dognose avatar image
dognose asked

DC-Coupled PV, Charge Limit


i'm currently doing some reading for an eventual system upgrade. I'm currently using some Fronius inverters with a total output capacity of ~ 16kW. So, when reading about ESS, i noted that pure AC-coupling is possible, but may lead to a stuck situation.

Hence I'd move some of the PV Strings to a dc-coupled mppt, maybe all in the long run. (Also Fronius won't live forever)

So, in the documentation about DC-coupled PV I read two different statements about the charge-current limit, which is confusing me:

Please note that when enabling this option, the DVCC charge current limit configured under Settings → Limit charge current won't be active. The solar charger will operate at full power for maximum feed-in into the grid. It's advisable to configure a safe limit on the solar chargers when used with a small battery bank.


Feed-in will only occur if there is sufficient surplus PV production to fully supply the loads, and the battery is charged (or at it's charge current limit).

So, how exactly is that fitting together now?

Assuming, The batteries recommended charge is 70A, i would (naturally) set the limit to 70A, even if it is capable of 150A peak.

What will now happen, if all DC-coupled MPPTs deliver, say 100A in Total?
- Will the charge Limit of 70A beeing ignored, and the battery feed with 100A, according to statement 1?
- Will the inverters take 30A and feed the grid - according to statement two, >>or at it's charge current limit<<?

Or does the additional feedin only happen, when the battery hit's it's physical (peak-)charge limit?

This also leads to a second question: If part of the PV Power stays AC-coupled, how does this work for Current Charge limits?

Will it be:
- MPPTs operating at their maximum ALWAYS.
- Inverter using their configured limit to provide UPTO for example 20A additional.
- Will the charge limit take both into account and adjust the inverts DC current accordingly? (i.e. if MPPTs delivering 70A, will the inverters top up 70A as well, because that is "the limit", or will ESS consider that the desired Limit of 70A is already reached through the MPPTs and not transform any of the AC Power to additional DC Current?

ESSAC PV Couplingdc-dc charger
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1 Answer
Alexandra avatar image
Alexandra answered ·


CCL is ignored when feedback is switched on. So if the battery is full technically it will be sending 0CCL to the system. It will be ignored. The system will produce.

You can charge the battery and feedback at the same time, the clue is in the word excess. (Extra to local needs). In which case it will try to obey the CCL to the battery. I will note though that if you have an undersized bank or an unsupported bms it does not always work well.

Yes you can limit feed in and it will limit production through throttling. Solar is used like a grid, theoretically megawatts are available but you only switch on a 3W bulb, that is all you draw. it powers the demand when programmed correctly.

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