On-grid domestic battery storage

Doobedoo

mmmmikey wrote: »

Err - hold the press - think I may have been talking nonsense when I wrote that last post. The effects I'm seeing around accuracy and background import could be explained by the frequency response / grid services functionality doing it's thing. Will check with Powervault later and post an update

Would love to hear more from your experiences Mikey as I’m very close to pulling the trigger on a 8.2kwh battery from Powervault.

mmmmikey

Doobedoo wrote: »

Would love to hear more from your experiences Mikey as I’m very close to pulling the trigger on a 8.2kwh battery from Powervault.

Hi,

All going well here with the PowerVault working as expected (now I've properly understood what I'm looking at).

Today has been a much better test, with the cooler day meaning more demand for electricity for heating and the sun going in and out with the April showers. I've had a low power convector heating on all day in the living room, powered directly by the solar panels when the sun's been out and the PowerVault when it rained. The PowerVault quickly recharged itself when the sun came back out, and this cycle repeated itself a few times during the day, leaving me with a full battery to cook my dinner and do some ironing this evening. ASHP is warming the bedroom now, ready for bed. I expect to have used just under 4kWh from the PowerVault by the end of the day. Saturday looks like being an even better test with less sun and more heating demand - I'm hoping to squeeze 6kWh or so out of it.

So all in all working well and although a week isn't enough to prove decisively that this is going to pay for itself, it's been enough to test various assumptions and my confidence that this is going to work well for me is growing daily. I've imported 2kWh of peak rate electricity during a week when I would have expected to import about 18kWh so a decent saving - keeping in mind that it's been an exceptionally sunny week so low demand has limited potential saving for me.

As far as the accuracy of the monitoring is concerned, it's actually only a few % adrift of what I'm seing on the generation and supply meters. The thing that confused me was looking at the Consumed figures which were about twice what I was expecting them to be. The penny has now dropped though. I was looking at this during a time of high solar generation (peaking at about 2.4kW for me) and low demand (about 0.1kW). The solar generation displayed by the PowerVault is about 4% out - i.e. about 01.kW at a power of 2.4kW. Because the Consumed figure is not measured directly but calculated from other measurements (e.g. when the battery is idle, any power not being used is being exported, so Consumed = Generated - Exported) then 0.1kW error in the solar generation = 0.1kW error in the Consumed figure, so actual consumption of 0.1kW could show as 0.2kW when you factor this in.

I hope this makes sense. The bottom line is that if you want accurate data for spreadsheet analysis, you'll need something like Zarch's emonpi, but if you just want to look at the screen and know what's going on it's easily up to the job. I really do like the simplicity of the display.

Assuming you've done all the sums, there's nothing I've seen that would make me think twice about pulling the trigger and placing the order


p.s. any specific questions, just ask....

zeupater

Hi

I've come across a number of articles recently regarding automated low charge remedial action, battery conditioning charge cycles and temperature pre-conditioning within various battery systems which create import requirements that reportedly don't appear as a defined technical value in specification or sales literature ...

Has anyone with grid connected domestic battery storage come across this happening and investigated the levels of energy required, especially so as this directly impacts efficiency?

HTH
Z

1961Nick

zeupater wrote: »

Hi

I've come across a number of articles recently regarding automated low charge remedial action, battery conditioning charge cycles and temperature pre-conditioning within various battery systems which create import requirements that reportedly don't appear as a defined technical value in specification or sales literature ...

Has anyone with grid connected domestic battery storage come across this happening and investigated the levels of energy required, especially so as this directly impacts efficiency?

HTH
Z

The only thing I've noticed out of the ordinary, is what looked to be battery charge balancing - 4 batteries were charging & one was discharging...lasted about 10 minutes.

I guess if the batteries are in standby for a long period then it's good that they maintain a minimum charge for longevity? I'm unlikely to see that scenario until the depths of next winter.

zeupater

1961Nick wrote: »

The only thing I've noticed out of the ordinary, is what looked to be battery charge balancing - 4 batteries were charging & one was discharging...lasted about 10 minutes.

I guess if the batteries are in standby for a long period then it's good that they maintain a minimum charge for longevity? I'm unlikely to see that scenario until the depths of next winter.

Hi

That sounds like it's within the normal management schedule and falls within the control of the battery system's internal management overhead, probably around 5W for most systems (and not within the charge/discharge efficiencies) ...

What I've come across is pre-conditioning thermal management of batteries on some systems (PW2 etc) which must have an efficiency impact, as well as charging from the mains when SOC in a fully PV environment falls a certain level below the minimum supply threshold, the example I came across being no generation for a number of days as a result of snow on the panels ...

In the case of the SOC, the battery depleted to minimum supply threshold, cut out and sat doing nothing for a number of days except monitoring & managing itself. This self consumption depleted the batteries to a point where the battery charged from the mains to avoid running flat, even though being set to run in PV mode ...

I've also come across reports that battery/PV combinations will 'randomly' charge to high levels from mains power after cycling at the lower end of their capacity for an extended period ... think winter when there is excess PV to charge, but not often & not much!

Additionally, I've also seen a number of mentions of 'external' system management system and data gateway combinations consuming 10W-15W in addition to the basic inverter/battery setup, for example, remote access into PW2 data having an overhead of ~11W, obviously another efficiency related consideration ...

HTH
Z

joefizz

zeupater wrote: »

What I've come across is pre-conditioning thermal management of batteries on some systems (PW2 etc) which must have an efficiency impact, as well as charging from the mains when SOC in a fully PV environment falls a certain level below the minimum supply threshold, the example I came across being no generation for a number of days as a result of snow on the panels ...

In the case of the SOC, the battery depleted to minimum supply threshold, cut out and sat doing nothing for a number of days except monitoring & managing itself. This self consumption depleted the batteries to a point where the battery charged from the mains to avoid running flat, even though being set to run in PV mode ...

I've also come across reports that battery/PV combinations will 'randomly' charge to high levels from mains power after cycling at the lower end of their capacity for an extended period ... think winter when there is excess PV to charge, but not often & not much!

The only time Ive ever recorded this was when I introduced the two new batteries (almost fully charged) to the system with the current batteries at 20% levels.
There was a brief period when it charged the lower level batteries from the mains and it appeared to be to a level where the 'normal' balancing could take place.
This was an undocumented 'feature' and I cycled the inverter a couple of times and it kept repeating the behaviour and although I didnt record the actual data it seemed to stop for me at around 20% difference. In the process of all the inverter cycling I had reduced the new battery levels down to about 60% by just using those and disconnecting the older batteries and then reconnecting the lot up.
It was only drawing power from the mains for about 20 mins or so in the end but again its not a 'feature' I found documented anywhere.
Ive been through empty to full cycles since and no repeat so put it down to real out of sync battery mismatch which shouldnt really occur under normal conditions for the setup in stasis, (sofar/pylontech) but which may occur later in the life cycle if theres a failure in one of the cells or just degradation of one of the batteries in general. I would suspect the ac coupled systems do this slightly differently, hence why I havent really noticed it (enough to put some charge in the batteries every day, only 3 days ever with no charge).



It wouldnt really surprise me if this was an undocumented feature of the tesla or the larger capacity battery systems as a lot of ups and other battery systems use trickle chargers and burst chargers to do exactly the same thing under marginal conditions (as well as load discharge if run at peak for days etc).
Id really want to know though if it just decided to draw from the mains itself...

1961Nick

zeupater wrote: »

Hi

That sounds like it's within the normal management schedule and falls within the control of the battery system's internal management overhead, probably around 5W for most systems (and not within the charge/discharge efficiencies) ...

What I've come across is pre-conditioning thermal management of batteries on some systems (PW2 etc) which must have an efficiency impact, as well as charging from the mains when SOC in a fully PV environment falls a certain level below the minimum supply threshold, the example I came across being no generation for a number of days as a result of snow on the panels ...

In the case of the SOC, the battery depleted to minimum supply threshold, cut out and sat doing nothing for a number of days except monitoring & managing itself. This self consumption depleted the batteries to a point where the battery charged from the mains to avoid running flat, even though being set to run in PV mode ...

I've also come across reports that battery/PV combinations will 'randomly' charge to high levels from mains power after cycling at the lower end of their capacity for an extended period ... think winter when there is excess PV to charge, but not often & not much!

Additionally, I've also seen a number of mentions of 'external' system management system and data gateway combinations consuming 10W-15W in addition to the basic inverter/battery setup, for example, remote access into PW2 data having an overhead of ~11W, obviously another efficiency related consideration ...

HTH
Z

Where this is likely to happen, it would be useful if manufacturers would include a section in the manual on what to expect from the BMS.

The monetary impact of an occasional grid charge is only the battery capacity x round trip loss x p/kWh...but, the psychological harm caused by seeing your battery bank charging from the grid, could be severe...if you don't know why,:D

The longest period without any grid import so far is 8 days. During that time the batteries didn't do anything untoward other than a few watts during the daily switch from charge to discharge as the solar runs down.

I'm waiting for the winter months to work out a strategy for the batteries. It may be advantageous to fully charge a couple & hibernate them. The 3 left in service would then have to work a bit harder...& I'd have 4kWh for the EPS.

Reed_Richards

zeupater wrote: »

In the case of the SOC, the battery depleted to minimum supply threshold, cut out and sat doing nothing for a number of days except monitoring & managing itself. This self consumption depleted the batteries to a point where the battery charged from the mains to avoid running flat, even though being set to run in PV mode ..

The day after it was installed, my battery SOC fell to 5% whereupon it charged itself up to 15% from the mains. Since the minimum SOC is normally 10% I don't think this has happened since.

Reed_Richards

My electricity usage has dropped to about 1.3 kWh per day since I had my PV + battery system installed at the beginning of March. I am wondering if I should switch my energy supplier to one that offers a zero standing charge tariff. What has everyone else done?

zeupater

Reed_Richards wrote: »

My electricity usage has dropped to about 1.3 kWh per day since I had my PV + battery system installed at the beginning of March. I am wondering if I should switch my energy supplier to one that offers a zero standing charge tariff. What has everyone else done?

Hi

... and you're asking because you're paying more in standing charge than electricity? ... what about gas? ... what about dual fuel discount etc? ...

Fire your estimated figures into a couple of comparison sites to get an idea, but be realistic in your consumption assessment ...

We average electricity imports of around 3.5kWh/day, with gas normally being slightly below this & it's by far the cheapest option for us!

HTH
Z