FoxESS Community - Owners & Installers Forum

I had a new EP12 installed recently, and everything has been working really well - heating, charging from grid and solar, etc. Until last night, when the SoC dropped to 7% and the battery failed to charge from the grid.

I’ve had the SoC and Reserve capacity set at the recommended 10%, but have noticed the SoC occasionally dropping to 9% and sometimes 8% before the grid charge between 2.00am and 5.00am. But it’s never dropped as low as 7% before.

When I checked it at 7.00 am this morning its reported status was 12% and charging (despite being outside of the off-peak period). It has remained on 12% since then, despite it being on charge. (PS: Update as of 08.50am. Now on 18%. It appears to be charging okay, albeit very slowly. Battery temp now 14.9C.)

It isn’t perishingly cold on the UK south coast, although temps did drop to 3C last night (bit of frost this morning), and besides, I have the heating set to come on to bring the battery up to 14C two hours before the grid charge, which has been working perfectly.

The inverter status is normal, battery SoH is 100%, and I can’t find any error reports (though I’m new to this and not too familiar with the software yet).

I have now set the reserve capacity and minimum SoC to 15% as a safeguard, though Fox do make a big deal about the 90% discharge capability, so it's not so good if this isn't feasible in the real world.

I will of course contact the installers about this, but it would be brilliant to glean an idea of what might have been going on, as I’m keen to learn as much as I can about this new technology in my life.

Can anyone help?
Thanks.

There are a couple of effects here, firstly the SoC is calculated by the battery as a best guess of it's actual SoC and when new or out of balance the SoC may jump, make corrections or drop lower than you have set minSoC to - it's not a fault, just the way it works.

Once the battery has had a few full charge cycles 10% > 100% and back again, the battery master (BMS) will have much better accuracy.

The next point is around temperature, essentially the battery wants to be at 25C, that is it's rated capacity - anything lower it will slowly reduce it's capacity but more importantly it will start to throttle it's maximum charge and discharge power - this can end up with a battery that won't charge (the first effect), and eventually a battery that won't discharge (as it reaches 0C).
The throttling is very minor to start and you wouldn't likely notice it, but as it passes through 15C it will start to become more noticeable and below 10C very noticeable.

If the EP12 has a heater (it would have a 'Warm' sticker), you definitely should use it when it is cold, the small amount of power it uses to stay warm keeps the battery operational - on older batteries many people have insulated enclosures to reduce the effects of cold.

The final thing to say is when your battery reaches minSoC it will occasionally take a small amount of power to charge itself so that it remains above minSoC. The battery is always being used to keep the inverter from going into standby, it's only a handful of watts but it slowly discharges the battery and so it needs the charge cycles to keep itself above minsoc (That's why it was charging when you looked).
It's better to charge the battery overnight on a low tariff if you have one, as certainly at this time of year there isn't enough solar to charge them and it avoids you taking power at the higher tariff.

There is a link here to a video that Will has done that explains temperature effects https://youtu.be/nDAw0KAYvcE?si=1foywrbZiTK3vx0n

And a video guide here around battery heating https://youtu.be/_9hNCI1bTP4?si=FLT5Hq-YA5rv20jJ

Many thanks for that, Dave – much appreciated.
I had heard of ‘throttling’, but your detailed explanation makes it easier to understand, as does your explanation of why the battery was in charge mode outside of the set time period this morning.
My EP12 does have a heater, and this has worked perfectly since installation. As said, everything has worked perfectly – until last night, when the battery failed to charge until I put it into force charge around 8.00am this morning.
Most strange!
Everything seems to be working as it should now, so I shall see what tonight brings.
Thanks again.

Perhaps you can share a picture of your mode scheduler setup, that might show something?
Also, verify on the front screen of inverter that date/time are spot on also.

Thanks.

I'll try to do that later when I get back from work.