occur , if it does regularly the system is sized incorrectly.

A better situation is to break the dc panel into high and low priority loads and to only disconnect low priority loads first but it’s an unnecessary complication

Also my own view ( based on tests ) is cell balancing is unnecessary in pre balanced Li batteries. It’s something that may be needed once or twice a year. As all it results in is a slight loss of capacity, the evidence is with fractional C charging and discharging , LFp actually stays remarkably in balance over long periods. Hence your bms should be focused on LVC , HVC and temp . A better name would be a cell monitor

Preferentially , the Bms should centrally control all Li charge sources including alternator , mains and solar rather then each device deciding on its own. Solutions like Cerbo Gx can integrate these solutions. , Victron mppt also has a remote off solution to aid this type of control.

As I said LI disconnect should be viewed as a major fault. It should never ever happen in the normal course of events, charging should stop well before HVC and loads should be preferentially disconnected well before LVC. In my view also overcurrent protection should be handled by fuses not the BMS. Li disconnect should be mechanical with extremely high interrupt ability and arguably be “fail safe “ , ie opens automatically if the monitoring input fails.

Your diagram contains many correct and appropriate solutions just missing a few key points

I’m a professional retired EE with a background in industrial battery systems amongst many other fields. I have read the relevant ABYC and ISO lithium standards.
Agreed on the 3 way switch. That had been mentioned earlier in the thread. But although neither ABYC or ISO allow it, I have yet to hear any technical reason why it isn't allowed. I know if many installations that parallels different chemistries without any significant issues. Rumors of reduced cell life, but certainly no reports of a hazard.

Agree on the alternator protection. Unfortunately, that feature is only available on what I otherwise consider ridiculously overpriced and over complicated BMSs. And I don't consider it a significant issue, because the BMS should never ever disconnect anyway. Years ago when the technology wasn't well understood, maybe then. But now we know how to setup charging and build the system so that disconnect just won't happen. On the rare case it does, the alternator protect is adequate.

Agree on the smart switch/battery protect.

Disagree on the BMS controlling charge sources. Victron chargers/MPPT controllers are sufficiently advanced that they will do just as well or better job than a BMS. Having the BMS control charging adds complexity(and chance of failure), and the BMS is no longer a failsafe. Note, that with a smart shunt or BMV meter, they will use battery voltage and tail current from read from that device. There is an argument the BMS should control charge sources in high C rate applications so that if the cells are out of balance it can slow charging to prevent a cell HV disconnect. But in fractional C rate applications the cells will always be nearly perfectly balanced.

FETs have their weaknesses, but it is a very rare event that voltages will ever be as high as you suggest. Overall, relays are far less reliable. I have changed 100's of failed relays, and only a very few FETs. Nearly every application I have changed relays in, have now been replaced by FETs, and never failed again since. I would suggest a relay in a starting application, and that's it. Even with a REC BMS, I would suggest an SSR of some kind over a relay.

I disagree on the need for a disconnect alarm, although it is a requirement of ABYC and ISO, so if you need to meet those standards, you do need it.
 
Agreed on the 3 way switch. That had been mentioned earlier in the thread. But although neither ABYC or ISO allow it, I have yet to hear any technical reason why it isn't allowed. I know if many installations that parallels different chemistries without any significant issues. Rumors of reduced cell life, but certainly no reports of a hazard.

Agree on the alternator protection. Unfortunately, that feature is only available on what I otherwise consider ridiculously overpriced and over complicated BMSs. And I don't consider it a significant issue, because the BMS should never ever disconnect anyway. Years ago when the technology wasn't well understood, maybe then. But now we know how to setup charging and build the system so that disconnect just won't happen. On the rare case it does, the alternator protect is adequate.

Agree on the smart switch/battery protect.

Disagree on the BMS controlling charge sources. Victron chargers/MPPT controllers are sufficiently advanced that they will do just as well or better job than a BMS. Having the BMS control charging adds complexity(and chance of failure), and the BMS is no longer a failsafe. Note, that with a smart shunt or BMV meter, they will use battery voltage and tail current from read from that device. There is an argument the BMS should control charge sources in high C rate applications so that if the cells are out of balance it can slow charging to prevent a cell HV disconnect. But in fractional C rate applications the cells will always be nearly perfectly balanced.

FETs have their weaknesses, but it is a very rare event that voltages will ever be as high as you suggest. Overall, relays are far less reliable. I have changed 100's of failed relays, and only a very few FETs. Nearly every application I have changed relays in, have now been replaced by FETs, and never failed again since. I would suggest a relay in a starting application, and that's it. Even with a REC BMS, I would suggest an SSR of some kind over a relay.

I disagree on the need for a disconnect alarm, although it is a requirement of ABYC and ISO, so if you need to meet those standards, you do need it.
Yeah, pretty much completely disagree with you on most of those points. Having a good smart BMS has made my system a dream to use. I can easily remotely kick my boat into storage mode, or back into usage mode, and because everything talks to everything else, I don't need to rely on voltages which are really hit or miss with LFP due to the flat charge curve (until you get to the knees). In terms of complexity, other than some initial programming and linking up some RJ45 connectors, again it was dead simple. The system has been live on my boat for 3 months now, and nary a hiccup. Even if it all does go tango-uniform, all the devices have a good fallback that will allow me to limp home.

Lastly, if my BMS takes a crap and dies, I can override it by manually actuating the 7713, and/or overriding on the BatteryProtect.

Plus, since I sleep on top of the whole thing, I just simply can't trust those FET based BMSs and running all the power through the FETs. Yes, there are FETs in my BatteryProtect, but I'm only running maybe 20A through it tops. All the high power stuff (inverter/charger, Alternator) runs through the 7713.

In terms of cost, the REC stuff with all the bells and whistles, was under $750, which in terms of stuff for boats, is pretty cheap. Hell, I spent nearly that much on new wire and terminals for the project. The functionality and integration it provides is miles away better than a Daly or other cheap FET BMS.
 
The OP stated the project was for a boat so insurers will insist on adherence to either ABYC and or ISO depending on jurisdiction , I know in correspondence with mine it has. ( and a professional survey ) so designing a non compliant system is a waste of time.

By the way for Europeans or any boat subject to the RCD , a complete replacement of your battery system IS regarded as a “ major craft modification “ especially if the system is NOT Diy , ie professionally installed. Hence in theory and legally you are required to perform a post construction CE assessment and re certify the boat !! ( this is not cheap )

So advance disconnect alarms are a must

I’m an EE my natural inclination would be mosfet disconnects ( I have a design on my computer for 400v capable 400A mosfet switching but it’s expensive , hence I remain convinced high interrupt capacity relays are best. Mosfet switches require power to switch off. Whereas relays can be configured fail safe if required , an added benefit. It’s worth noting EVs use relay disconnects my Leaf has a nice “ clunk “ when it connects.
I disagree with distributed charging intelligence. Mppt controllers don’t coulomb count and hence only have voltage set points to guide them. A bms os supposed to track SOC , so it should therefore logically make all charge initiation and termination decisions. ( I have the Victron ve.smart network system ) ONE master unit should be tracking SOC and it should make the charge decisions.

On alternators , it’s a given that any Li install needs to upgrade the stock internal car regulator. This will inevitable give you field current control hence. This therefore removes the need for alternator protection

If you do fit alternator protection such as the new balmar , you need to ensure any electronics that’s remains connected to the alternator during a protection sceanario is capable of handling at least 32V. ( the Victron stuff is very sensitive to battery voltage ) Alternator protectors are there to protect alternators not everything else. So it can still fry down stream devices. By all means use then but understand their weaknesses.

As someone who lost 90% of all electronics in a recent lightening strike. ( all the cheap Chinese lighting survived! ) any design needs to be protected from disaster of the control electronics gets hit. Looking at my failed gear in my lab indications are the strike coupled about 100v into the DC line.
 
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But although neither ABYC or ISO allow it, I have yet to hear any technical reason why it isn't allowed. I know if many installations that parallels different chemistries without any significant issues. Rumors of reduced cell life, but certainly no reports of a hazard.

Cell failure in the LA can lead to very high currents flowing into the battery from the Li with catastrophic results , especially on high capacity high current Li systems. Fault modes in either battery are excerbated when interconnected. Hence the spec bodies don’t allow them. I would not accept your comment re experience as we have not enough installation history to draw definitive conclusions.

Since the system can easily be developed to isolate the starter it’s not an issue anyway and I think medium term the starter will go Li also anyways as Li makes a better starter battery ( smaller lighter )
 
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