Shunt Connects for an Inverter with built-in Charger?

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Gilbert

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May 29, 2021, 5:38:06 PMMay 29
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Hi guys, I own a Xantrex Inverter/Charger, and I am in the stage of creating a detail schematics for my camper's electrical system, and I just realized if my inverter/charger uses the same 12v cable to the battery, how can I connect it to the SHUNT correctly for the BMS to read LOAD/Charge seperately?  or this cannot be done?
Thank you!!

Dacian Todea

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May 29, 2021, 9:26:55 PMMay 29
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The battery shunt is bidirectional so it will see both the charge and discharge current. Also your inverter charger will either work as an inverter or as a charger can not do both at the same time.
Your bigger problem is to find a way to allow SBMS0 to control ON/OFF the inverter and separately the charger as that is needed in order for the SBMS0 to be able to protect the battery.

Gilbert

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May 29, 2021, 11:37:05 PMMay 29
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For the ON/OFF control from the SBMS0, it should be ok with the arduino Nano Every.
Since the display controller of my inverter/charger has a PUSH ON/OFF button seperately for Inverter and Charger, so I just need 2 arduino Nano Every, one for inverter, one for charger.  that should do the trick.

For the Shunt, I am still confused.  
I thought the reason we need 2 Shunts connected to the + of the battery, Load to 1st Shunt, Charge to 2nd Shunt, is to get a separate reading of charging Amp and Load Amp.  In my case, if I connect my Inverter to 1st shunt, when it is charging, isn't the charging current will go through the 1st shunt, not 2nd?
In another way of saying it, it is effectively using only 1 SHUNT for LOADS and CHARGE (from Shore power).

This is my beta version of my electrical system setup, but it shows my concerns on the SHUNT connections.
Camper 2.0 Electrical System.png

Am I making sense here?
Here an example: LOAD 30Amp and CHARGE 60Amp
1) With 2 SHUNTS, the BMS will read 30A Loads and 60A Charging Current
2) With only 1 SHUNT, the BMS will reads 0 Loads and 30A Charging Current.

Everything still works fine, but I will not able to read the current flow information correctly from the BMS info.

Gilbert

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May 29, 2021, 11:39:22 PMMay 29
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Seems like the picture was reduced in size in my previous post, just posting it again here and also attaching the PDF file so it is readable.
Thanks.

Camper 2.0 Electrical System.png

Camper 2.0 Electrical System v0.9.pdf

Nils

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May 30, 2021, 1:53:37 AMMay 30
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Hi Gilbert,

as Dacian already said: Bat Shunt is bi-directional, PV shunt is uni directional. As SBMS was primarily developed as a solar charge controlling BMS, PV as a current source got its on stats. (At least that is my interpretation).

You drawing looks good from my understanding (related to the shunt question). In your case alternator / pv energy flowing through the PV shunt will be counted as "PV" in the SBMS0 stats. You get the benefit of higher shunt resolution, as the PV shunt can use the full ADC resolution in one direction of current flow (uni directional, see manual for more details). 

You would also be fine to connect you alternator to the bat shunt. The SBMS will notices once energy flows into the battery or if current is taken from the battery. Or to put it easy: Energy flowing into the system will be accounted for no matter what, when connected to the PV shunt. You just will have to live with the fact, that you won't have separated PV energy stats, when combining multiple charge source on the PV shunt.

You have to know that SBMS can only see two current flows (pv = always positiv and bat current = pos / neg). Load current is then calculated as a difference (and Dacian also respects the consumption of the SBMS0 itself). So any consumption happening before current is flowing through the battery shunt will not be monitored, of course. It simply limits the current flow into the battery.

Example: Your inverter-charger is in charge mode and charges with 50 A. Parallel you fridge takes 10 A. Than the SBMS will only see 40 A going into the battery.

Hope that makes it a little more clear :)
Best,
Nils

Gilbert

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May 30, 2021, 11:36:40 AM (13 days ago) May 30
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Thank you so much for the details Nils!   

I just realized I ordered 500Amp/50mv Shunt (instead of 75mv)   I read online that it will be much less accurate is this correct? 
Should I return it and order a 75mv Shunt instead?
Thanks again!!

Nils

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May 30, 2021, 11:54:08 AM (13 days ago) May 30
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Hi Gilbert,

I was looking for the same answer a few days ago. You have to know that you will always have a trade off between dissipated heat by the shunt (= lost engergy) vs. accuracy of the shunt reading. A shunt with a higher voltage drop (e.g. 75 mV) will have a higher loss, but better resolution.

As you are planning a 12 V system, which potentially will see some high currents, I personally would stick with the 50 mV shunt.

Dacian shared this link with me, which is great for all the details about shunts and SBMS0: https://groups.google.com/g/electrodacus/c/ReL-z6vGFdw/m/vsmKm0grAQAJ

So basically: A 500 A / 50 mV Shunt will give you a resolution of: 50 mV / 90 mV * 2^15 steps = 18.204 steps. 500 A / 18.204 steps = 0.027 A resolution.

A 500 A / 75 mV would be 75 mV / 90 mV * 2^15 steps = 27.306 steps. 500 A / 27.306 steps = 0.018 A 

At the same time the heat dissipation will be 0,075 V *500 A = 37,5 W (75 mV version) vs. 0,05 V *500 A = 25 W (50 mV version).

Hope I didn't make any mistakes with these calculation. (Maybe Dacian can check?)

In case you are wondering where the 2^15 steps are coming from: The ADC has a resolution of 16 Bit in each current "direction".

All in all: You should be fine with the 50 mV shunt, I would assume. :)

Best,
Nils

Dacian Todea

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May 30, 2021, 12:36:36 PM (13 days ago) May 30
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Gilbert,

The diagram looks OK other than I do not like the relay idea as your alternator will sure be damaged in this configuration.  The alternator is OK charging a small 280A Lead Acid battery as that battery even if empty will get to 14.4V within a few seconds at most very few minutes and then current will drop to reasonable levels for the alternator. Alternators are just around 50% efficient so even at 280A (the alternator rating) amount of loss as heat is extreme 280A x 13V = 3640W and yes this will be the charge power but since efficiency is only 50% it will also be the heat loss and 3640W is more than a large space heater so Alternator since is a large chunk of steel, copper and aluminium will absorb that for a few minutes but if it needs to continue to push that for many minutes then it will overheat and just fail.
A LiFePO4 like yours can easily handle 300A (maybe alternator will push even more depending on your cable voltage loss) and will be able to do that for even hours depending on state of charge so alternator will just fail.
It is best to use a Battery to Battery charger and not a relay (the relay will be OK if you had a Lead Acid battery to charge but not a LiFePO4).
The 500A switch on the inverter will need a precharge circuit in parallel unless that is the special switch that has the precharge build in.

1) In your example the PV shunt will read 60A and the Battery current will read a battery charge current of 30A then the SBMS0 will calculate Load current as PV curent - Battery current = 30A load.
2) Yes with only one shunt the battery shunt the SBMS0 can only see the 30A battery charge current. That is sufficient to calculate the battery state of charge and all will work correctly but you will not know the PV current and the Load current with just one shunt.

The 500A 50mV shunt is fine as Nils explained the current step resolution is a bit lower but not that relevant it will still be more than good enough.
You are only allowed to use 66% of the shunt rating continues so about 330A is the max continuous current than the 500A rated shunt can handle.



Gilbert

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May 30, 2021, 2:37:45 PM (13 days ago) May 30
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Fabulous! 
Thank you all.  I will look into the DC-DC charger.  other then that, I will continue with the setup, and update how the end products goes!  
Now I just need to wait for all the parts to arrive, cannot wait to see my new BMS arrival.  =)

Nils

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May 30, 2021, 3:22:43 PM (13 days ago) May 30
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Gilbert, regarding the alternator topic. Victron has a good video showing part of the problem described by Dacian above. Might be worth watching: https://www.youtube.com/watch?v=jgoIocPgOug

Best,
Nils

Gilbert

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May 30, 2021, 4:12:28 PM (13 days ago) May 30
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THX Nils!!
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