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Small Boat BMS/LFP with Backup

@Rick-admin  we (4 sailors who have an interest in LFP and many who are EE or have electrical skills) have been working on a diagram and design for a small boat bms/lfp that uses REC Active BMS, Wakespeed WS500 and CerboGX along with other Victron Equipment.

This is intended as a solution for a small boat without much room available for the equipment.

This system has a 1-Off_2 Switch that is used in the the event of BMS/LFP failure. SK-2 has a wiring diagram and explains the intent of the switch.

Of course if you notice any problems, improvements or clarifications we would appreciate your thoughts.

We have some questions about what is expected in the case of an LVD or HVD shutdown and if the response Procedure shown on SK-2 is acceptable and appropriate?

Some people who have reviewed this diagram think that the Lithionics Battery BMS  or Victron BMS will have better reactions and directions that require less user interference and involvement than the REC Active BMS. Is this true?

One other aspect that has directed and governed this design is ABYC compliance, therefore because ABYC wants the battery shutdown in the event of a failure, we have not used a dual bus but have a single main relay contactor.  Is it true that compliance with ABYC requires the use of a single relay to shut the battery down?

Some people who have reviewed the diagram comment that a dual bus with charge and discharge relays (as REC BMS has designed) is the only way they would accept a BMS to work, but wouldn't a dual bus design not be compliant with ABYC requirements?  If not, please explain why.

Please note that we have configured the 1-Off-2 Switch with AFD contacts and some relays to provide the following:

  • 1 - LFP Disconnect and SLA Emergency Feed
  • Off - Normal LFP
  • 2 -LFP/BMS Override for LVD Recovery

Is this a reasonable way to configure the switch? If not, why not?

Thank you for your excellent forum and good information.

Rick

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Rick-admin and Adam Sherman have reacted to this post.
Rick-adminAdam Sherman

Some good stuff here.  Thanks for sharing!

Rick,

Thanks, I had lots of help from some very knowledgeable guys,   @Goboatingnow @svsagres @wholybee and John Harries.

I had several questions, I hope you might know or confirm the answers.

Regarding dual bus and single bus configurations.

I understand (please see this post) that  "The key point both under ABYC and ISO is the presence of a BMS , a complete disconnection of the Battery from both loads and charge sources , and advance warning of such an action" hence ABYC does not require any complex  charge and load bus separation.

Thus we do not really need dual buses or the second relay.  What capacity are we loosing by just having a very simple single bus and single relay, (since LFP disconnects are intended to be very rare).

Also if you happen to build a dual bus, isn't it true that ABYC still requires disconnection of the Battery from both loads and charge sources?   So what advantage is that second bus?

This is why we are using a single bus and single relay.

Is the REC Active BMS able to be configured this way as a single bus with single relay?

Also,  we have some questions about what is expected in the case of an LVD or HVD shutdown and if the response Procedure shown on SK-2 is acceptable and appropriate?  (Some people who have reviewed this diagram think that the Lithionics Battery BMS or Victron BMS will have better reactions and directions that require less user interference and involvement than the REC Active BMS.)  Is this true?

Rick Gleason

Hello Rick Gleason,

I only had a quick look to your diagram, I didnt check or all the details are correct!!

To make a one contactor system you will safe costs.

I think a two contactor system is much better.
The reason for this is that IF a HV event is there, only the charge bus will switch off.
NOT your full boat into dark!!, big difference.
Even a good dropin battery, will not cuttoff the LOAD at a HV event.

I have seen, more and more because off the quality off the cells, that one cell hits the cell-MAX already at a charge voltage from 13.6-13.8V. THIS means with one contactor the LOAD is also switch off.
Yes off course at that time there is a huge cell unbalance.

With your setup, with a REC and Cerbo and using the full communication, a HV event will VERY unusually because, IF the first cell reach cell-balance Voltage, your chargers will directed through Cerbo- DVCC, to lower the charge.
This result in that non off the cells goes higher as end-off-charge Voltage, and balancing can do there work.

From this point off view, the one contactor setup will never trip on HV.   (A little bit the same as the Victron Lyxn BMS setup)
BUT, IF something fails in this charge cycle, with a one contactor setup, this will open, and your boat LOAD is OFF.
Only saving cost would be a reason to don't install a second relay, just my personal view.
A second relay makes a safer system.

The REC has this extra safety option, IF something fails in the communication, and gives a HV,  why not use it? (For example the Victron Lynx, if charge communication fails, this can result in a HVevent, it will switch off the Main contactor  🙁   .

 

This charge communication is more and more IMPORTANT, because the lesser quality off the cells AND because the higher charge and discharge currents that I see in systems.  The good thing, you have already this communication with the REC and Cerbo, WS500 in your system design.

 

 

ONE exception, one off your charger has NO communication, this gives another view on your setup, that a CHARGE bus is more needed,

OR as a MINIMUM required solution could be to switch off this charger with one off the REC outputs. This will avoid a trip from the main contactor.

 

 

Your other question:
Your REC can be programmed for a single relay and for two relays.

The Lithioncs BMS I dont know.
The Victron BMS, in general it can only be used with Victron batteries.
REC has plenty off possibilities for setup, it will be a learning process if it is new for you. Or hire a installer, there are many here on the forum, like me. :):)

Ben

 

 

Another thing that not clear in you diagram, is where you install the voltage sense from the WS500.

Think about that IF the REC is the "system battery" monitor in the Cerbo, you can not use SVS in the Cerbo.

With all "voltage sense" think about that the voltage sense, should NEVER disconnected from the charger.

 

Also there is not a main switch on your starter battery.

 

According ISO, IF you connect to batteries together, there need a kind off amp limiter in between. Depend on the length off cable, this will be limiting the amps. Good to do a test, with extreme low and high level, from the 2 batteries, to test or the amps are not to high, when you switch the batteries together!!

 

Your Orion is on the DC panel, this will work, but if the user dont switch it ON, your starter battery will be empty. I like design that the user cant make mistakes, or forget things. Therefore, have the orion connected to the LOAD busbar, and all goes automatic. Soon or later, sea sickness as example, users will forget things!!

 

from WS500 to cerbo is VE-CAN, diagram says VE-bus

from REC to WS500 it says BMS can, is not BMS CAN but VE-CAN (Victon has BMS-CAN and VE-CAN, you need to use VE-CAN)

 

 

I like your diagram, which software you use to make it?

 

 

Ben

Dear Ben,

Thank you so much for your reply and the corrections and clarifications and the answers to my questions.

I have been using DIYsolarForum.com  "Small Boat BMS/LFP with Backup" thread as my main development communications.  I've been using the diagram tools suggested on the REC BMS website  under Create your own diagrams. I have found drawio to be quite good and have gradually gotten used to it.

I am an Architect, but with a physics major and an engineering interest, probably from my father. Where are you located?

To clarify:

  1. 1-Off-2 switch never parallels the LFP and SLA batteries to comply with ABYC.  "Off" is "Normal LFP" therefor there is no "amp limiter" needed. Please see SK-2 diagram and notes for operation.
  2. As I and others understand it "The key point under ABYC and ISO is the presence of a BMS , a complete disconnection of the Battery from both loads and charge sources , and advance warning of such an action."  See Diysolarforum thread Post #371  Therefore (recognizing that dual bus might be more acceptable) however both charge and discharge buses must be shutdown on BMS failures, so there is no real advantage to having dual bus in this situation, since failure theoretically will occur only rarely.
  3. We currently have the BMS sense leads connected to the LFP as they should be, and those also power the BMS, so the BMS remains connected and powered after a failure, as I understand it. I think this is acceptable by ABYC.
  4. The CerboGX is powered from the SLA so that it's functions remain powered, which I think is also acceptable by ABYC.
  5. The Wakespeed leads  #9, #10 & #11 are going to the Alternator.  I believe the REC Active BMS shunt is also used by the regulator.
  6. You advise:  "IF the REC is the "system battery" monitor in the Cerbo, you can not use SVS in the Cerbo. With all "voltage sense" think about that the voltage sense, should NEVER disconnected from the charger."
  7. Sorry but I don't understand your first point, what is SVS?  We want the CerboGX communications and control to have access to the current from the shunt, which I believe is accomplished by the configuration. Of course I agree about the second statement.  Keep in mind that when the 1-Off-2 switch is on "LFP Disconnect & SLA Emergency" the regulator defaults to "Limp home mode" and charges the SLA at a parameter setting of 13.2 - 13.8".

I am going to be making additional revisions to the diagram in the next couple of days, and your suggestions and corrections will be added.

Best Regards,

Rick Gleason

PS: I found the new Victron documentation for Wakespeed WS500 and it is showing a separate current shunt in the Alternator red cable to the Lynx bus.  The temp sensor goes on the alternator and it does appear we should use VE.bus.

https://www.victronenergy.com/media/pg/Cerbo_GX/en/wakespeed-ws500-support.html

This diagram uses the red cable. Why the red cable? Can't we use the same shunt as the REC Active BMS which is closer to the battery and on the black cable?

comments to your questions.
1: from ISO TE13:
""NOTES:
If batteries are paralleled in an unbalanced state, extremely high paralleling currents can flow. If used automatic paralleling devised or on/of switches, should have an appropriate current limiting capability.
manual switch 1, 2 both and automatic parallel systems MAY NOT BE appropriate. ""

As example, your starter battery is empty and you use the "emergency switch" is good to know that the amp limiting from the resistant in the cable is enough, to not blow the fuses.
That why I wrote do some tests.

2: I explain already why 2 relays systems are better, it is just my view, with explanations why.
You wrote "since failure theoretically will occur only rarely""
My vision:
In theory it is not rarely happen, in theory it NEVER happen at all.
But in practical, yes it happen, when there is a cell imbalance. Just the practical experience what I have seen, more and more!!

3.I didn't reread this at the moment where it is is ABYC.
But a BMS need to be powered from the battery its controlling. My logical sense say this. If your starter is empty, it would switch off the Lifepo4, that's not logical, even dangerous.

4.If the Cerbo stopped working, because empty starter battery, the BMS still controls the lifepo4, therefore it is safe to do. The risk on a disconnect from HV will be higher, because the charger have No communication. Some off the chargers stopped working in most cases, when communication is lost. Other chargers go to there internal charge algorithm which can give HV on one cell , but bms will shut down, therefore it is safe.

5. 10-11 need to be close as possible to the battery for best performance, but not disconnect by any contactor. This count for every brand external alternator regulator and any other charger.

6 SVS= Shared Voltage Sense, see Victon Cerbo manual

PS: The WS 500 can setup for shunt in negative or in positive, I prefer the negative. You dont need to install a extra shunt in general.
If VE.bus is written than that is a not correct, I didnt have see it in that manual. WS500 and REC need to be on VE.CAN

Ben

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Rick Gleason

Thanks Ben for your response.

Ben wrote

1: from ISO TE13:
"NOTES:
If batteries are paralleled in an unbalanced state, extremely high paralleling currents can flow.

Ben I don't believe you saw the 1-0ff-2 Diagram on SK-2.  The LFP and SLA Batteries are Never paralleled.

1 Position = LFP Disconnect and SLA Emergency Feed

 Off = Normal LFP

 2 Position = LFP Bypass

2: I explain already why 2 relays systems are better, it is just my view, with explanations why.

Yes,  2 relays and dual bus are better because LVE relay can independently disconnect the load bus and HVE relay can independently disconnect the chargers,  but ABYC requires that the LFP be totally disconnected, according to my understanding.

3.I didn't reread this at the moment where it is is ABYC.
But a BMS need to be powered from the battery its controlling. My logical sense say this. If your starter is empty, it would switch off the Lifepo4, that's not logical, even dangerous.

Yes, the REC BMS is powered by the LFP Battery as stated above.  The CerboGX cannot be powered by the LFP as that will violate ABYC, so to keep it powered for monitering purposes we have shown it powered by SLA Battery.

4.If the Cerbo stopped working, because empty starter battery, the BMS still controls the lifepo4, therefore it is safe to do. The risk on a disconnect from HV will be higher, because the charger have No communication. Some off the chargers stopped working in most cases, when communication is lost. Other chargers go to there internal charge algorithm which can give HV on one cell , but bms will shut down, therefore it is safe.

Yes, I believe that is what I indicated.  Good point about the HV disconnect and charger control, I had not thought about the loss of communication if the SLA went dead. Thanks for the more complete explanation.

5. 10-11 need to be close as possible to the battery for best performance, but not disconnect by any contactor. This count for every brand external alternator regulator and any other charger.

I agree, right now I am using a fluky ARS-5 and the sense leads are essentially on the battery for + and on the shunt for - . I think the closest I could put the positive sense lead in this configuration is at the 1-off-2 switch and the negative sense lead at the shunt.

6 SVS= Shared Voltage Sense, see Victon Cerbo manual

Thanks I will check the manual. I think that is some of the information I am missing and need to understand. I expect there is a lot of configuration needed for BMS, WS500 and CerboGX.

PS: The WS 500 can setup for shunt in negative or in positive, I prefer the negative. You dont need to install a extra shunt in general.
If VE.bus is written than that is a not correct, I didnt have see it in that manual. WS500 and REC need to be on VE.CAN

I got it incorrectly named. Thanks.  I am going to change that as you suggest. WS500 is quite flexible, their manual is very lengthy and has many different configurations and parameters.

I am going to revise the diagram and post it.

Discussion about rules are not always straight, because you can interpreted them sometimes different.

see comments below:

1. If never paralleled than all is ok.
As I mention I didn't read all the details.

2. With 2 relays there is also a complete disconnect!!!
They rules don't say it need to be one contactor.  It only says disconnect completely, when needed thats done with 2 contactors!!
Off course it is good to have in the ""user manual"" a comment about this. That the system has 2 contactors..
Name the contactor like  main contactor, 1/2 and 2/2 as example and labeled like that.

Dont forget this part from the rules.
ISO 13297 9.3 says:
""Remote controlled battery disconnect switches, if used, shall also permit safe manual operation.""

The contactor in your system can be seeing as a remotely controlled contactor?.
This means there need a manual control from the contactors also!!!
This means if there is a faulty BMS, you can still operated the system.
ONLY in case off a fault in equipment, and NEVER for normally use!!!. It should labeled as this also!!

This seams contradiction with other rules, BUT I see this as a extra safety if lightning damage a processor in the BMS. You can have long discussions with technician about this which rules overrules another rule or not!!

3. What is the exact discretion from BMS?
Battery Management System.
In your setup the REC AND the CERBO together are the battery management system.
I interpreted the Rules like that.

Yes the REC has the name, BMS, the Cerbo,  is named different. BUT only together the have the function from a complete battery management system.

What they like to say, in my personal interpretation, as example, don't connect any other equipment like  chargers, mppt direct to the battery without discontactor.

 

About settings: Every device has settings, not that difficult, but if you are new in this, it is lot off reading.

I hope this info will help you.

 

Ben

 

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Rick Gleason

I contributed in a small way to refining the RGleason schematic.

one point I feel very excercised on and I see it pop up across many forums

HVE and LVE should be configured to be serious safety trip points and if they occur should be treated so. Both should NEVER occur in a properly functioning Li system. Low SOC alarms should trigger well before LVE and charge stop should occur well below HVE

Hence consistent with ABYC and ISO I would argue on these events the battery is disconnected from all load and charge sources.

then after manual inspection and cause determination should the appropriate overrides be activated which can (a) switch in alternative power sources (b) reconnect charge sources ( c) reconnect loads.

a bms should NEVER decide to allow charge sources to remain connected on LVE or loads on HVE. The BMS HAS NO IDEA what caused the lve or hve and hence to have it leave charge sources or loads connected on any combination of these safety trips is not “ safety first. “

we must get away from the false notion that LVE or HVE are benign common situations in a normal system. They are not . On HVE LVE over temp and over current which all result in complete LI disconnect, no automatic re connection should occur without manual inspection and then manual intervention process depending on what’s discovered.

Rick Gleason has reacted to this post.
Rick Gleason

Thank you again @Goboatingnow for keeping my little project on track!  Much appreciated.

Revision 10-04-2022 which is 99% there.

 

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