question

mfred68 avatar image
mfred68 asked

Should charging turn off at 100% soc?

I have a 42kwh lifepo4 battery bank 6x 7kwh each, and for the first time since using it, the batteries have reached 100%soc.

The monitoring is via the internal bms's

I am charging them with just my mppt and solar in order to balance them as each battery was originally a different soc.

So now they have reached 100%, why is it still charging?

Can the mppt be set to stop charging at 95%soc? As I understand that 100% is not good for the batteries, and surely charging after they have reached 100% is even worse.

Can anyone advise please? 1000039795.jpg

battery charging
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2 Answers
nickdb avatar image
nickdb answered ·

Because SOC is a calculated metric not a measurement, and it drifts on many batteries, so, while it says it is at 100%, it is in fact not and below the CVL, so it will keep charging until the battery decides it is actually properly charged and then adjusts the CVL/CCL.

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mfred68 answered ·

Thanks for the explanation. I have now found that the charging has stopped, so all 6 batteries must be at 100%.

Is it OK to charge them to 100% though, and can the mppt stop at 90% or so to prolong the usable battery life?

Or does this not affect lifepo4 cells?

I know too much dod does affect them, so I only discharge to 20%

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nickdb avatar image nickdb ♦♦ commented ·

There is no need to stop charging earlier. Commercial batteries have designed this into their lifespan and warranty calculations.

In most cases it is counter productive as it limits balancing which is only really effective at top of charge.

Most wear is related to time and temperature, not keeping the batteries at full charge.

There are good reasons for some batteries to not fully charge them, but if you did not build your own, and are using a battery designed for ESS usage (which pretty much all lithiums are), then the manufacturer has already done the work for you.

Just let the system do what it needs to do.

Since yours seems to drift a bit, not fully charging will only make it worse.

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niklas-schauberg avatar image niklas-schauberg nickdb ♦♦ commented ·

fully agree. additionally to what you have said I think you reduce battery stress most easily if you don‘t charge them higher than 3,45V and discharge not further than 3,0V per cell if chemistry is LiFePO4.

means:

45,0V - 51,75V for a 15s battery

or

48,0V - 55,2V for a 16s battery

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