FoxESS Community - Owners & Installers Forum

Hi!

I have a couple of questions about a new installation. I have a KH7 with an EP11 battery, 7kWp of PV.

Over the last couple of days, I've been watching the battery behaviour particularly when at minimum SoC (which I've left as 10%).

First observation is that there's a sort of SoC oscillation where (despite zero charging taking place) the SoC recovers from 10% up to 14-15% and then discharges again. Please see my attached screenshot. Is this normal? Is the BMS algorithm just "settling" a little?

Second observation is that the system has a "Generation Meter" (OB112) connected to the RS485-Meter port. This meter is at the isolator between the PV system and grid. With zero PV, and the battery at minimum SoC (so the system should be doing nothing) I see ~300W at this meter. The inverter HMI agrees with this, displaying (eg) "Power: -324W" on the screen. The grid CT is working fine, as the load displayed in the Fox Cloud App aligns with that measured by my smart meter.

I should note I do not think the battery has had a chance to charge to 100% yet. Is that advised?

Firmware versions:

Version_BMS (EP11) - 1.007
Version_Master - 1.41
Version_Slave - 1.01
Version_ARM - 1.40

Behaviour on "good" days is quite normal - charging and discharging work as expected.

Thanks!

Hi,

Your inverter maintains itself by drawing power from the DC circuits i.e. PV/Battery there is a constant trickle of power to maintain inverter / BMS and batteries, the scale of this usually relates to the number of phases and size of the inverter but a typical inverter will take around 100 watts to maintain itself.

As the battery powers the house it will fall towards minSoC and the battery discharge will include the 'parasitical' loss mentioned above, however when the battery reaches minSoC the inverter will initiate a maintenance charge to stop the drain pushing the battery below minSoC.
The maintenance charge is at 5A DC (so with EP11 that's approx 1.7kW) and you will see the battery charge back above minSoC usually by a few % and then it will cycle like that until you either charge the battery from grid or solar.

The battery performs some balancing at the bottom and so a battery that is new needs a few minSoC -> 100% cycles before the BMS has trained itself on the actual limits of the battery, if you haven't done a 100% charge yet you may see SoC jumps and pauses in the charging / discharging where the BMS is balancing the cells within the battery pack.

On the 100% battery charge, yes this is advised to be done at least once a month for the cell balancing reasons I mentioned, if you don't the battery SoC accuracy will get worse over time and it may not provide the full capacity you expect and you'll see jumps in SoC as it re-assesses the inter cell voltage balance.

If you have a low tariff it is always advisable at this time of year (assuming you are UK?) when solar isn't as strong to make sure that the battery is charged occasionally from the grid using a low tariff, that ensures you don't pay the higher tariff and avoids the battery maintenance charging when it reaches minSoC which may be at the higher tariff.

On the 300 watts, that seems high for an inverter of this size, i'd normally expect ~ 100-150 watts - do you have the EP11 battery heaters enabled ? - what does your grid usage over time look like?, does the 300 watts instantaneous translate into 300 watt/hours - the inverter is responding to house load which could be changing quickly (as is the case from infra red heaters, induction hobs, some fridges etc..) and what you measure over time may be much lower.

There is always slight bias from grid when the inverter is in self use workmode and no solar to force the bias in the opposite direction (export). There is however an (installer only) option that allows you to set a bias point for the grid (called grid compensation) - setting this can reduce the amount of power the inverter draws from the grid and allows you to get to as near a minimum bias point as your normal house loads will allow but less than 1kWh should be possible.

Your firmware versions for inverter and battery are a little behind, it's usually advisable to get updated shortly after install as this will likely be to correct various bugs discovered in the field - there is a firmware version list here viewtopic.php?t=1927.

Unless you have installer access rights to your system, you won't be able to update it your self, but your installer can update you remotely, as can Fox service - get in touch and ask for your inverter and battery versions to be updated.

Welcome, I have same system as yours, except 6.2kWp of PV on the roof.

As mentioned, it will take a few cycles to calibrate your battery bottom/top end points.
Ideally you should at this time of year, have the battery charge overnight on a cheap tariff, that would get it to hit 100% and you drain this down during the day, and you can maximize your export.

I use for example Octopus Go to fill battery each night, and if there is some sunny weather, the exported energy is better for ROI calculations.


This is assuming you are in UK.

Thank you both for the replies - very helpful.

That's interesting, and makes total sense. I have a clamp meter and will take some current measurements to confirm that here. The observations I made were during a period with no activity in the house (obviously the occasional fridge compressor starting etc). I'm quite certain the 300W was constant, but it does seem to track with the charging/discharging cycle I described.

I will reach out to my installer and ask that they upgrade the inverter & battery firmware to the latest builds too.

I'm in the process of arranging a change to a time-of-use tariff so regular nightly charging should be possible soon.

Thanks again,
Ret.