Lithium battery series bank charging unevenly

flowingspringspower · November 2021

Hello all, novice here and I have a question regarding lithium battery bank series charging.

A brief overview of my system.

Running an MPP solar LV6548 off grid using 20 295 watt REC panels powering 20 100AH lithium batteries purchased a month ago.

Batteries are wired 4 in series x 5 banks.=500 AH
been monitoring the batteries since I installed them with a combination of a volt/amp meter, inverter data and the watchpower app of my phone.

Batteries are fully recharged by around noon each day after which they enter float charge.

Batteries were fully balanced and charged to 14.6v before installation in parallel then reconfigured in series to connect to inverter.

I’ve been noticing that each bank of 4 will show 58.4v once fully charged but the individual batteries in that bank are not charging evenly. The sum of the battery voltages equals 58.4 but one battery in the bank will read as high as 17.4 volts. I know this is higher than technically they should reach. I believe this high voltage is preventing the other three batteries in each bank from getting fully charged. Do I make sense here? I have removed a series bank from the system and rebalanced it before reinstall action and it does the same thing again after a day or two. The battery reading high voltage prior was also placed in a different position in the series and did not repeat the high voltage but again, one of the batteries in the series will read high voltage by the end of the charge cycle.

Any thoughts? Thanks for your input.

BB. · November 2021

Welcome to the forum FSP,

Do you have a BMS (battery monitor system) for your bank?

With lead acid batteries we typically place batteries in series (for 48 volts or whatever), then connect those strings in parallel (for your total AH rating of bank). And Lead Acid batteries are "lossy" and forgiving enough that Equalize Charging is enough to rebalance the cells). And keeping the cells "separate", it is easier to find problems with a voltmeter (bad cell/battery has high/low voltage with respect to all the others).

With Li Ion, you can do separate strings (which you are doing here?) and it makes it easier to find weak cells/batteries--The ones that have too high or too low of voltage vs the others.

With Li Ion BMS, these are not cheap and have lots of extra wires (one wire for each cell)--And connecting the cells in parallel first, then in series--Then you only need one BMS per bank (vs one BMS per parallel string). This makes it easier/cheaper to wire and monitor--But can "hide" problem cells.

In your case, you have 12 volt batteries. What brand/model of batteries are these? Many (some, most, all?) 12 volt Li Ion batteries have an internal BMS inside each 12 volt pack. Depending on the type of BMS (there are different types--Passive monitoring, to active cell balancing, to active battery shutdown if voltage limits are exceeded), you may see different things happening. And we need to know the details of the 12 volt battery BMS configuration to better understand what is happening.

At this point, if it is only one battery that is showing this high voltage problem--I would suspect you have a bad battery or a "tripped" BMS that is shutting down the battery (to protect against over voltage).

If you have a DC Current Clamp Meter--You can charge (or discharge) the bank (provide current flow) and use the clamp meter to measure the current in each series string of 4x batteries. I would suspect that the 17.4 volt battery is possibly "open" (BMS tripped) and there is no current flow in that string.

Generally, "matched" cells and batteries should match voltages among all in the bank (pretty closely--more than a few 1/10's of volt difference could be an issue). In this case, you are not seeing matched voltages? (you are top balancing the batteries, so they should match very closely nearing 100% state of charge--As you approach 0% SoC, the voltages may diverge a bit).

There are other BMS functions that could affect battery performance... It is possible now that some 12 volt Li Ion batteries even have electric heaters inside them to keep them >~40F -- That is great for the battery, but if series strings of them in a 48 volt bank--Could very easily unbalance the batteries. (I am just guessing here--Tossing an example of what could happen).

Can you contact Tech Support for your battery supplier and ask them? Not all 12 volt Li Ion batteries can be used in 48 VDC battery banks (because of BMS issues and such)...

By the way, there is a "right way" to parallel connect battery banks (I don't think this is 'your issue'). This website tells how:

http://www.smartgauge.co.uk/batt_con.html

-Bill

mike95490 · November 2021

> Batteries are wired 4 in series x 5 banks.=500 AH

Well, anytime you place Li batteries in series, you need the cells to be closely matched in capacity, to prevent a small capacity cell, filling up first and shutting off charging to the rest of the pack. A BMS can help some with balancing while charging, but only the larger and more advanced BMS systems can manage 10-20Amps in bypass to allow the rest of the cellls to fill up. Many BMS's don't allow series to larger than 24V nominal bank.
You have to work this through with your battery vendor to get this straightened up. With overvoltages cells, you are in line for some exciting failures of the battery with the damages cells.

A lead acid system using L-16 floor scrubber batteries could be quickly put into play . It would use eight, 6V 400Ah batteries in series

flowingspringspower · November 2021

BB. said:

Welcome to the forum FSP,

Do you have a BMS (battery monitor system) for your bank?

With lead acid batteries we typically place batteries in series (for 48 volts or whatever), then connect those strings in parallel (for your total AH rating of bank). And Lead Acid batteries are "lossy" and forgiving enough that Equalize Charging is enough to rebalance the cells). And keeping the cells "separate", it is easier to find problems with a voltmeter (bad cell/battery has high/low voltage with respect to all the others).

With Li Ion, you can do separate strings (which you are doing here?) and it makes it easier to find weak cells/batteries--The ones that have too high or too low of voltage vs the others.

With Li Ion BMS, these are not cheap and have lots of extra wires (one wire for each cell)--And connecting the cells in parallel first, then in series--Then you only need one BMS per bank (vs one BMS per parallel string). This makes it easier/cheaper to wire and monitor--But can "hide" problem cells.

In your case, you have 12 volt batteries. What brand/model of batteries are these? Many (some, most, all?) 12 volt Li Ion batteries have an internal BMS inside each 12 volt pack. Depending on the type of BMS (there are different types--Passive monitoring, to active cell balancing, to active battery shutdown if voltage limits are exceeded), you may see different things happening. And we need to know the details of the 12 volt battery BMS configuration to better understand what is happening.

At this point, if it is only one battery that is showing this high voltage problem--I would suspect you have a bad battery or a "tripped" BMS that is shutting down the battery (to protect against over voltage).

If you have a DC Current Clamp Meter--You can charge (or discharge) the bank (provide current flow) and use the clamp meter to measure the current in each series string of 4x batteries. I would suspect that the 17.4 volt battery is possibly "open" (BMS tripped) and there is no current flow in that string.

Generally, "matched" cells and batteries should match voltages among all in the bank (pretty closely--more than a few 1/10's of volt difference could be an issue). In this case, you are not seeing matched voltages? (you are top balancing the batteries, so they should match very closely nearing 100% state of charge--As you approach 0% SoC, the voltages may diverge a bit).

There are other BMS functions that could affect battery performance... It is possible now that some 12 volt Li Ion batteries even have electric heaters inside them to keep them >~40F -- That is great for the battery, but if series strings of them in a 48 volt bank--Could very easily unbalance the batteries. (I am just guessing here--Tossing an example of what could happen).

Can you contact Tech Support for your battery supplier and ask them? Not all 12 volt Li Ion batteries can be used in 48 VDC battery banks (because of BMS issues and such)...

By the way, there is a "right way" to parallel connect battery banks (I don't think this is 'your issue'). This website tells how:

http://www.smartgauge.co.uk/batt_con.html

-Bill

I purchased Lossigy batteries from Amazon. I am aware that they are bottom of the barrel batteries. 350 bucks give or take. They are rated at 100 AH each. They do not have low temp cut off but where they are installed that isn’t an issue. They were each charged to 14.6 volts then wired in parallel to balance, again charged that way to 14.6 before being installed in series.

All 20 batteries (5 banks of 4=48v) where purchased within a few week of each other.

I’ve noticed that this high voltage only occurs during charging. As soon as a load is applied to the batteries the voltages all drop into line with one another.

I used the battery cables that came with the batteries to connect them in series. As best as I can tell (without being able to read Chinese) the battery cables are probably 4 AWG. I’m running 2/0 cables from each bank to a buss bar which subsequently drops to a 1/0 cable to the inverter. Would running smaller gauge wires in the series wiring cause enough resistance to present this problem?

BB. · November 2021

From reading the Amazon information, these batteries do have a BMS that appears to cut off the battery in high/low voltage situations, over current cutoff, and balance cells internally--No high/low temperature cutoff, no Bluetooth or other external BMS communications. There may be some BMS equalization when up to 4x batteries are in series (Amazon listing was not explicit).

Have you measured the current flow into the string with the one battery at ~17 volts? If the BMS tripped (to protect the battery), I would expect that string to have zero current while other strings continue to charge. This would be "normal" operation.

Does the battery have some indicator to show the status of the BMS (LEDs or something else)?

From what you have said, there is nothing you are doing wrong. The BMS may be "turning off" the battery with high voltage because it has reached (what the BMS "thinks") 100% State of Charge (internal battery cell max voltage, turn off output or "cutoff").

About the only thing you could possibly do--We have one poster here with LiFePO4 batteries and internal BMS where the BMS was not correctly keeping track of battery State of Charge--And was cutting off the battery at something like 50% of capacity. They took it back to their supplier, and ran the batteries through several discharge/charge cycles and the BMS (apparently) "re-calibrated" to allow the battery to provide 100% of rated AH capacity.

If you have a "smart phone", there is a Google Translate app. (at least for Android) that you can load that (among other things) will let you point your camera at a foreign language and get a translation. Works pretty well (at least most of the time).

That is about all I know/can think of...

-Bill

PS: There is always the possibility of a bad cable connection (loose, poor crimp on cable ends, etc.) that can cause issues like this.

Don't think this is your issue, but checking connections and using a volt meter to check for "bad" wiring is also something to check.

Lithium battery series bank charging unevenly

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