Bms hvd vs mppt absorbtion value

[rmcrowe]

I finally got my new sbms0 and installed it. Testing now and I have a question ( 12v 280ah)

Sbms0 is set for hvd at 3.5v ,. Victron default lithium profile set for 14.2v absorbtion. I have one cell that charges a bit faster than the others - I'm not worried much about this, as it only manifests at the very high end of the charge cycle.

What happens is the one cell gets to 3.5v. While the others are around 3.4 (this is at 99% soc). So the pack voltage is around 13.9. sbms0 shuts off the mppt due to hvd on the one cell, so it never goes into float (this is a victron 100/50 smart solar mppt).

My question is:. Would it be better to set absorbtion voltage lower on the mppt so that it goes into float mode and doesn't get get shutoff, or just leave it like it is where it toggles between bulk/off as the battery charges and discharges?

The way this system is currently configured I don't think the mppt would ever go to float even if the cells were perfectly balanced (4x3.5 = 14 vs 14.2 absorbtion value). I'm not that concerned about getting every watt of power from this system, more focused on optimizing life.

Bob

 

[snoobler]

BMS should be set to protect the cells, not to manage utilization.

You should manage your usable upper voltage limit with the charging device.

You should manage your usable lower voltage limit with the discharging device.

If the BMS is activating at a limit, you're likely doing it wrong.

BMS should be set to balance during charging only and above 3.3V.

If you want to limit your individual cells to 3.5V, you should set the absorption value to 3.5*4 = 14V, float to 13.2V and raise your BMS limits to 3.65V. After enough use, balancing should get all cells up to 3.5V. Make sure you set your tail current in expert mode to 14A, i.e., once you hit 14A at the absorption voltage, your battery is full, and it should transition to float.

 

[GXMnow]

Sbms0 is set for hvd at 3.5v ,. Victron default lithium profile set for 14.2v absorbtion. I have one cell that charges a bit faster than the others - I'm not worried much about this, as it only manifests at the very high end of the charge cycle.

What happens is the one cell gets to 3.5v. While the others are around 3.4 (this is at 99% soc). So the pack voltage is around 13.9. sbms0 shuts off the mppt due to hvd on the one cell, so it never goes into float (this is a victron 100/50 smart solar mppt).

This sounds like the one cell has a bit lower capacity than the other 3. Have you tried doing a solid "top balance" on the pack? If the cell is still good, but just has less capacity, you should get good performance by getting all 4 cells to fully charged. Then during discharge, the one weaker cell will still drop down a little faster, but as long as you only use a bit less than that weakest cell can handle, it will work fine. When the pack charges again, the weak, faster charging cell, will climb quicker, and "catch" the other 3 cells as they all reach full charge again. That should eliminate the problem and allow you to run with the 14.2 volt absorption setting again. If that cell is too weak for this to work, then I would suggest replacing the cell as it is failing. In any case with cells in series, you will have to limit your use to the weakest cell in the chain. If 3 of the cells are truly 280 amp hour, but one is just 190 amp hour, you can only use the 190 amp hour from all of the cells. So if this issue is that bad, it will pay off to replace that bad cell.

 

[rmcrowe]

Thanks @snoobler I was thinking the same regarding bms controlling the charge/discharge function - its really just there for safety. Im not worried about the 2ah or so top of this thing, so I'm just going to reduce my absorption on both the multiplus and the mppt to 13.9.

@GXMnow thanks also -the lower cell is only about 1% less SOC than the others. I think my challenge stemmed from the low default HVD on the SBMS0 - compared to the absorption value for the Victron.

Just out of curiosity - how many AH do you see between 3.45v (pack volt of 13.8) and 3.65 (14.4) ? By my reckoning its maybe 2-3AH, not worth getting close to that upper limit especially if this stretches out battery life.

 

[snoobler]

Thanks @snoobler I was thinking the same regarding bms controlling the charge/discharge function - its really just there for safety. Im not worried about the 2ah or so top of this thing, so I'm just going to reduce my absorption on both the multiplus and the mppt to 13.9.

@GXMnow thanks also -the lower cell is only about 1% less SOC than the others. I think my challenge stemmed from the low default HVD on the SBMS0 - compared to the absorption value for the Victron.

Just out of curiosity - how many AH do you see between 3.45v (pack volt of 13.8) and 3.65 (14.4) ? By my reckoning its maybe 2-3AH, not worth getting close to that upper limit especially if this stretches out battery life.

It depends on the capacity and the charge current. If you're charging at the rated 0.5C, you'll lose a lot. If you're charging at smaller fractions of C, then SoC drives higher once the "absorption" voltage is hit. It's more accurately expressed as a % of capacity vs. a hard Ah, and it varies with usage profile.

 

[GXMnow]

Well Said snoobler. It is not so much about how many amp hours you have in the cells, it is about getting all 4 of them to hit the knee at the same time. Once you get the cells to all top balance at the knee, they will probably work fine together for a long time before getting out of balance again. Even if the capacity of a cell deviates by 10%, it should still work just fine, as long as you never run the weak cell too low. So your 3 good cells are 280 AH, and the weak one is 250 AH. If you discharge 140 AH from the pack, the 3 good cells are left at 50%, and the weak sell is down at 44%. Now when you charge it back up, the cells all take 140 AH and they all hit 100% SOC at the same time again and again.

The weak cell will age a little faster and the balance at low charge will get worse, but as long as the lowest cell is not pushed too low, it will work, just at lower total usable capacity. When that weak cell does get too bad, then the BMS can step in and shut things down when it goes too low, but on the top end, they should all hit the knee together, and the charge controller should do it's job of absorb charging the pack and as the absorb current falls off, the balancer can top balance the cells each time they come up to full.

Most built in BMS balancers just are not powerful enough to do this on large cells, so you probably need to do the initial top balance yourself. Once they are all close, then the balancer should be able to keep them there.

 

[rmcrowe]

When building the pack I did do a capacity test on all the cells. I was getting between 176 to 180 ah from each. I did do a top balance before building the final configuration. 3.65 for each individual cell then I paralllel at 3.65 for 48 hours. Bit since the bms was back ordered I ran these without for a few months with very conservative settings (charger absorbtion value at 12.7). Maybe they got a bit unbalanced. Now that I have a bms I feel more comfortable doing a full capacity test so I'll see what I can actually pull.

This rig is mostly in shade now so the solar charge is only running at about 15 amps.

 

[John Frum]

When building the pack I did do a capacity test on all the cells. I was getting between 176 to 180 ah from each. I did do a top balance before building the final configuration. 3.65 for each individual cell then I paralllel at 3.65 for 48 hours. Bit since the bms was back ordered I ran these without for a few months with very conservative settings (charger absorbtion value at 12.7). Maybe they got a bit unbalanced. Now that I have a bms I feel more comfortable doing a full capacity test so I'll see what I can actually pull.

This rig is mostly in shade now so the solar charge is only running at about 15 amps.

176-180 ah from a 280 ah cell is no good at all.

 

[rmcrowe]

176-180 ah from a 280 ah cell is no good at all.

Oops. That was a typo... 276 to 280 is what I meant to write . This was in an ambient temperature of about 95f so I thought it was okay.