question

tastenspecht avatar image
tastenspecht asked

midpoint alarm on two lifepo4 12 (serial to 24v)

Dear community,

I observe a strange behavior (regarding midpoint deviation) for while now, and I didn't find a thread explaining it.

First I like to describe my setup:

  • 2x 200Ah 12v lifepo (both the same manufacturer and the same BMS = 200A)
  • SmartShunt 500A
  • the batteries connected serial (resulting in a 24v 200Ah batterie bank)
  • before connecting the batteries I charged both to 100% separately

I connected the AUX of smart shunt to the plus pole of the batterie of which the minus is connected to the smart shunt. Now to the behavior: under load and or charging no alarm is triggered midpoint is 13.35V and bank is 26.7V. The magic happens always if the battery reaches SOC 100% then deviations of up to 5.7%.

What do you think about this?

1) Is this a critical issue?

2) May it be caused be the batteries BMS?

Thanks in advance for your support.

regards

Andre

Battery Protect
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4 Answers
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derrick thomas answered ·

You need a battery balancer

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tastenspecht avatar image
tastenspecht answered ·

@derrick thomas Thank you for your quick response!

I'm a bit surprised that such a simple battery bank (of only two batteries in series) needs a battery balancer. Also I would expect that if a midpoint deviation occurs on heavy load (when more energy is drawn from one of the batteries) or during charging (one battery consumes more energy than the other). Or do I miss something and the described scenario in my first post (~ SOC 100% -> midpoint deviation up to 5.7%) is the usual when midpoint deviations occur?

Has anyone an explanation for that?

I think I will also contact the battery provider (especially on behalf if this is a critical issue). And if I get any answers to that issue I'll post it here.

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derrick thomas avatar image derrick thomas commented ·
This is not an uncommon occurrence with series connected Lifepo4. When the batteries reach 100% most likely one of them is stopping charge due to total voltage or cell high voltage. Lifepo4 batteries in series will not self balance. I had the same issue on series connected 12v lifepo4 and once I installed the balancer I saw the deviation drop from 6% to under 1 within a few charge cycles. Lithium has very low internal resistance so as the cells reach full SOC the cell imbalance can climb pretty quick. Without a balancer each charge cycle will only continue to compound the issue.
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tastenspecht avatar image tastenspecht derrick thomas commented ·
@derrick thomas Thank you very much for your explanation!
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Paul B avatar image
Paul B answered ·

for Lifepo4 I would be monitoring cell voltages in each bank and mid point is a totally irrelevant reading as per derricks advice.

so watch cell voltages. ie depending on brand specs 3.50 is a full cell so once a cell reaches this voltage a small 1amp draw is place across that cell thus stopping it charging or allowing the other cells to fill, once all cells reach 3.5 volts then the batteries are full and also considered BALANCED. and thus 28 volts is seen but you could have one bank at 13 volts and the other at 15 volts - this points to unbalanced cells in the lower voltage bank.


anyway monitor the cell voltages that's the key

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derrick thomas avatar image derrick thomas commented ·
Having different upper and lower battery voltages may not necessarily point to cell imbalance in either battery. The upper and lower battery may simply have slightly different internal resistance which will cause differences in charging/discharging characteristics. The longer this goes unchecked the greater the imbalance will get. Lifepo4 batteries connected in series should always have a way to balance, if they do not have built in communication between batteries. (Those people reading need to understand the difference between the terms 'battery' and 'cells' or this will all be confusing)


With my first set of lifepo4 batteries (12v in series parallel for 24v) it took almost a year before I realized that there was a midpoint deviation, as I was not monitoring midpoint voltage. I started noticing the charged voltage at which point the batteries bms disconnected and no longer took a charge was getting lower and lower. I found the SOC to be about 10% difference between upper and lower strings. New balancer and all was right with the world again.

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tastenspecht avatar image tastenspecht derrick thomas commented ·
@derrick thomas Thank you! I think, now I understood how things are related and that a cell imbalance is not the only reason for a midpoint deviation. I've ordered a battery balancer.
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tastenspecht avatar image
tastenspecht answered ·

@Paul B First of all thank you for your answer!

You mentioned that midpoint deviation for lifepo4 is irrelevant. And therefore using a battery balancer is not useful (or even destructive?). Did I get this right?

I decided to use batteries shipped with an internal BMS. A bit more detailed: I use two "Redodo LiFePO4 200Ah Plus Akku 12V" batteries. I was hoping the internal BMS is taking care of cell voltages (at least each battery for its cells) and I don't have to care about this. Under this circumstances would you still say that I have to monitor the cell voltages?

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Paul B avatar image Paul B commented ·
a bms usually only balances a cell when it gets to its balancing point ( most cases this is between 3.4 to 3.5v)

in my opion you should monitor the cells and make sure they all reach 3.5v at least once a month. you can only check this when the charge voltage is around 28 to 28.4volts in your case. (this is called top balancing) all cells must reach 3.5v at least once a month. or on a regular base's the better quality the cell the less they will move out of balance over the mth.

of note a 12.8v lifepo4 battery has 4 cells in it and a fully charged cell is between 3.4 and 3.5v (depending on manufacturer specs) I myself use 3.5v a cell can usually go as high as 3.7 and even 4v without damage but there is NO POINT as they are effectually full at 3.5v. going higher all the time is not helpfull to the cells


at the other end of the scale ie FLATE - no cell should ever go below 2.5v if you go below 2v EVEN ONCE this BADLY damages the cell . AND I DO MEAN BADLY DAMAGES THE CELL maybe even making the cell usless

so yes alkways monitor the cell voltages, a bms is not the only controlling factor.


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