Charging and top balancing with 10A bench power supply

aclosson

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Feb 15, 2021
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After doing a top balance I assembled my 280AH grade B batteries with an Overkill 120A BMS which is being charged by a Renogy DCC50S 50A DC to DC MPPT charger. The charger is set for Lithium batteries but when being charged, one of the batteries exceeds the over voltage threshold and the BMS turns off the charge input. Question, what it the best way to try and get cell 3 to stay in the proper voltage range? Would a separate active cell balancer help in this situation? As you can see in the settings I currently have "Balancer enabled" but this appears to only work when charging and it does not appear to work when a cell over voltage has been reached.

When started the charger is running with solar it was doing 14 amps which makes me think the battery bank is not at fully charged but I am currently doing a significant discharge to try again with a lower SOC.

I know over voltage is a common problem with grade B cells but I would appreciate any suggestions on how best to manage it.

I have attached a pdf with the BMS settings and some pictures of the installation.
 

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time2roll

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Consider a lower charging voltage set point on the solar controller. What is the full pack voltage when the BMS cuts off?
 

aclosson

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Feb 15, 2021
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Consider a lower charging voltage set point on the solar controller. What is the full pack voltage when the BMS cuts off?
Unfortunately the Renology charger does not give you the option to change the charge voltage other than changing the battery type. The full pack over voltage is currently set at 14.6V.
 

time2roll

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Lithium setting looks like 14.4 volts and you are getting cut off at 14.07 volts. You could use it as is and wait for the BMS to balance. This might take a lot of cycles. An active balancer would speed up the process. You could manually discharge that one cell using a 1 to 3 ohm resistor to speed up the process. Could also use the power supply used for the initial top balance to boost up the low cells. Maybe use the GEL setting at 14.2 volts for a while to see if the BMS will balance.

Which of these are you inclined to do? Effect in the end is the same. I would lean to getting the active balancer for $12 to $20 as it might help control a chronic issue.
 

GXMnow

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I would connect a few 12 volt car tail light bulbs (old school filaments, not LED) on Cell 3 and monitor it with a meter. Pull that cell down to about 3.5 volts. It will take a while, the more bulbs the faster. But check it often, get an idea how fast it is falling. Once you get it down, then you can start charging the pack again, maybe even leave a few bulbs connected to keep that cell from running again. Hopefully you can get the whole pack into the CV range and then the balancer can sit and work a while.
 

OnTheRoadAgain

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Feb 22, 2021
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Adding this for the benefit of anyone who might read this thread in the future looking for info on top balancing.

Just an update, It took the better part of 3 days to top balance my 206AH pack with a 10 amp bench power supply
(I've ordered a 30 amp Victron). A couple of key observations here.

Whether charging a single cell or a complete pack, set the voltage on the power supply before connecting to the cell/pack and don't disturb it, even if it reads low when you first connect the cell/pack. The power supply will change from constant current to constant voltage pretty quickly and then, slowly, the voltage will come up to the intended level and the amps will decrease over time.

Be patient. The cell and pack voltages are very slow to respond to either charge or discharge due to the very flat voltage curve of the lithium technology. It seems like nothing is happening until you approach either end of the state of charge curve, then things happen quite quickly.

I also did a capacity test on my 206AH pack using Will's suggested Chinese load tester (can you say that or should I just say "load tester"?) It took 15.5 hours for the BMS to detect <2.5 volts on one of the cells and shut down the party but the pack ultimately gave up 218 AH (according to the tester). Much happy. I'm sure that capacity will come down over the next couple of cycles but I'm still happy.

With an exception.....

If at any point you notice the voltage dropping, then you may have an actual power supply issue.

On my new 120Ah cells, I could never get over 3.7 - 3.8v no matter what.
Once the voltage reached that range, the voltage would suddenly start dropping back, ultimately to the resting voltage of the battery.
Where I would wait for the charge to resume assuming it was just filling the void so to speak.

After switching to a different power supply, the batteries immediately resumed charging to full with no further issues and never dropping back in voltage.

This was a brand new, Will recommended 10a Dr. meter power supply from Amazon. (And I was charged a 20% restocking fee to return it)
So trust your power supply, but if the voltage reverses, verify it.
 

aclosson

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Feb 15, 2021
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Once I start discharging the cells they quickly equalize with only .02V of difference. I am going to go through a few cycles on a camping trip next week and I will watch this closely but my guess is that I am going to need to add an active cell balancer. If nothing else it will be an interesting experiment. Does anyone have a specific balancer that they like? Being a geek, it would be nice to be able monitor what it is doing. Another option would be to increase the cell protection voltage on the BMS to something like 3.7V but I am hesitant do this for fear of damaging the other cells.
 

GXMnow

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Don't raise the voltage. 3.6 volts on a cell is plenty high and is actually already stressing the cell. What is happening, is that one cell is full, and the rest are still in the flat part of their curve. The other cells could be as much as 20% below full charge. When you start to discharge, the high cell comes down off the knee, and the low cells just move a bit down on the very flat part of the curve, so it looks like they are close, but they are not.

You do need to actually top balance, or set your charge completion voltage lower and live with less capacity until you can top balance them. Load down the high cell, then charge the pack, keep doing that until all of the cells hit the knee and climb up to 3.5 to 3.6 volts. 3.5 x 4 = 14 volts for all 4 cells. Set the charge current very low so the balancer has a chance. I have the JK BMS with 2 amps of active balancing, it works well, but still is only 2 amps for less than 1/2 the time, so you would need to charge at under 1 amp for it to truly balance a mismatched pack. Many BMS balancers can only pull 30 milliamps. That will take forever to balance a pack. Once they are balanced, it can help hold them balanced.
 

HRTKD

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Check all your connections. At least half the strange voltage issues seen here on the forum are due to bad connections. Using one finger, check the temperature of the bus bars, lugs and fasteners when the battery is under load. If you feel a hot spot, you've found the root of the problem.
 

aclosson

New Member
Joined
Feb 15, 2021
Messages
18
After doing a top balance I assembled my 280AH grade B batteries with an Overkill 120A BMS which is being charged by a Renogy DCC50S 50A DC to DC MPPT charger. The charger is set for Lithium batteries but when being charged, one of the batteries exceeds the over voltage threshold and the BMS turns off the charge input. Question, what it the best way to try and get cell 3 to stay in the proper voltage range? Would a separate active cell balancer help in this situation? As you can see in the settings I currently have "Balancer enabled" but this appears to only work when charging and it does not appear to work when a cell over voltage has been reached.

When started the charger is running with solar it was doing 14 amps which makes me think the battery bank is not at fully charged but I am currently doing a significant discharge to try again with a lower SOC.

I know over voltage is a common problem with grade B cells but I would appreciate any suggestions on how best to manage it.

I have attached a pdf with the BMS settings and some pictures of the installation.
Thank you everyone for all of your help. I wound up ordering an 5a to 8a active balancer to compensate for my poorly matched cells
based on a recommendation from the Freely Roaming YouTube channel which can be found here. This definitely solved the problem and I now can use the Renogy DCC50S built in Lithium profile. Another update is that if you purchase the Renogy wi-fi module you DO have the ability to create a custom user charging profile per Time2rool and Gary's suggestion which is what I did until the balancer arrived.
 
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