Electrodacus using the Victron shunt from BMV712 battery monitor

[Chris R8]

Hi all,

I am installing 840AH Lithium Lishen cells in my catamaran with Electrodacus BMS.

I am struggling to get the shunts delivered to Canary Islands.

Can I use the Victron 500A/50mv shunt from the Victron battery monitor BMV 712 instead as battery shunt for the Electrodacus till the 2 shunts (500A BAT and 300A PV)arrive?

If yes where do I connect on the shunt?

If yes can I run the BMV 712 connected to the shunts board (like it normally connects) and the Electrodacus at the same time on top with a ringcable shoe?

I am asking because with the first input I could monitor the lifepo4 additionally (not really needed but) the 2nd input of the BMV I could still control one of my 2 lead starter/service banks (they act as backup). For the 2nd input to work on the BMV I need to connect the 1st too. The BMV712 normally monitors my 2 lead hybrid starter/service banks, oversized with 300AH (input 1/main with the shunt) and 180AH(2nd input) but they are new and are a good backup to power the cat in case the lifepo4 bank has a problem and switches off.

Thank you.

Christian 

[Jan Paauw]

Hi Cristian, The shunt from BMV712 is connected to negatieve, and the shunt from SBMS0 to positive... 

Op dinsdag 7 september 2021 om 00:29:19 UTC+2 schreef kampfsc...@gmail.com:

[Dacian Todea]

Christian,

You can use the Victron 500A 50mV shunt but just the shunt not his electronics as that is already inside the SBMS0

So you will take just the metal part basically just the shunt not use the electronics.

In the past their shunt looked like this https://victronenergy.co.nz/products/shunt-500a-50mv-45-x-28-x-120  so is easy to see the two screws for voltage drop sensing where ADC1p and ADC1n will connect on the new one the electronic is in a plastic box and not sure how will that disconnect from the shunt 

A shunt is nothing else than a power resistor and you measure the voltage drop on it to calculate the current.

You can not use them at the same time as the SBMS0 requires shunt to be installed on battery positive side.

[Chris R8]

Great, thank you. Sure shunts on positive with Electrodacus…

I have the old version with the electronic just on the side, so I will take this off and use the Victron till I get my new ones.

If the new arrive can I put the new 2 (bat and PV) in the positive and the Victron shunt with electronic and BMV 712 then on the negative side?

Would like to use the high/low voltage cutoff function of the BMV 712 (this is Marine use certified and will work) as last borderline to steer a relay to shut off the cells in case the Electrodacus fails due to any reason (user error, faulty or just salty air does the job…) and bat exceed the limits.

[Dacian Todea]

Yes you can have the Victron shunt on negative but there is no advantage as the BMV712 is less accurate than SBMS0 and provides way less info.

SBMS0 has a type 5 for EXT IOx and that is a better fail safe as that will react if any cell gets to secondary levels 3.75V default and that means a charge source failed to turn OFF then that type 5 signal can trip a breaker.

Since BMV can only see total battery voltage it can not protect a single cell and so if you set limit to low it may trip when there is nothing wrong and if limit is set to high then it will protect to late when one or more cells are already damaged.

As an example say you set the BMV 712 to stop the system at 28.5V (3.55V * 8 + 0.1V) then if you have a slightly lower capacity cell or a cell with slightly higher internal resistance all cell may be at 3.4V except that cell thus 3.4V x 7 = 23.8V and then 28.5V - 23.8V = 4.7V well above cell damage while the SBMS0 EXT IOx set as type 5 will stop the system if that or any other cell exceed 3.75V (still a safe voltage and cell will not be damaged).

You can just have a trip type circuit breaker like I shown in one of my posts and that can be tripped by the type 5 if something like a charge source ever fails to turn OFF for some reason.

All EXT IOx are open circuit for OFF so if any of the wires breaks the devices (charge source or Load) will be OFF so if say SBMS0 remains without power for some reason everything will turn OFF if it is correctly connected.

You can just install the SBMS0 in a sealed box so that is protected from high humidity and salt.

[Chris R8]

Thank you.

I will be faced mostly with high voltage cutoffs as I am massively overpaneled on solar (bifacial panels delivering real 1200W in Summer, so charge 100A and even on a rainy morning I get around 15-20A in). Have the space and cheapest solution ( no noisy windigen, expensive hydrogen or unreliable generator needed) so even on a cloudy day with minimal solar harvest it is still enough to cover the cats average energy consumption on passage where the highest consumption happens and sails shade some of the panels. So on anker batteries will be full at latest 3pm incl. 2h just dumping all solar amps into the hot water boiler. And I have a daily consumption of around 150Ah (without hot water boiler dump) and 12V/ 840AH capacity, so full 5 days without any recharge.

I am aware that BMV 712 is less accurate but it’s marine certified and I can count on it to work due to its spec. I rely mainly Electrodacus to do all the security and managing the bank but major systems like the bank are all double if not triple fallback security on my cat.

It’s double security and if the BMV 712 shuts the bank wrongly down I don‘t care. Better shut 10times wrongly off then 1 time it should but doesn‘t do. Plan is that if the BMV712 trips it just completely disconnects the lifepo4 bank and it automatically switches to the 300AH lead hybrid starter/service, so I still have power on all important systems. These are all on legacy house circuit which is switched on/off by Electrodacus via a Victron BP220 smart.

Just don‘t know how I can automate that, researching this at the moment. Till now I will have a spare 1 or 2 switch going in before the BP220 and flip then manually to the lead till I figure it out and get the parts to automate it). Yes I know I have to make sure the lifepo4 and lead is not connected at the same time.

[Dacian Todea]

What I was saying is that BMV712 can only turn OFF the battery when is already to late (is still maybe of some benefit better late than never) but if you get to 28.5V then more than one thing went wrong and at least one cell is damaged. If you set the limit lower than 28.5V then it will disconnect most of the time not allowing you to charge the battery.

The SBMS0 already offers dual level protection for overcharge if you use two separate EXT IOx.

You will have the regular EXT IO4 set as type 1 that will stop all chargers when battery is full any cell at 3.55V and if for some reasons chargers are not responding and continue to stay ON (supper unlikely) then the say EXT IO6 set as type 5 can be used for secondary level protection so that if any cell gets to 3.75V it means the EXT IO4 was unable to stop one of the chargers and then EXT IO6 signal (open circuit) will trip a breaker that physically disconnects all chargers and all Loads isolating the battery.

Not many people use the type 5 for secondary level protection as if all is installed correctly it is super unlikely (never heard to happen) that charger will not turn OFF. The only case (fairly recent) where I seen a charger not stopping was a badly made install where the EXT IO4 wires getting out of SBMS0 had a large part of the isolation removed so much that the EXT IO4+ and EXT IO4- shorted and so circuit was always closed but if install will have been made more carefuly a short will have never happened and the more likely scenario is one of the wires breaking so open circuit that means OFF so safe way to fail.

Below is a zoom in that photo from the user. He used stranded wires (not for those types for connectors that where designed for solid core wires) and more importantly the isolation is removed way to much allowing wires to short circuit. On EXT IO4 he even had a a solid core wire with no insulation at all. 

Below is a SBMS with properly connected wires solid core and isolation removed only as much as needed. Also that CAT5 or CAT6 cable will need to be secured next to the SBMS0 or to the SBMS0 on the new model.

[Chris R8]

Thank you again for all the info. Very useful to know its connectors are for solid wire. The Cat6 cable I have has stranded wires and all cables in marine environment too. It’s forbidden by ISO and marine installation guidelines to use solid wire in marine environments due to it much more prone to break through permanent vibration, connection gets easier fouled and the smaller surface area is much more impacted by oxidation/corrosion. I will crimp isolated ferrules to the stranded ends and then isolated ferrules into the Electrodacus. That’s how the marine guidelines states it should be installed.

 I am not mistrusting the Electrodacus and the 2 level protection is indeed very useful.

It’s more like you said a last borderline that doesn’t hurt but helps, have the BMV712 anyhow installed and monitor the 300AH lead starter/service with its 2nd input. I assume if Electrodacus gets fried/out of order through eg a short it automatically disconnects everything if I understood the manual right…

I am running a 12V system, so I plan to use in your example EXT 04 with 3.50V and EXT IO6 then 3.6V for Electrodacus and 14,5V for the BMV 712. need to try out. I am willing to trade some capacity for safety and not charge to the limit/100% and be save, I have enough overhead. Running an 3P4S config so Electrodacus sees the 3p pack as one cell. Also as I explained i think I have to even lower the limits a bit more as I am massively overpaneled on solar and battery will be hitting the 3.50V nearly every day. Have 2x 50/100 MPPT by Victron and solar runs on 40V. Is it possible if I hit eg 70% SOC Electrodacus shuts on the 750W hot water boiler and at 80% shuts one of the MPPTs off so top charge takes longer with less amps coming in? Need to play with that, wanna cycle them between 60 and 80% SOC most of the time.

Charge is 0.1C regularly and up to 0.2C with engines+solar running. Discharge i am having frequently up 0.25C when cooking for maybe 10-20min and seldom peaks to 0.4C so the cells drifting apart should be significantly reduced.

[Dacian Todea]

The EXT IOx are all based on Toshiba TLP172GM solid state relay and so if SBMS0 for any reason will lose power all EXT IOx will be open circuit meaning add controlled devices will be OFF including the type 5 level 2 protection if used the breaker will be OFF so manual re-connection will be needed.

The default settings 3.55V for over 6 seconds consecutive for fully charge battery so fairly conservative and 3.75V for any amount of time even a few ms as level 2 protection called overvoltage lock.  This larger distance of 200mV is needed as the cell balancing if active can create a voltage drop delta of up to 150mV so the level 2 needs to be large enough not to be triggered due to cell balancing.  14.5V should be fine if you want to use the BMV712 assuming it measures the voltage accurate enough.

There is no choice with LiFePO4 other than to charge to 100% as you will not be able to know any intermediary state based on voltage see below SOC vs cell voltage

 

 For good LiFePO4 0.2C charging is not a problem and if fact is the ideal ratio between battery capacity and solar if you have the space.

You mentioned 3P 300Ah cells that means about 900Ah thus there is no reason to ever disconnect one of the two 50A MPPT's. In fact it will be better to have around 180A of solar charging available if there is space for that.

You can use one of the extra EXT IOx to stop a load like water boiler when battery is below 70%.

You do not need to care much about the cycling of LiFePO4 as most of the degradation is calendar aging and not cycling so no matter what you will still need to replace the batteries in about 10 to 20 years mostly depending on battery temperature (about the same with ambient temperature) as that influences the calendar aging the most.

There is an option on SBMS0 to keep the SOC at what value you want after the first charge of the day say 90% for the rest of the day. SOC is not based on voltage is calculated based on energy in and out of the battery and a full charge is needed to re-calibrate the SOC.

Bellow is a typical cycle life vs DOD and you can see that doing a 5 or 10% DOD gets you half a million such cycles so basically the same as a super capacitor and so you do not need to care about this sort of micro cycles. 

 You can check how my battery degraded over time here https://groups.google.com/g/electrodacus/c/nKWp_6tjMNM/m/1f66fW6-BwAJ