ESS why does it use battery percentage SoC to determine charge stop point instead of voltage/current

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djdemond asked Nov 7, '22

ESS why does it use battery percentage SoC to determine charge stop point instead of voltage/current

I understand why using voltage to measure the state of a battery under load can be unreliable, since voltage drop is expected.

However, when charging under ESS I have an installation where charging stopped at 95% as programmed but this was actually a lower voltage, well below fully charged. This continued to get out of shape over a few days.

Why does ESS use SoC instead of voltage to determine when scheduled charging should stop? Whilst charging the absorption voltage is gradually reached in CC mode, then it switches to CV mode and the current gradually reduces as the battery reaches the voltage set.

So voltage would be a perfectly suitable means to determine when to stop scheduled charging. It would also be more reliable, as it would always reach a full charge, then set SoC indicator and then begin to discharge.

battery charging ESS SOC

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Answer 1 · 2022-11-07T17:36:35Z

wkirby answered · Nov 07, 2022 at 05:36 PM

Absorb Voltage is used at the extreme end of the Voltage curve where is is much steeper and more is a reliable way to determine that the battery is full. It is also useful to linger at this Voltage for a while to allow the individual cells to balance.

In the middle between 20% and 80% the Voltage is very flat and it very difficult to tell the state of charge from the Voltage, even at rest. This is why SOC is more reliable than Voltage is for this purpose. Using these ESS set points is intended to get a certain amount of energy during a cheap tariff for example, not for fully charging the battery and balancing the cells. SOC does drift and that is why things go out of shape over the days. A 100% charge is required every so often to balance the cells and to reset the 100% SOC point.

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