Can LifePO4(Zooms) battery be damaged from floating at 57.5V?

Riley

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For the USE2 setting from Growatt to solve a charging issue, they bound floating and bulk to the same voltage. I found if I set my bulk charge voltage at 54V, it stops charging after 54V and will not come back until it drops to 52.3V, and the voltage would drop to 53V in a few minutes after charging is stopped. Which means I can't top up the last 10% of the battery.

What if I set them to 58V and limit the charging amp to 2A to make it float at somewhere around 57V until sunsit? Would that be bad for the battery?
 

pellicle

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I'm not an expert here but LiFePO4 doesn't really benefit from a "float" (or even have such) as does Lead Acid tech.

As far as I know leaving the cells at higher charge levels reduces their capacity over time. Instead its better to keep them lower. However this (again as far as I know) results in cells being "out of balance" and there being problems with ensuring that when charging the BMS can actually fully charge each cell without 'over charge' of one cell bundle triggering BMS declare charging is over.

So for maximum longevity of the cells (as far as I know) you want to keep them at about 80% capacity and not take them really deep into discharge.
 

JoeHam

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Yes, that would be bad for longevity.

There is very little useable capacity that high in the charge curve.
 

Riley

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It turns out I can't hold it to 54V even I am floating it at 57V...
every time after sunset, it just drops to 53.1 to 53.3V right away.
 

time2roll

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It turns out I can't hold it to 54V even I am floating it at 57V...
every time after sunset, it just drops to 53.1 to 53.3V right away.
See if charging to 56 and float 55 still drops to the same ~53.2
I think you are virtually full.
 

RCinFLA

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3.43v cell voltage for LFP is the magic voltage. Any float voltage below this will have zero cell current (well maybe very small cell leakage current).

It takes 10-30 mV above or below 3.43v before any (mA's) of current starts to flows through cell. This is over-potential voltage required to push lithium ions to move through cell required for current to flow. So fully charged is really about 3.45v of no-load rested voltage.

You do not want continuous charge current to be imposed on a LFP cell. This will happen if cell if floated at greater than 3.45v. When you have continuous charge current, even a little, the SEI protective layer will continue to grow. This is a slow process, but it consumes some available lithium that will no longer contribute to cell capacity. It also increases cell impedance over time as the SEI layer gets thicker. Both these accelerate aging of cell.

When you put multiple cells in series, there will always be some variations between cells so 16x 3.45v = 55.2v overall charger float will have some cells above 3.45v. To provide some tolerance margin you should not continuously float above 16x 3.40v = 54.4 vdc. 53.8-54.0v is even better, giving more margin for cell matching variance.

A different problem that needs to be addressed is BMS balancing. If you continuously float at 3.40v per cell or below (54.0v) you will have very little balancing by BMS since most BMS's do not balance until a cell gets above 3.4v. So periodically you need to charge battery up to about 3.50-3.55v per cell (56.0v to 56.8v) for a few hours to give a chance for BMS balancer to operate. It should be done at least every month or two and soaked at 16x 3.5v = 56.0v for a few hours. You need to keep up with proper balancing or you will pay the price later when BMS keeps shutting down for over or under cell voltage with a lot of effort required to fix the large cell imbalance that grew over time.
 

Bud Martin

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OP, so the Voltage drops down to 53V with loads connected?
Is that Voltage measured right at the battery terminals or at the Load ends.
 

Riley

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OP, so the Voltage drops down to 53V with loads connected?
Is that Voltage measured right at the battery terminals or at the Load ends.
With about 100watts load, for Growatt and my laptop.

I just read voltage on the screen from Growatt.
 

robby

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3.43v cell voltage for LFP is the magic voltage. Any float voltage below this will have zero cell current (well maybe very small cell leakage current).

It takes 10-30 mV above or below 3.43v before any (mA's) of current starts to flows through cell. This is over-potential voltage required to push lithium ions to move through cell required for current to flow. So fully charged is really about 3.45v of no-load rested voltage.

You do not want continuous charge current to be imposed on a LFP cell. This will happen if cell if floated at greater than 3.45v. When you have continuous charge current, even a little, the SEI protective layer will continue to grow. This is a slow process, but it consumes some available lithium that will no longer contribute to cell capacity. It also increases cell impedance over time as the SEI layer gets thicker. Both these accelerate aging of cell.

When you put multiple cells in series, there will always be some variations between cells so 16x 3.45v = 55.2v overall charger float will have some cells above 3.45v. To provide some tolerance margin you should not continuously float above 16x 3.40v = 54.4 vdc. 53.8-54.0v is even better, giving more margin for cell matching variance.

A different problem that needs to be addressed is BMS balancing. If you continuously float at 3.40v per cell or below (54.0v) you will have very little balancing by BMS since most BMS's do not balance until a cell gets above 3.4v. So periodically you need to charge battery up to about 3.50-3.55v per cell (56.0v to 56.8v) for a few hours to give a chance for BMS balancer to operate. It should be done at least every month or two and soaked at 16x 3.5v = 56.0v for a few hours. You need to keep up with proper balancing or you will pay the price later when BMS keeps shutting down for over or under cell voltage with a lot of effort required to fix the large cell imbalance that grew over time.
Spot on Post IMHO.
I recently started to use the Equalization feature in the battery. I set it to go on every 14 days for one hour at 55V to allow the Inverter to recalibrate the SOC. Since I started doing that there is zero almost zero drift in the SOC vs Voltage Match. I just wish I could go back to being on closed Loop but I need the port for Solar Assistant.
 

pellicle

retired ex-professional dingbat
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It turns out I can't hold it to 54V even I am floating it at 57V...
every time after sunset, it just drops to 53.1 to 53.3V right away.
again, as far as I know, the BMS protects the battery by shutting off the charge to the battery (so you aren't usually seeing the actual voltage of the battery on the other side of the BMS, but most likely the charge voltage.

When that goes away I understand that the BMS opens the gate and when even the smallest of load is applied the battery will be seen for the voltage that it is. Its a 13s configuration isn't it? If it is that means the BMS is shutting off charge at 4.1v per cell (a good amount)
PS the above was relating to Lithium, but just redo the maths for FePO4
 
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