Overvolt when battery reaches full charge

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I shut down charging as it was getting pretty high pretty quick.

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Glad I could see this today as this is usually happening while I am at work. Will lowering the charging voltage help to mitigate it going to high? Again, thanks to all for your help.
 

MisterSandals

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I shut down charging as it was getting pretty high pretty quick.

Glad I could see this today as this is usually happening while I am at work. Will lowering the charging voltage help to mitigate it going to high? Again, thanks to all for your help.

Try 27.6V which is 3.45Vpc. I charge to 3.45Vpc which i consider a gentle 100% charge.
 

Bud Martin

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Aug 27, 2020
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Shouldn't the Equalization be set to 0 Volt instead of 28.80V? I know that the EQ time is set to 0 but may something going on to let the Voltage go higher than 30V.
 

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I dropped it again to 27.4V as it started ramping again. The system is balancing without taking charge and the solar is holding the load. Voltage diff is 0.158V and dropping. Will hold here for the day and see how this turns out. Thanks for all those that have helped. Hopefully I can keep this under control now.
 

GXMnow

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That 0.150 volt difference from the highest to lowest cells is a huge imbalance, especially for LFP cells. What amp hour rating are the cells? How many cells are in parallel? Are you sure the cells are evenly sharing the current?

With the cell voltage numbers you showed, I would certainly be using some external load to pull down those high cells. Many BMS's only have 30 or 60 milliamps of balance current. You need a couple amps to get those back in line. Maybe look into an active balancer that can move over an amp between cells. My original battery bank is using a JK-BMS, also sold as Heltec. It can pull or push up to 2 amps, but only one cell group at a time. That works great for 1 or 2 runners like you have. It will pull 2 amps from the highest cell, then push 2 amps to the lowest cell. It just keeps doing that, as long as the cells are above the minimum voltage setting. My cells all now stay within 0.005 volt of each other at all times. Over 100 mv difference is way too much.

The "Problem" with LFP is that their discharge curve is nearly flat for about 70% of their capacity. So from 20% charge up to 90% charge, the cells seem to be closely balanced. But in reality, you could have one or two cells at 90%, while the other cells are at just 50% and yet they are only 5 millivolts different in voltage. The whole point of top balancing is bringing all the cells into the top knee where they climb fast up to 3.6 volts or so. Then when you discharge the pack, even if they ae slightly different capacity, they should still all charge back up and hit the top together. You said you did a top balance. How high up did you bring the cells, and did you hold them there for a while? If you balanced at less than 3.4 volts, some of the cells may not have truly been full.
 

Any name you wish

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That 0.150 volt difference from the highest to lowest cells is a huge imbalance, especially for LFP cells. What amp hour rating are the cells? How many cells are in parallel? Are you sure the cells are evenly sharing the current?

With the cell voltage numbers you showed, I would certainly be using some external load to pull down those high cells. Many BMS's only have 30 or 60 milliamps of balance current. You need a couple amps to get those back in line. Maybe look into an active balancer that can move over an amp between cells. My original battery bank is using a JK-BMS, also sold as Heltec. It can pull or push up to 2 amps, but only one cell group at a time. That works great for 1 or 2 runners like you have. It will pull 2 amps from the highest cell, then push 2 amps to the lowest cell. It just keeps doing that, as long as the cells are above the minimum voltage setting. My cells all now stay within 0.005 volt of each other at all times. Over 100 mv difference is way too much.

The "Problem" with LFP is that their discharge curve is nearly flat for about 70% of their capacity. So from 20% charge up to 90% charge, the cells seem to be closely balanced. But in reality, you could have one or two cells at 90%, while the other cells are at just 50% and yet they are only 5 millivolts different in voltage. The whole point of top balancing is bringing all the cells into the top knee where they climb fast up to 3.6 volts or so. Then when you discharge the pack, even if they ae slightly different capacity, they should still all charge back up and hit the top together. You said you did a top balance. How high up did you bring the cells, and did you hold them there for a while? If you balanced at less than 3.4 volts, some of the cells may not have truly been full.
This BMS has a 200mA current for balancing. It can do multiple cells, but it does it on and Even/Odd basis from what I can tell.

Each "cell" is a 20P configuration of this cell: https://batteryhookup.com/products/new-battery-hookup-lifep04-32650-3-2v-5000mah-cells

When I did the top balance I did it to 3.65V and held it at 0A for 15 min to be sure. Verified with PS and stand alone voltmeter.

I have the charging turned off right now and it has dropped from 200mV down to 150mV in about 1.5 hours. I will likely keep it this way until it balances back to where it is supposed to. The discharge is still in effect so I won't lose power to the AC components. Once it is back in balance (probably tomorrow morning), I will turn charging back on again. Note, when I did turn off charging this time with the lower voltage, it did not trigger overvoltage on my inverter so at least that is a plus.
 

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Incase anyone wants to know, the 30A SCC has been my work horse, backup, and now is dual charging the battery with the new 40A one. I have learned a bunch since watching the videos and working on my setup. My hope is to one day make at least 1MWh with them both. over 1/3rd there.

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