Is it normal for my BMS to report over voltage cells?

[delvxe]

Hi all,

I built a 120Ah battery using the Overkill Solar BMS (also sold in many flavors from Alibaba) and installed in my camper van. Once the battery is at 100% and the van still running (alternator charging) the "cell over voltage" protection from the BMS frequently kicks in and stops charging. The over voltage protection is set to 3.65 per cell. Is this normal and simply how the BMS controls charging or is something wrong?

Separate question, I expected that charging would slow as the battery approached full, but this does not seem to be the case for me. I am charging at 300-400 watts while the van is running right up until the battery is full.

Thanks for the feedback and help.

 

[Bob B]

You'd need to provide a screen shot from the app that show the other cell voltages and the overall pack voltage..... or list them all as close as possible to the time of disconnect.

 

[gnubie]

As far as I can tell this BMS can not control charging per se. Its sole control of battery charging is to detect when a cell reaches the over voltage threshold or when the voltage across the battery in general reaches the over voltage threshold and at that point disconnect.

How is your alternator connected to the battery? If it's direct, ie you've taken steps to ensure the battery does not kill your alternator, what voltage is it set up for? If it's at a voltage that will result in cells rising up to the over-voltage threshold what you are seeing is what to expect.

Are you seeing just one cell hitting the threshold? How far away are the others when this happens if so? Another possibility is you have a mismatched / weak cell.

 

[John Frum]

Hi all,

I built a 120Ah battery using the Overkill Solar BMS (also sold in many flavors from Alibaba) and installed in my camper van. Once the battery is at 100% and the van still running (alternator charging) the "cell over voltage" protection from the BMS frequently kicks in and stops charging. The over voltage protection is set to 3.65 per cell. Is this normal and simply how the BMS controls charging or is something wrong?

Separate question, I expected that charging would slow as the battery approached full, but this does not seem to be the case for me. I am charging at 300-400 watts while the van is running right up until the battery is full.

Thanks for the feedback and help.

What are the cell voltages as indicated by the bms?
Does the bms voltage agree with your dvom?

 

[Ampster]

It is normal for Amps to taper unless one cell is a runner and hits hits 3.65 before others. There is no "reporting" if you mean an email, text or buzzer?

 

[delvxe]

I guess when I think about it, my real question is "is this the proper way the battery is supposed to be charged and is it safe and healthy?" I assumed the BMS or charge controller would prevent the cells from ever hitting the 3.65 and that when this happens the cell is either unsafe or being over taxed, but maybe this is a wrong assumption.

As far as I can tell this BMS can not control charging per se. Its sole control of battery charging is to detect when a cell reaches the over voltage threshold or when the voltage across the battery in general reaches the over voltage threshold and at that point disconnect.

Thanks for confirming. This seems to be how the BMS acts. Maybe this is fine and how a BMS is supposed to behave?

How is your alternator connected to the battery? If it's direct, ie you've taken steps to ensure the battery does not kill your alternator, what voltage is it set up for? If it's at a voltage that will result in cells rising up to the over-voltage threshold what you are seeing is what to expect.

I am using the Renogy DC550S controller between the alternator and battery. This unit is set to the charge profile attached.

Are you seeing just one cell hitting the threshold? How far away are the others when this happens if so? Another possibility is you have a mismatched / weak cell.

It hasn't always been the same cell. So far only cells 1 and 4 in the pack. They were all top balanced before putting in the van.

 

[delvxe]

What are the cell voltages as indicated by the bms?
Does the bms voltage agree with your dvom?

The BMS seems to be correctly reading the cell voltage an shutting off the battery appropriately. I don't remember the exact voltages of each cell but when the over voltage protection kicks in one cell has been at 3.65 and the other three pretty close to that.

Yes, my multimeter agrees with the BMS.

 

[gnubie]

What's your battery terminal voltage when you see this happen, use a multimeter? Making sure the Renogy is actually not going over 14.4V. You might also consider reducing the Renogy to 14.2V. That's still enough to give bring your battery to a good charge but gives you more head room to play with slightly mismatched cells. It'd also compensate for any inaccuracies in the Renogy's voltage regulation being low, ie it's actually producing a higher voltage than it should.

As far as it being safe and healthy, the BMS will prevent things from getting out of hand but you should not be relying on the BMS to do this, it's not what the intent is. It'd be like crashing your car into a wall to stop instead of using the brakes. If everything is set up properly the charger will stop charging before the BMS has to step in.

*edit for more*
The reason I an suspicious about your Renogy charger is your cell voltages. If the renogy is doing its thing properly the average cell voltage is 3.6V. If you have one cell hitting 3.65 and the others are not far behind, I'm taking this as being 3.62 or more, you must be getting > 14.4V on the battery terminals, and that means the Renogy is going over the 14.4 setting.

 

[John Frum]

The BMS seems to be correctly reading the cell voltage an shutting off the battery appropriately. I don't remember the exact voltages of each cell but when the over voltage protection kicks in one cell has been at 3.65 and the other three pretty close to that.

Yes, my multimeter agrees with the BMS.

most bms set the high cell cutoff slightly higher than 3.65 volts which is the max cell voltage for lifepo4.
what is the high pack cutoff?
I would be more comfortable if you could quantify "pretty close".
What charge profile is the renogy using?

 

[John Frum]

Oh and are you using the BVS leads?
If so, which battery are they connected to?

 

[delvxe]

Thank you @gnubie and @smoothJoey

This is all very helpful. I think I may be on to something but it raises more questions. I have screenshots from the BMS app from two different days. The first screenshot from about 20 days ago shows the cells well balanced. I think this was a>24 hours after the initial top balance. The second screenshot is just now when the over voltage protection kicked in. In the one now, you can see the cells are out of balance
Cell 1: 3.650
cell 2: 3.537
cell 3: 3.588
cell 4: 3.646

Does this indicate that my cells are out of balance now and causing the problem? If so, what could cause this? Are the cells maybe bad? I assumed since I got them fully charged and were still balanced after a day or two that they were good to go.

 

[gnubie]

Image 2, 0.14V over the supposed voltage the Renogy should be putting out. It's not necessarily the actual voltage the Renogy is attempting to put out either as the cells are still taking current and the balancer is active. The final voltage may be higher.

It also shows your system is capable of controlling the differences in the cells. Image one shows they aren't closely matched. While I'm not a cell and BMS expert by any means so take this with a grain of salt and listen to what everyone else says about it, image 1 says the cells are reasonably matched as far as I'm concerned. At the top end of charging the cells quickly move up in voltage with the difference between cell 1 and 2 being about 2% SOC.

The charger may not be the whole problem but it's certainly contributing to it.

 

[delvxe]

most bms set the high cell cutoff slightly higher than 3.65 volts which is the max cell voltage for lifepo4.

That makes sense. I used the setting recommended by the BMS reseller. What would you say is "slightly higher"?

what is the high pack cutoff?

I set it to 14.60 for the cutoff for the entire pack.

I would be more comfortable if you could quantify "pretty close".

I put screen shots in my other response and am interested in your thoughts.

What charge profile is the renogy using?

I am using the Lithium setting on the Renogy controller. The profile is shown above for this.

It seems like maybe the Renogy unit is functioning properly and that I am getting a single cell hitting 3.65 before the whole pack hits 14.40 where the Renogy controller kicks in and stops charging. Does that seem right? If so, what can or should I do to deal with this?

 

[MrNatural22]

After many OVD issues I also had one cell that took off and shut the BMS down every time when the charger was set at 28.8v (24v battery so that would be 14.4 for 12v. Ever since I’ve reduced the charge voltage down to <28.4> or 14.2 in your case and the OVD has come close 3.60v but the charger current drops to .01a and stays at CV and it no longer hits that OVD threshold. Battery is full and loving it. If possible maybe set your charge voltage down a tad you’ll have a happy battery for a longer time.
I also am using the overkill BMS.

 

[John Frum]

Make sure your bms only balances on charge and only at 3.4 and above.
You didn't answer about the battery sense leads. I'm gonna guess that you are using the. If you disconnect them I bet your charge will terminate a bit lower.

 

[gnubie]

It seems like maybe the Renogy unit is functioning properly and that I am getting a single cell hitting 3.65 before the whole pack hits 14.40 where the Renogy controller kicks in and stops charging. Does that seem right? If so, what can or should I do to deal with this?

Image 2 (IMG_0868) disproves that. Look at the maximum voltage, 14.54. Above your chargers 14.4 setting.

 

[delvxe]

Make sure your bms only balances on charge and only at 3.4 and above.

Thanks. Just confirmed that I am using these setting.

You didn't answer about the battery sense leads. I'm gonna guess that you are using the. If you disconnect them I bet your charge will terminate a bit lower.

Sorry. I don't understand the question. Are you suggesting I disconnect the balance cables and bus bars and read the cells?

 

[delvxe]

Image 2 (IMG_0868) disproves that. Look at the maximum voltage, 14.54. Above your chargers 14.4 setting.

That screenshot may have been when the battery was still on the workbench and resting after charge and top balance.

 

[John Frum]

Thanks. Just confirmed that I am using these setting.


Sorry. I don't understand the question. Are you suggesting I disconnect the balance cables and bus bars and read the cells?

The renogy charger has sense leads designated bvs.
If they are connected remove them.

 

[gnubie]

That screenshot may have been when the battery was still on the workbench and resting after charge and top balance.

It might have been when an elephant was standing on the battery too.