question

WildSilo avatar image

SOC reaches "hard limit" of 88% whilst charging at more than "C10" rate for an hour

Hi there

I have 4x3.5kWh Pylontech batteries, and they perform very well. This is more or less 280Ah @48V.

On a number of occasions, I have seen the SOC remain at 88% as per the Venus remote console, even when charging at 30-40A (28A is about the C10 rate). Charging at 28A should result in SOC moving up by about 10%/hour or 1% every 6 minutes (more or less).

But SOC never goes above 88% that day. I have observed reported battery volt go up from 50.8 to 50.9 to 51.2, but the SOC remains at 88%. It only ever gets "stuck" at this specific 88% SOC. IIRC 51.2 is about 95%.

Since the battery voltage does go up, I assume the batteries are charging (they are accepting the current). Is this perhaps an SOC algorithm/BMS issue?

I have not been able to test if they stay at 88% longer as well when they start to discharge at the end of the day.

ESSVenus GX - VGXbattery chargingSOCPylontech
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3 Answers
WildSilo avatar image
WildSilo answered ·

Thanks for the answer. Firmware is up to date (not sure about the batteries' firmware?) as of last week.

With the above info, I am not too worried.

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WildSilo avatar image
WildSilo answered ·

It seems I was right. The battery is accepting charge. A while after my post above, I took a screenshot of the control panel showing 88%/51.2V (bulk). By this time of the day, the charge current had already dropped to about 7% per hour.


8 minutes later, I took another screenshot showing 95%/51.9V (absorption).


So it seems the SOC sometimes just takes a while to catch up?


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Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

I don't have any firm information and haven't noticed this myself. I would think you are right though.

Assuming all is correctly connected, configured and up to date with firmware - I would suspect there is an internal process to the Pylontech battery that is doing more balancing or calibration than capacity charging for that time, and if current was maintained, you would see the SOC rise again.

As you say, the current has to go somewhere, and it could be that the battery BMS itself is surprised at the current / voltage relationship and holds on the SOC.

Perhaps next time you notice it, you could bring in a generator or additional charge source and see if you can watch what happens if current continues to be applied (after the end of the solar day).

This is a lot of speculation on my behalf, but unless you are seeing other symptoms of a problem, I would keep an eye on it, and not worry.


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