Induced AC on LiFePO4 Battery Sungrow SH5K Solar Hybrid

[Animalector]

Hi all,

I have a first generation Sungrow SH5K inverter - First generation meaning no firmware updates and limited functionality. it's almost 8 years old, so it has been a reliable system.

I originally had it running a 48V lead acid battery, but now have a DIY LiFePO4 using Voltax 302Ah cells with CAN integration to a Batrium BMS running Project Lychee comms protocol.

The SH5K has some issues identified in the video above, like you cannot actually run it to 100% SOC... Also the cells that came out of the Voltax 12V batteries were.. err.. not A-Grade..
Some shots of the cells here:

Anyway, the reason for the post is to ask why I am getting AC voltage on the battery. It measures about 70V ish.

I don't want to earth the Negative of the battery pack, because I figured if this was desired, they would have earthed it in the inverter. The Sungrow SH5K Manual is silent on earthing for the battery. I didn't think it was necessary and once it is tucked away in it's box it's all fine..

The reason I am asking is because I want to DC-couple a second MPPT to the battery to "boost" it in the morning so it wil charge to full throughout the day, currently the solar is insufficient to charge it fully thoughout the day unless it's zero clouds.... plus it's hot here, and we like Air-con.

QUESTION: Will I have an issue DC Coupling a MPPT 150/45 to the battery?

Thanks all

Andy
#300SunnyDays

 

[sunshine_eggo]

Anyway, the reason for the post is to ask why I am getting AC voltage on the battery. It measures about 70V ish.

First, check your voltmeter battery.

Second, are you setting your voltmeter to AC and then measuring? If yes, this is incorrect use of the tool and the behavior of your voltmeter. I just measured AC voltage on a standalone (connected to nothing) 12V battery. Read 26V.

What does it say when you measure DC voltage?

 

[Animalector]

Measures 52V DC. As it should. There's definately AC there. It's enough to give a tingle / buzz.

But I will replace the batteries in the meter anyway it's possibly time.

The main thing I want to avoid is frying the MPPT when I connect it.

 

[sunshine_eggo]

I would hope that you're aware that 52VDC is enough to give a tingle/buzz depending on the conductivity of your skin.

Suggest you disconnect battery from the inverter where there is no possibility of AC feedback and re-measure.

Measuring DC voltage with AC voltmeter = erroneous reading.

Is your lead acid battery still around? If so, what does it measure with voltmeter set to AC?

 

[42OhmsPA]

I would hope that you're aware that 52VDC is enough to give a tingle/buzz depending on the conductivity of your skin.

Suggest you disconnect battery from the inverter where there is no possibility of AC feedback and re-measure.

Measuring DC voltage with AC voltmeter = erroneous reading.

Is your lead acid battery still around? If so, what does it measure with voltmeter set to AC?

Yup.

 

[Animalector]

I would hope that you're aware that 52VDC is enough to give a tingle/buzz depending on the conductivity of your skin.

Suggest you disconnect battery from the inverter where there is no possibility of AC feedback and re-measure.

Measuring DC voltage with AC voltmeter = erroneous reading.

Is your lead acid battery still around? If so, what does it measure with voltmeter set to AC?

That's not the point.. When the battery is disconnected from the inverter it's just a battery, it has no AC on it. no tingles, nothing measures 52VDC as you expect..

The issue is when it is connected, it still measures 52VDC but there's also about 110V to earth. (when measured across the terminals the reading jumps around but settles on 0VAC..) This is the scenario I want to DC Couple a second MPPT to the Battery / DC Bus...

I read the articles / watched the vids about non-isolated PV terminals, and mains voltage on the MPPT terminals. This feels the same, the Inverter circuits are not isolated, and there's AC on the DC Bus..

My feeling is that DC Coupling of a different MPPT to the DC Bus would not work in thise case, since the DC side is not isolated.

Can anyone confirm?

Cheers,
Andy