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Victron inverter startup current tripping BMS overcurrent protection


Surge
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At some point in the near future I'd like to set up a hybrid grid connected system with minimal battery backup.

I want to use minimal battery for purely backup/UPS to keep the costs down but what concerns me is that it seems that some hybrid inverters like Victron have a very high start up current which trips the BMS overcurrent protection on Pylontech's (and probably other lithium packs too). The Victron forums are scattered with comments like one needs a minimum of 3 x Pylontech US3000 batteries for a Multiplus II 48/5000 system in order to not trip the BMS overcurrent protection mechanism. That prevents the idea of having minimal battery capacity for lighting, TV, alarm, etc. and makes the system very expensive. Victron even have a page describing the requirements. https://www.victronenergy.com/live/battery_compatibility:pylontech_phantom

The Multiplus II 48/3000 is too small for my needs so downsizing to a smaller inverter in order to have less batteries is not an option. It seems to be a rather silly restriction in my opinion. Why can't Victron build in a "slow-start" mechanism to limit the start up current of their inverters? As this is only a startup issue it seems that if one can pre-charge the capacitors in the Victron before switching it on it would solve the problem. In fact I just stumbled upon such setups. https://community.victronenergy.com/questions/38299/phoenix-inverter-2.html

What I'm curious about is why I don't see this issue mentioned in most of the DIY builds or even the kits being sold locally. How do other people get around it? Do other hybrid inverters have the same issues or does everyone build their own pre-charge circuits? I must be missing something.

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Are you sure the surge current that trips the BMS isn't just when you connect the inverter to the battery for the first time? The inverter will have a fairly large DC bus capacitance which will cause a big current spike when you connect it to the battery. Turning pylontech batteries on with the Inverter connected will also cause a trip no-matter how many batteries you have because the Master battery turns on first and will trip before the others have started. The solution to this problem is to add a pre-charge circuit. The simplest way is to use a removable fuse between the battery and the Inverter (you must have a fuse anyway, so why not use a cartridge fuse or similar fuse that is easy to remove and insert) and then wire an normally-open momentary contact push button in series with a resistor (I would say 47 Ohms, 20W or so should be ok) across the fuse holder. Then to start the inverter, begin with the fuse open, turn the battery on, then press and hold the push button for 10 seconds or so before closing the fuse. This will eliminate most of the current surge and prevent tripping the BMS.

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35 minutes ago, Bloubul7 said:

Why not look at the SunSynk inverter range?  They offer excellent value for money along with great features.

What he said. 

The BMS of the pylontech batteries manages the charge and discharge current so even if you only have one pylontech battery on the 8kw sunsynk inverter it will not charge or discharge more than the limit of the BMS irrespective what the inverter charge and discharge settings are. 

I have 3 US3000 pylontech batteries connected to my 8kw sunsynk inverter and it cannot charge or discharge more than 111A (37A*3)

Screenshot below from the inverter. The charge current limit will drop to 22A when the battery is at 88% and then down to 11A as it gets closer to 100% SOC. Yesterday's changes to charge current limit as the SOC increased. 

Screenshot_20201028-092340_solarmanPro.jpg

Screenshot_20201028-092554_solarmanPro.jpg

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3 hours ago, Stanley said:

Are you sure the surge current that trips the BMS isn't just when you connect the inverter to the battery for the first time? The inverter will have a fairly large DC bus capacitance which will cause a big current spike when you connect it to the battery. Turning pylontech batteries on with the Inverter connected will also cause a trip no-matter how many batteries you have because the Master battery turns on first and will trip before the others have started. The solution to this problem is to add a pre-charge circuit. The simplest way is to use a removable fuse between the battery and the Inverter (you must have a fuse anyway, so why not use a cartridge fuse or similar fuse that is easy to remove and insert) and then wire an normally-open momentary contact push button in series with a resistor (I would say 47 Ohms, 20W or so should be ok) across the fuse holder. Then to start the inverter, begin with the fuse open, turn the battery on, then press and hold the push button for 10 seconds or so before closing the fuse. This will eliminate most of the current surge and prevent tripping the BMS.

Yes, that's exactly what I mean. The once off inverter startup current trips the BMS (something like 200A surge when switching on a Multiplus II 48/5000).

So the "normal" solution is to build a pre-charge circuit? I've just never seen this mentioned in any kits being sold. Thanks for the good idea. Seems like a very affordable way to solve the issue.

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I have been using this type of pre-charge for years. It is cheap and works really well. The nice part is that you only need to use it very rarely. After the initial startup when you install the inverter, the only time you would need to pre-charge it again is if you had to completely shut the system down for some reason and disconnect the battery, so it isn't worthwhile trying to find a fancy solution. (A fancy solution would be something like a contactor with the pre-charge resistor wired across it, and a circuit to close the contactor when the voltage on the inverter side of the contactor is high enough, but this could be very expensive since the contactor would need to be able to carry the full DC current, which could be hundreds of amps depending on the inverter size.)

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23 hours ago, Bloubul7 said:

Why not look at the SunSynk inverter range?  They offer excellent value for money along with great features.

It's not really a Victron issue. I suspect that other brands of inverters would do the same to Pylontech BMS' on initial startup.

Thanks for the recommendation about Sunsynk though. They look like really good value for money. Unfortunately finding a solar installer who supplies Sunsynk seems to be a problem - they all seem to be selling Deye kits which appear to be a watered down Sunsynk (from what I've read on Power Forum).

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2 hours ago, Surge said:

they all seem to be selling Deye kits which appear to be a watered down Sunsynk (from what I've read on Power Forum).

The Deye and Sunsynk are the same hardware. (Watch the SunSynk factory tour video, and you may notice that the factory is branded Deye.)

That said, the Sunsynk UI is definitely better, although, as far as I can tell, actual functionality is the same.

Local support for both Sunsynk and Deye are handled by the same company.

Second line support is where the real difference is. Queries to Sunsynk international are generally answered directly by Mike, and usually within a day.  Queries to Deye international go straight to China, and although they do respond, a lot is lost in translation.

That said, the local agents have now got very good first line support, so you should seldom need to go to the international support contacts. As long as you get the WiFi logger, the Chinese engineers will generally log in within an hour or two, and sort out any configuration issues, or let you know if there are external or hardware issues.

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22 minutes ago, JustinSchoeman said:

The Deye and Sunsynk are the same hardware. (Watch the SunSynk factory tour video, and you may notice that the factory is branded Deye.)

That said, the Sunsynk UI is definitely better, although, as far as I can tell, actual functionality is the same.

Local support for both Sunsynk and Deye are handled by the same company.

Second line support is where the real difference is. Queries to Sunsynk international are generally answered directly by Mike, and usually within a day.  Queries to Deye international go straight to China, and although they do respond, a lot is lost in translation.

That said, the local agents have now got very good first line support, so you should seldom need to go to the international support contacts. As long as you get the WiFi logger, the Chinese engineers will generally log in within an hour or two, and sort out any configuration issues, or let you know if there are external or hardware issues.

Isn't there a difference between Deye and Sunsynk when it comes to contacts for an earth-neutral bonding relay with the Sunsynk having the feature built in? I recall reading something like that somewhere.

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3 minutes ago, Surge said:

Isn't there a difference between Deye and Sunsynk when it comes to contacts for an earth-neutral bonding relay with the Sunsynk having the feature built in? I recall reading something like that somewhere.

The UI option was not there. When I asked for it, they updated the firmware and pushed it to my inverter in 3 days.  That was still a fairly new feature on the SunSynk firmware, and they had not enabled it in the Deye firmware yet.

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5 minutes ago, JustinSchoeman said:

The UI option was not there. When I asked for it, they updated the firmware and pushed it to my inverter in 3 days.  That was still a fairly new feature on the SunSynk firmware, and they had not enabled it in the Deye firmware yet.

Excellent! It's seems to be a key feature to avoid a floating neutral when Eskom dies.

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