SBMS0 Resistors and Flyback Diodes

[Andre Samson]

Hi, hoping someone can explain when and where (and what values) to use resistors and flyback diodes in an Electrodacus SBMS0 setup? I can't seem to find a clear explanation.

I'm using this in a campervan with 3 panels, and will have BlueSeas 500a disconnects for load and charge. They will have optocouplers between them and the SMBS0 to bridge the switching load. Also have a Midnite Solar MPPT panel controller that's already installed in the van, so I will keep using that to run the panels. The SMBS0 will only see a charging source from them.

I believe the flyback diode has to go on any relays/optocouplers that aren't DSSR20s or 50s. Not sure where the resistors go. No idea on type or values.

[Andre Samson]

Also, what is recommended for an SSR or other relay to control the BlueSea relays?

[Dacian Todea (electrodacus)]

What is the exact model of that BlueSeas relay ?

The SBMS0 EXT IOx outputs are all optoisolated and they will be either Open circuit (for OFF) or Close circuit (for ON) and max current allowed when close circuit is 50mA.

The SBMS0 requires that charge sources are controlled ON/OFF separately from loads and SBMS0 needs to be able to control ON/OFF all Loads and separately all charge sources else it can not protect the battery.

I do not know what model of Midnite MPPT you have but some have a AUX port that can be configured to be able to be controlled by an external BMS.  The SBMS0 needs to control ON/OFF your MPPT as the MPPT has no idea when to stop charging as it can not see the individual cell voltages.

[sailingharry]

If you are talking about the Blue Seas RBS (Remote Battery Switch), I'm about 95% (or more!) sure that the control wires (which you connect the SBMS0 to) are truly control wires -- their current is measured in mA or even microamps.  They don't actually operate the switch, they activate internal circuitry and the power for the operation comes from the connected battery.

[Andre Samson]

The BlueSeas relay is an RBS 7713. I have 2 of them; one for loads, one for charge. I *think* they use more than 50ma for control switching, but I'm not sure. If so, I would need to use an SSR between the RBS and SBMS0. 

I have the Midnite Kid solar controller, not sure if it has an aux control. If not, I can either control the panel output with the SBMS0 through an SSR, or switch to DSSR50. I like the Midnite Kid as it's already installed and cut into my cabinetry, and I can see the solar output at a quick glance when I'm sitting in the main part of the van.

[Dacian Todea (electrodacus)]

That 7713 model will work OK and can be directly controlled by the SBMS0.

Below is the answer by Gemini with a bit more details but it is as I suspected

So you will just connect the red signal wire trough the EXT IOx So the one you use for Loads trough EXT IO3 set as type 2 so just cut your red wire and insert one end in EXT IO3+ and then other end in EXT IO3- (polarity is irrelevant as the optoisolator in the new SBMS0 version is not polarized).

My question :

What happens if the Control red wire is disconnected on a RBS 7713 ? Will it turn OFF or will it remain ON due to latching ? Also do the 7A for 20ms when turning ON are trough the control wire or is that only trough ground wire and the positive power connection ? 

Gemini answer:

Here is exactly what happens with the Blue Sea Systems ML-RBS 7713 under those conditions.
1. Disconnecting the Control Red Wire: Will it turn OFF or stay ON?

If the Control (red) wire is disconnected while the switch is ON, the 7713 will turn OFF. It will not remain on.

While the Blue Sea ML-series switches utilize a magnetic latching mechanism to mechanically hold their states with very low power, the 7713 model specifically features an "Auto-Releasing" coil function.  

Unlike standard bi-stable latching switches (like the Blue Sea 7700 or 7701) which require a pulse to turn ON and a separate pulse to turn OFF, the 7713 acts more like a standard, normally-open relay from a user standpoint:  

    To turn and stay ON: The red control wire must receive a continuous +12V signal.

    To turn OFF: If that +12V signal is broken—whether by flipping your remote dash switch or if the red wire physically disconnects or breaks—the internal circuit automatically releases the magnetic latch, forcing the battery switch to open (turn OFF).

2. Where does the <7A (40ms) Inrush Current Flow?

When the 7713 switches states, the brief high-amperage draw (<7A for up to 40ms) travels through the positive power connection and the ground wire, not the control wire.

Inside the unit, there is an internal relay circuit that acts as a gatekeeper. Here is how the path breaks down:

    Control Wire (Red): This is purely a low-current signal wire. It only carries a few milliamps (8mA when OFF / 13mA when ON) to tell the internal logic board what state it should be in. It is not sized or designed to handle the heavy shifting current.  

Positive Power Connection: The heavy mechanical shifting mechanism pulls its high current directly from the main high-amperage battery studs on the device.

Ground Wire (Black): The current completes its circuit and returns to the system ground through the black wire in the control harness.

[Andre Samson]

Okay, that's great news, thank you. It simplifies things for sure.

So, when do I need a resistor on EXT IOx? Where would it be placed and how do I choose a value?

[Dacian Todea (electrodacus)]

The resistor is to limit the current and protect the EXT IOx and the wires in case of fault (wire shorted to GND(battery negative)).

For example you have a 2A fuse from Battery+ going to EXT IO3+ so you can replace the 2A fuse with a resistor or add a resistor in series with the fuse.

Then you connect the control wire to EXT IO3- but if you instead connect the EXT IO3 to GND(battery negative) you just shorted the EXT IO3 and while the recommended 2A fuse can protect the wires it can not protect the EXT IO3 thus the resistor.

The control wire likely requires 5 to 10mA max so resistor should be low enough value to provide that but high enough value so that it will not exceed 50 to 60mA in case of fault (short to GND).

Assuming you have a 12V battery that fully charged is around 14.4V / 0.05A = 288Ohm so a 270Ohm in this case will work fine.

And 14.4 * 0.05A = 0.72W in case of fault thus a 1W resistor or larger will have no issues and the 2A fuse will become useless so not needed as resistor can protect the wire by limiting the short circuit current to 50mA

[Andre Samson]

Okay, makes sense. And that resistor would basically go right at the EXT IO on the SBMS0 I assume.

When do I need a flyback diode? Do I need one on the RBS 7713? On other SSRs like CPC1706Y or these on Amazon: https://www.amazon.com/SSR-25DD-3-32VDC-Output-5-240VDC-Plastic/dp/B08GNSPCND?th=1

I also have a Stinger 500a solenoid I'll be using to combine batteries for alternator charging (https://www.amazon.ca/Stinger-SGP35-500-Amp-Relay-Isolator/dp/B004WDPZEW) with an SSR on the control ground for BMS control. Will this need one? How do I know when one is necessary and how do I pick a diode value or type?

[Dacian Todea (electrodacus)]

The resistor will be installed where the fuse was or is installed so closest to the source (battery).

The RBS7713 likely has all the protection required build in.

I do not think is a good idea to parallel batteries or try and charge a LiFePO4 directly from alternator without current limiting.

Alternators are constant voltage limited sources and they will push as much current as required limited only by the circuit resistance that includes the battery internal resistance.

Since small Lead Acid starter battery has relatively high internal resistance and small capacity the alternator will not normally overheat thus less likely to get damaged.

With a large LiFePO4 connected directly to alternator the alternator will work way to hard and likely will fail.

You will sure need a diode for the Stinger 500A since that coil when disconnected will create a high voltage that will damage whatever you use to drive that solenoid.

Diode will need to be rated at least 50V and a few A depending on what is the current trough the coil of that solenoid. There is energy stored in that coil will be dissipated on the diode when the circuit is opened.

You can use the 10A10 diode a bit of an overkill and it is to large or expensive look at 1N5404 or similar.  They will be installed directly at the terminals of the Stinger 500A coil and will protect your SSR that you want to use to drive this. 

[Andre Samson]

Thanks very much for the help with the diode.

I'm using a water cooled alternator with thermal protection so there should be no concern for it failing due to heat. If I don't like the behaviour, I will add a Balmar or Wakespeed external regulator.

Concerning the Midnite Kid solar controller, if I wanted to keep it for the reasons I stated above, how should I control it with the SBMS0? Would I cut the output (20 to 22a) between the Kid and the batteries with a SSR-40DD or something similar? Or is there a better way to do it? Or cut the panels to the Kid? The 3 panels are in series so they make about 5a max at 63v.

[Dacian Todea (electrodacus)]

The Midnite Kid unfortunately does not have the AUX input like the larger Midnite Clasic.

You may be able to use the temperature sensor. I think that is 10kOhm NTC and if you parallel 1KOhm it will think it is over temperature and will stop charging.

But you need to test this see if it works. Then you will need a small normally close SSR that then you control with the SBMS0 EXT IO4 set as type 1

The EXT IO4 set as type 1 is open circuit for charging OFF and close circuit for charging ON thus the need for extra normally close SSR to add the 1KOhm in parallel with the 10KOhm NTC to trick the Midnite (it will think is battery over temperature and stop the charging) .

[Dave McCampbell]

We have used this simple trickle charge circuit for almost 30 years to keep our start battery at full charge.  No need for combining batteries or using other expensive devices to keep them charged.  https://www.svsoggypaws.com/electricalsystems.htm#tricklecharging

We also use Blue Seas 7713 500 amp relays to control our separate load and charge busses.  They are directly controlled by our SBMS0 to shut off at Dacian's default HVD Lock.  All our various charging devices are controlled by the SBMS0 via DSSRs to be shut off at Dacian's default HVD.  If your system voltage is 12v nominal and your panels are 24v nominal or greater you will need to use the controller to reduce the voltage.  See our setup here:  https://www.svsoggypaws.com/electricalsystems.htm#lifepo4
Dave

[Andre Samson]

This is a great workaround for controlling the Kid; thank you! I hope it works. How would I implement it? Would I need to dissect the Midnite BTS to add the resistor somewhere? Or would I need to make one from scratch with a 10kOhm NTC and a resistor in parallel? I guess for the NC SSR I could use a small CPC1705.

Dave: That's a great way to trickle charge the starter! Simple, smart, and effective. Thanks.

[Dacian Todea (electrodacus)]

The temperature sensor has a resistance of 10kOhms at +25C to create an over temperature error temperature will need to be higher. For a resistance of 1kOhm the MPPT will think that temperature of the battery is around +80C and stop the charging.  You can first just install a 1KOhm resistor instead of the temperature sensor to see if this works as I expect and stops the charging. I guess just disconnecting the sensor will not stop the charging.

So then you need to find a way to combine the EXT IO4 set as type 1 witch is close circuit when charging should be ON with the 1KOhm resistor and 10KOhm NTC temperature sensor so you get the correct ON/OFF conditions.

Like for example have a normal close SSR in parallel with the 10K NTC so that is normally shorted and 1Kohm resistor in series so MPPT will see over +80C battery temperature and stop the charging. Then use the EXT IO4 to control ON/OFF the NC SSR so that way it will switch between 1KOhm (+80C) and (10K+1K=11kOhm) a bit more than +25C

[Andre Samson]

That should be easy to build. Thank you!

Do you have any tricks to trigger an inverter/charger that has no power switch? I have a Xantrex Freedom HF 1800 and I don't know of any way to trigger it without using another huge BlueSeas switch on the 4/0 wires, which I really don't want to do. I know that if the remote display is unplugged, the unit shuts down completely, so maybe there's a wire in the RJ11/telephone connector I could use to trigger. The biggest problem there is that when the display is disconnected, none of the AC plugs in the van work anymore, as they run through the inverter. So when I'm plugged into "shore power", I won't have AC once the batteries have topped up, which wouldn't take very long. 

[Andre Samson]

I do NOT mind opening the Xantrex, or drilling a hole for access if necessary. Whatever it takes to enable a control switch. Especially for charge. I don't often use the inverter anyway. 

[Dacian Todea (electrodacus)]

See user manual https://xantrex.com/wp-content/uploads/2021/12/975-0390-01-01_Rev-JFreedom-HF-Owners-Guide_ENG.pdf

Search for "Ignition Control" as there is a jumper under that display plate and you should be able to just remove that jumper and connect the EXT IO3 set as type 2 to control the inverter ON/OFF

This is designed for ignition signal in a vehicle so that inverter is ON only when the vehicle is running. Same can be used by the SBMS0

[Andre Samson]

Oh wow. Can't believe I missed that; thank you. Easy solution for loads. My research says it does not affect the charger, however; only the inverter. Any ideas for stopping the charger?

[Dacian Todea (electrodacus)]

I do not think it has a separate control for charger. My guess is that both charger and inverter will be OFF if ignition is OFF but not sure you need to test. The charger is just the inverter run in reverse.

To control the charger separately you will need to add an AC SSR on the AC input line and control that with EXT IO4 set as type 1

[Andre Samson]

I hope you're right, but here's what Gemini said when I asked:

1. Does ignition control affect both the inverter and charger?

No. The ignition control feature (often referred to as "ignition lockout") only disables the inverter function.

Its primary purpose is to prevent unnecessary battery drain by shutting off the inverter when the vehicle's engine is turned off. It ensures that heavy 120V AC household loads don't accidentally run off the house or chassis batteries while the alternator isn't actively generating power.

2. Can you use the charger while the ignition has shut off the inverter?

Yes, absolutely. The charger function operates completely independently of the ignition control status.

• How it works: When you plug the vehicle into shore power (or fire up an AC generator), the Freedom HF 1800 automatically activates its built-in 30A automatic transfer switch and the battery charger.

• The Result: Even if the vehicle's ignition is completely off (meaning the inverter is locked out and cannot drain the batteries), the unit will pass shore power straight through to your AC appliances and safely charge your 12V battery bank.

Essentially, the ignition lockout is a one-way safety valve: it blocks the battery-to-AC conversion (inverting) when the engine is off, but it never blocks AC-to-battery power (charging) when an external shore power source is available.

If Gemini is right, I'll have to use the AC SSR like you suggested, which is a good idea as well. Problem with it is that I won't have AC power to the van at all when the SBMS0 shuts it down.

[Dacian Todea (electrodacus)]

Most of this inverter chargers where not designed to work with Lithium batteries and so no separate charge and discharge BMS control.

Victron multiplus has separate remote ON/OFF controls for inverter and charger so it can be controlled by SBMS0 and work like a UPS

Adding the SSR on the AC input will stop the charger when battery is full then run your loads from the battery until charging is needed again.

If you have access to grid AC power then you should just connect your AC loads directly to grid.

[Andre Samson]

Yes, this inverter is an older design from long before lithium was in general use.

Most common scenario for needing AC is running an electric heater in the shoulder seasons. To run it off the grid I would have to leave the door open a crack to let the cable in which is impractical and not private when sleeping. Wife won't have that!  Normally I plug the van in and then plug the heater in inside. Maybe my only solution is an RBS switch on the huge cable to battery.

[Andre Samson]

Oh yeah, and my fridge switches to AC when I'm plugged in and since we keep it full much of the summer, so it's plugged in more than it's not, between trips. So, AC in the van is crucial. Another RBS may be necessary to make it work, they have a low draw, but those switches are very expensive. Could put that towards a used better inverter/charger maybe. 

Or, maybe I could somehow use an AC automatic transfer switch to bypass the Xantrex. Would it hurt it to have AC back fed into the Xantrex AC out when the in is bypassed? 

[Andre Samson]

No, it wouldn't have to be back fed if I put an SSR on the Xantrex AC input, and used the auto transfer switch after the Xantrex with the Xantrex AC out as primary power, and the direct shore power as secondary. It would use the Xantrex as first choice, but when the SBMS0 triggers the SSR and the ATC sees no power from the Xantrex, then it would auto switch to shore power.