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SOC goes down when selling power

Posted: Tue May 03, 2016 1:08 pm
by jgreen1024
Hi all-

Just had a FlexPower One with the GS4048A inverter installed last week. This is set up to be primarily grid-tied, but with battery backup in case the grid goes out. I am having a little trouble getting my head around the theory of operation of this unit - any "getting started" pointers would be appreciated.

One immediate issue that I can't make sense of: When the system is selling power back to the grid, my battery state of charge value starts dropping. However, the battery voltage stays constant (right around 55.4) and after I manually caused it to start charging the batteries last night, it only took ~45 minutes to show fully charged - seems way too fast if the batteries were actually down to 64%. So I'm not sure the batteries are actually being drained. How is the SOC value computed?

Here's a view of today's activity in the attachment - you can see that the SOC started dropping as soon as it started selling power. Right now it's showing the batteries at 66%. Any ideas?

Thanks!

Re: SOC goes down when selling power

Posted: Mon May 23, 2016 6:40 am
by Mike Curran
I think SOC = [(Batt capacity - AH out)/Batt cap] x 100%.

Batt capacity must be entered into your Mate3 based on your bank's 20 hour rate.
AH out is what's being measured by your system's primary shunt.

My SOC also goes down while I'm selling but only ever so slightly. I have a small PV array with a large battery. Not sure what you have but suspect your bank capacity is not entered correctly into your system's Mate3.

Re: SOC goes down when selling power

Posted: Mon May 23, 2016 7:12 am
by jgreen1024
We figured it out - one of the leads from a shunt to the FNDC was not screwed down tight - came that way from the factory. Once I tightened that screw, problem went away.

Re: SOC goes down when selling power

Posted: Mon May 23, 2016 3:20 pm
by larrywa
jgreen1024 wrote:We figured it out - one of the leads from a shunt to the FNDC was not screwed down tight - came that way from the factory. Once I tightened that screw, problem went away.
Nice!

Good catch!

After wiring my FlexPower II from scratch I went over every connection again. After about six months I shut it down and did the same thing again. I needed to reverse my two shunts to match my CC order anyway.

It doesn't take much when dealing with big currents or sensitive low voltage inputs to make things wrong.