BMS and MPPT solar charger controller configurations

[juan3211]

Hi, I already have a Victron MPPT 75/15A solar charger controller. And I am trying to build my own powerwall with LifePo4 32700 battery cells.
Because I like a lot the DIY philosopy, I will try to make also the famous DIYBMS v4 or similar. but it is the same, and it is not key to this question.
This post is because I have a doubt about configurations.

I will have to configure the MPPT solar charger controller and the DIYBMS to work togheter

My plan is 8 LIFePO4 in series, so I will have 3,2 x 8 = 25.6 V nominal, and maximum: 3,65 x 8 = 29.2 V (it is not object here to discuss whether it should be 3.55, 3.6 or 3.65 or 3.7, consider it as an example)

DIYBMS (an others) does balance in the "up voltage", I mean, it tries to get all batteries with 3.65 V, no more.

But we also have the MPPT configuration about maximum voltage of battery.

So the question is: what is better for the "29.2 v and 3.65v" example?

1. MPPT: max 29.2 and BMS: max per cell 3,65 - gap. And what gap: 0.1V or more or less?
2. MPPT: max 29.2 - gap and BMS: max per cell 3,65. And what gap: 0.1V or 0.2V or more or less?
3. MPPT: max 29.2 + gap and BMS: max per cell 3,65. And what gap: 0.1V or 0.2V or more or less?

with option 1, theorically, MPPT will NEVER stop charging when all 8 cells are well balanced at 3.65 - gap, as they doesnt reach 29.2 V
with option 2, theorically, MPPT will stop charging before getting all cells at 3.65, because of the gap
with option 3, theorically, MPPT will NEVER stop charging, all cells will be at 3.65 and the DIYBMS must to stop the charge of each of the 8 cells.
(option 1 and 3 are very similar in their final result, MPPT will not stop, and BMS will have to stop the charge of all the cellsl)

What is the better option? What about the gaps? and what about the accuracy between voltage measurements of MPPT and BMS?

Does anybody has a similar instalation? Do you think that is better to post in other forum?

Thanks a lot and happy new year.

 

[gnubie]

Victron's default lifepo4 profile when it detects you have a 24v battery ( the current latest firmware detects the battery voltage and stores it the first time it is connected to a battery, failing this it will treat the battery as 12v ) will pull the battery up to 28.4v, sit it there for 2 hours, then drop the charger output down to 27.0 volts.

If you configure your BMS to disconnect at 29v you will protect your battery and not disrupt the charging activities of the Victron SCC.

The latest thinking is that 2 hours is too long to leave your battery at 28.4v. Victron seems happy with that, but you may not. You can customise the charging profile via the Victron app on an android phone, or via their Windows software, to reduce the time at 28.4v or if you have a current shunt to sense charge in / out of the battery or small load that is connected to the SCC's output terminals so that the Victron can use its own internal shunt to monitor battery current you can set the Victron to use the current to sense when it should terminate 28.4v charging.

 

[juan3211]

Hi, thanks a lot for your quick answer, so your proposal is (within 3 options):

2. MPPT: max 29.2 - gap and BMS: max per cell 3,65. And what gap: 0.1V or 0.2V or more or less?

But with in your example with Victron default profile, we can rewrite it as:
2. MPPT-SCC: max 28.4 v, and BMS: 29 V, so max per cell: 3.625 V. Your proposed GAP is 0.6V.

Option 2 is to keep MPPT with less maximum voltage than BMS.

But what about BMS configuration for each cell?

With option 2. theorically, MPPT will stop charging before getting all cells at 3.625, because of the gap

In the worst case, we could have the SCC at 28.4v and 7 cells at 3.625 and the last one at 3,025 V. (7*3.625+3.025=28.4V)

May be we have to reduce the gap between SCC and BMS ? I mean: 28.4V SCC and 28.6V BMS?

What do you think ?

In the other hand, I can't understand what you say here:

if you have a current shunt to sense charge in / out of the battery or small load that is connected to the SCC's output terminals so that the Victron can use its own internal shunt to monitor battery current you can set the Victron to use the current to sense when it should terminate 28.4v charging.

 

[gnubie]

There's so much discussion and hand wringing about lifepo4 cell charging voltages on the internet that it's hard to sift it and find something useful. Victron has settled on 28.4 as being the voltage to charge up to so I'm happy with that. My battery's BMS has a cut off of 29.2.

In terms of cell balancing I'm not familiar with the DIYBMS but if you adopted 28.4 (3.55/cell) as being the highest charge voltage applied to the battery you might want to set the point at which it commences balancing cells at 3.52. TBH I'm no expert when it comes to the finer points of lifepo4 cell charging.

The Victron SCC you are using has battery terminals and load terminals. If your load is under 15 amps you can connect it to the load terminals and then the Victron can see where the current is flowing and use that to determine when the battery is charged. If you have higher loads an external shunt (BMS 712) will be needed if you want to do that. The other option is simply to fall back to a maximum time at 28.4 volts before the Victron goes back down to the float voltage.

 

[juan3211]

There's so much discussion and hand wringing about lifepo4 cell charging voltages on the internet that it's hard to sift it and find something useful. Victron has settled on 28.4 as being the voltage to charge up to so I'm happy with that. My battery's BMS has a cut off of 29.2.

In terms of cell balancing I'm not familiar with the DIYBMS but if you adopted 28.4 (3.55/cell) as being the highest charge voltage applied to the battery you might want to set the point at which it commences balancing cells at 3.52. TBH I'm no expert when it comes to the finer points of lifepo4 cell charging.

I think I understand that your proposal could be (more or less)

2. MPPT-SCC: max 28.4 v, and BMS: 28.2 V, so max per cell: 3.525 V

Then the MPPT-SCC will never stop (or implement your second parragraph solution to stop SCC). That is my doubt, what is better.

The Victron SCC you are using has battery terminals and load terminals. If your load is under 15 amps you can connect it to the load terminals and then the Victron can see where the current is flowing and use that to determine when the battery is charged. If you have higher loads an external shunt (BMS 712) will be needed if you want to do that. The other option is simply to fall back to a maximum time at 28.4 volts before the Victron goes back down to the float voltage.

Hi, my load is an inverter directly connected to batteries. I didnt know nothing about "the Victron can see where the current is flowing and use that to determine when the battery is charged". I will read the SCC manual, for me it is strange that how the Victron can see the battery state by seeing current in load terminals. I will read. With this solution option 2 as you have proposed is better, isn't it?

May be more people think about three different options about SCC + BMS (think about a DIYBMS or a common BMS, not Victron ones).

Thanks a lot

 

[juan3211]

Any one ?

 

[astronom]

Personnaly I would try to keep the BMS as the last resort fail safe, meaning it should only disconnect the battery from the system if all other regulations did not work properly. I would do that because I suspect that an MPPT-SCC would not be very happy from a sudden electric surge coming from the opening of the battery circuit (the power it was putting in has nowhere to go anymore). So I would rather have the MPPT stop charging first on its own, before the BMS goes into overvoltage protection. If your BMS has the right settings, I'm hoping that it should not prevent you from getting a proper balancing of your cells by setting the balancing voltage lower than the max charge voltage on your MPPT.

I intend to do an intermediate solution in my setup: I will use the Chargery BMS, which outputs "allow charge"/"allow discharge" signals that are supposed to command relays placed at the output circuit of the battery. I will wire the "allow charge" signal to my Victron system (will be an EasySolar, with MPPT and inverter integrated together) instead of a relay, this way the MPPT will be directly ordered to stop by the BMS if it overcharges. I guess it's better than opening the circuit while the MPPT is pushing current. I guess you should have such an input on your Victron, double check maybe.

I'm still in the process of designing/building my setup so all what I'm saying does not come from personal experience, but maybe you can find more direct testimonies around on the forum.

 

[juan3211]

Personnaly I would try to keep the BMS as the last resort fail safe, meaning it should only disconnect the battery from the system if all other regulations did not work properly. I would do that because I suspect that an MPPT-SCC would not be very happy from a sudden electric surge coming from the opening of the battery circuit (the power it was putting in has nowhere to go anymore). So I would rather have the MPPT stop charging first on its own, before the BMS goes into overvoltage protection. If your BMS has the right settings, I'm hoping that it should not prevent you from getting a proper balancing of your cells by setting the balancing voltage lower than the max charge voltage on your MPPT.

I intend to do an intermediate solution in my setup: I will use the Chargery BMS, which outputs "allow charge"/"allow discharge" signals that are supposed to command relays placed at the output circuit of the battery. I will wire the "allow charge" signal to my Victron system (will be an EasySolar, with MPPT and inverter integrated together) instead of a relay, this way the MPPT will be directly ordered to stop by the BMS if it overcharges. I guess it's better than opening the circuit while the MPPT is pushing current. I guess you should have such an input on your Victron, double check maybe.

I'm still in the process of designing/building my setup so all what I'm saying does not come from personal experience, but maybe you can find more direct testimonies around on the forum.

Thanks a lot for your point of view.

I think this post is very special because I hadn't found any other post discussion this type of installation (BMS + MPPT)

But in my case, my MPPT SCC hasn't got that kind of input.

At this time, we have just two opposite options, haven't we?

GNUBIE:
2. MPPT-SCC: max 28.4 v, and BMS: 28.2 V, so max per cell: 3.525 V

ASTRONOM:
3. MPPT-SCC: max 28.4 v, and BMS: 28.6 V, so max per cell: 3,575 V (in worst case, one of the cells will have 28.4-7*3.575 = 3.375 V
In this case, BMS will never stop charging, SCC will be the "responsible for it".

I hope more people read this post and we could discuss further.

Thanks to everybody.

 

[juan3211]

Hi, anybody has any opinion or experience ? Thanks.

 

[juan3211]

Anybody ?

 

[volkergi1970]

Anybody ?

I understand your problem. I had the same considerations. I set set the loading voltage near exactly of the summary from the maximum balance voltage. This leads to power on all balance modules until all cells are full.