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marv2097 avatar image
marv2097 asked

ESS Charging state misleads the SOC value

Hi,

My SOC increases even while the battery is discharging.


I have noticed that when using ESS the Inverter is always showing charging in the VBUS state similar to this post: https://community.victronenergy.com/questions/32283/ess-ve-bus-state-charging-behaviour.html

I have scheduled charge at 00:30 for 4 hours, so at 4:30 the MPII starts inverting to supply the house load. However the Charger side of things seems to continue the Bulk-Absorb-Float cycle. This causes the SOC to jump to 100% even though its been discharging for nearly an a hour. I suspect this is due to the 1 hour Absorption timer.

Is this expected operation? It doesn't always happen and is not consistent and causes different depths of discharge.


This can be seen in the graphs below with the jump in SOC happening at approx 05:30.


Many Thanks


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1 Answer
Alexandra avatar image
Alexandra answered ·

@marv2097

You will only be in inverting mode when you have no grid connected. You can check this by disconnecting your multi from power.

I can see though at your end of scheduled charge window the batteries are still drawing a heavy current suggesting that they need way more charge than you are giving them. Do you have solar to supplement the charge?

Basically what seems to be happening here is State of Charge Drift.

What batteries do you have? How is your battery SOC being measured?

If it is VE bus then you need to keep them fully charged for at least a day to sync. if it is the Victron BMV then you also need to make sure they have a good charge. If it is measuring its own SOC and reporting tot he GX device then it may also have had a SOC drift.

The different charge stages are usually triggered by the target voltages reached that you have set. But if there has been SOC drift then the graph will be inaccurate.

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marv2097 avatar image marv2097 commented ·

Hi @Alexandra

Thanks for your reply. The batteries are a DIY Nissan leaf pack which consists of 14S 6P. With their current health they are configured as approx 52.5v and 260Ah in the multi.

At the moment there is no solar, they just charge as much as they can in the off-peak 4 hour window and then discharge through the day until they reach 10% SOC. At 70amps they get to roughly 85-90% most nights.

All the SOC calculation is done by the multi and no BMV as I have no other DC sources of power or loads.

From what I can tell if the charger is still in the Bulk phase at the end of the scheduled charge window then its fine, the SOC decreases as expected. However if it makes it to Absorption stage then we have the same problem. Batteries start discharging as expected but regardless of this the SOC will always be set to 100% 1 hour after the start of Absorption even though it hasn't been actually charging during this hour.

If the batteries are being discharged then why would the multi think its correct to sync to 100% as we cant go from Absorption to Float if not actually charging?

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Alexandra avatar image Alexandra ♦ marv2097 commented ·

@marv2097

What bms are you using on the DIY pack?

What you may have to do (and I know it will cost more) is make your pack fully charge by letting it sit on charge untill it is no longer drawing the huge amps.

I hear what you are saying with the incorrect soc reporting but letting it get up to the full soc and sit for a few hours like that should solve the issue you are having.

And then I would recommend getting a shunt based system like the bmv for your DIY pack it works better for partial charge and discharge cycles like you are doing.

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