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Inverter power draw and battery discharge when on A/C bypass


SiliconKid
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Hi all

I'm new to all this and I'm still trying to understand a few specific technical things properly, so I'm hoping somebody can give me input here.

I've recently installed a 5Kva Growatt SPF5000 Inverter and 2 x Pylontech US3000 batteries. No solar involved, inverter is being used purely for UPS / Battery Backup and is on A/C for charging and auto switching to battery during load shedding.

Everything is hooked up and online now and after a bit of hassle with getting the inverter to connect to the BMS properly I've sorted that out too and the inverter is now reading values from the BMS over the comms cable and displaying percentage values instead of volts.

I've done a lot of research on all of this over the last 2 months and a lot of reading but I still can't find specific answers to certain questions.

One of those questions, is this: 

When the inverter is on A/C bypass (no lead shedding, batteries charging from A/C (grid)), does the inverter still use some power from the batteries and therefore cause the batteries to discharge?

I ask this because I am observing a slow discharge of the batteries from 99/100% down to around 94% SOC, over a period of around 6 hours, and then the inverter kicks the charger in and charges them back up to 99/100% (in about 25 minutes), and then it starts the slow discharge again. If you look at it on a graph it's a sawtooth pattern with peaks every 6 to 6.5 hours.

That would seem to indicate that despite the fact that the inverter is on A/C bypass and the load on the inverter is receiving power directly from the grid, and the batteries appear to be in an idle state (both power lights on the batteries blink slowly), something is still putting a small load on the batteries.

Is it the case that the inverter itself is always actually powered from the batteries? So it is always drawing current from the batteries to keep itself on? Even if the power being sent to the main load is on a/c bypass direct from the grid?

Are the batteries supposed to constantly slowly discharge like this even when the inverter is not in battery mode and supplying the main load from the batteries?

Or is there something else going on here? 

Any helpful input appreciated.

Allan

 

 

 

Edited by SiliconKid
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4 hours ago, SiliconKid said:

I've recently installed a 5Kva Growatt SPF5000 Inverter ...

These seem to me cloned Axperts, that then evolved on their own in slightly different directions to the Axperts.

Quote

When the inverter is on A/C bypass (no lead shedding, batteries charging from A/C (grid)), does the inverter still use some power from the batteries and therefore cause the batteries to discharge?

Essentially, yes. But it depends on exactly when the Growatts copied the Axperts. Very early Axperts had an AC power supply, so that if the utility was present, the utility would run the inverter electronics (some 35 W in a 5 kVA model, if the inverter proper isn't running, so that's processor, lots of electronics, LEDs, display, etc.). [ Edit: from about 2014 or 2015, this power supply was deleted, leading to the behaviour that you are seeing. The first Must clones started appearing late in 2015, so my guess is that the Growatt was copied from a later design without the AC power supply. ]

Quote

Are the batteries supposed to constantly slowly discharge like this even when the inverter is not in battery mode and supplying the main load from the batteries?

[ Edit: discharging like this is obviously not ideal. ] I believe that this is the reason for the 2 A utility charge option. The idle load is of the order of 1 A, so the 2 A of utility charging compensates for that, and even slightly chargers the battery. Of course, if you are utility charging at say 20 A, then the utility charger just pumps out say 21 A to make 20 A flow into the battery.

Does your Growatt have a maxmum utility charge current setting of 2 A?

Edited by Coulomb
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@Coulomb

Well that was one of the most useful and on-topic responses to a forum post I've received in a while, thanks for that.

This Growatt that I've got here has 2 settings that appear to be related to charge current:

  • Setting 2: Maximum total charging current - This is to limit the total charging current if both Solar and Utility are active and allowed to charge.
  • Setting 11: Maximum utility charging current.

Both of those settings were configurable before I managed to get the inverter to talk to the Pylontech BMS properly, but once proper comms was established both of those settings have been locked and now show as 10A, which I believe is simply the lowest value it can display.

I checked with Growatt themselves on this and the feedback from them was: Yes, it should be locked now, the inverter will decide what's best based on input from the BMS.

Any reference to charging current in this Growatt manual also refers to a minimum settings of 10A, not 2A.

I don't have a clamp meter so I can't verify what the current to the batteries is during idle or while on bypass, at the moment, but I'd love to verify the numbers.

I've just prepped a Raspberry Pi with ICC and I sourced a Pylontech cable for it, so I should have more detailed information via the Pi soon.

Thanks for the input.

 

 

 

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Just to follow up on this.

We had load shedding earlier and I was working from home, so monitored the behaviour.

Before LS I double checked the charging current values and they were both 16A (not 10A as they are when the system is at full idle, so the inverter was obviously sucking a little extra (fans maybe?).

Then after LS, when it started charging again, I went back to the inverter and took a look on the status screens again and observed that the charging current was now showing as 30A, fluctuating slightly up to 32A and back down to 30A, so hovering around 30A.

So this is good news from my perspective because although those fields are locked and no longer editable, they ARE actually being used to show dynamic values based on actual behaviour, live.

They haven't just become dead fields that show 10A because that's the lowest figure they can show, they are actually being updated in real time as per the actual charging current being delivered to the batteries at any point in time.

I'm very pleased about that.

Edited by SiliconKid
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Hi Siliconkid,

Your instructions were awesome on BMS configuration - thanks for the detail it saved a lot of time. 
 

I am having an issue with the same system not charging when Eskom is powering. also, batteries discharging with “analog” SOC showing drop but inverter showing 100%. 
 

Is there a setting that I am missing that The Growatt is assuming full charge so therefore doesn’t kick charging in?

 

thanks for your help. 

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  • 3 months later...

Hi,

I have Install 2x Growatt Inverters in Parallal. The Battaries is fully Charged. They are on Bypass Mode and will stay on Bypass Mode until tomorrow morning until PV is kicking in.  I installed to 3x Shoto 4.8 KW battries.  Why???

The purpose was to use the Battery as 1st and Eskom as Backup, but now Eskom is running the show... and not the Batteries. Please help??

 

 

 

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7 hours ago, Phillip du Bruyn said:

They are on Bypass Mode and will stay on Bypass Mode until tomorrow morning until PV is kicking in.

Sounds like you're using what would be SOL output priority mode in an Axpert. I just checked a Growatt manual, setting 01 is the same as the Axpert. Try setting 01 at SbU.

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  • 3 months later...
On 2020/02/19 at 10:17 PM, SiliconKid said:

Hi all

I'm new to all this and I'm still trying to understand a few specific technical things properly, so I'm hoping somebody can give me input here.

I've recently installed a 5Kva Growatt SPF5000 Inverter and 2 x Pylontech US3000 batteries. No solar involved, inverter is being used purely for UPS / Battery Backup and is on A/C for charging and auto switching to battery during load shedding.

Everything is hooked up and online now and after a bit of hassle with getting the inverter to connect to the BMS properly I've sorted that out too and the inverter is now reading values from the BMS over the comms cable and displaying percentage values instead of volts.

I've done a lot of research on all of this over the last 2 months and a lot of reading but I still can't find specific answers to certain questions.

One of those questions, is this: 

When the inverter is on A/C bypass (no lead shedding, batteries charging from A/C (grid)), does the inverter still use some power from the batteries and therefore cause the batteries to discharge?

I ask this because I am observing a slow discharge of the batteries from 99/100% down to around 94% SOC, over a period of around 6 hours, and then the inverter kicks the charger in and charges them back up to 99/100% (in about 25 minutes), and then it starts the slow discharge again. If you look at it on a graph it's a sawtooth pattern with peaks every 6 to 6.5 hours.

 

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It's been discussed at length but it seems that Li-Ion batteries don't need a float charge like the older types do from what I've seen mentioned in these forums. 

 

The 100% down to 95% and then back up, in a constant cycle, is by design according to Growatt, so it's not considered a problem. 

If Solar is involved the Solar will get cut off at 100% and the inverter will again wait until the SOC drops to around 95% before turning on the Solar input again. And again, that is by design according to Growatt, and lots of discussion about that on these forums too. 

Long story short, the Growatt is good for pure battery backup (no solar) and serviceable and ok for Solar, but not great it would seem. 

When I add Solar I will probably sell my Growatt and buy a more expensive Sunsynk. 

 

 

 

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  • 5 months later...
  • 2 months later...

@SiliconKid Found this post while looking around for battery discharge on AC bypass, seem to be following in your footsteps :) 

I know about the 100% to 95%, but do you still have the issue of a slow drain on the batteries while in AC bypass? I've picked up a 0.8amp drain using battery view.

Edited by Rooney Hat
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  • 2 months later...
On 2020/02/20 at 8:01 AM, SiliconKid said:

@Coulomb

Well that was one of the most useful and on-topic responses to a forum post I've received in a while, thanks for that.

This Growatt that I've got here has 2 settings that appear to be related to charge current:

  • Setting 2: Maximum total charging current - This is to limit the total charging current if both Solar and Utility are active and allowed to charge.
  • Setting 11: Maximum utility charging current.

Both of those settings were configurable before I managed to get the inverter to talk to the Pylontech BMS properly, but once proper comms was established both of those settings have been locked and now show as 10A, which I believe is simply the lowest value it can display.

I checked with Growatt themselves on this and the feedback from them was: Yes, it should be locked now, the inverter will decide what's best based on input from the BMS.

Any reference to charging current in this Growatt manual also refers to a minimum settings of 10A, not 2A.

I don't have a clamp meter so I can't verify what the current to the batteries is during idle or while on bypass, at the moment, but I'd love to verify the numbers.

I've just prepped a Raspberry Pi with ICC and I sourced a Pylontech cable for it, so I should have more detailed information via the Pi soon.

Thanks for the input.

 

 

 

@SiliconKid

Thanks for sharing. I have the same challenge. I use SBU setting and my back to AC/Eskom is set at 20%. Everyday when I wake up my battery is down to 10%. Clearly the inverter is draining my battery. 

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