Multiplus II GX 24/3000
Redodo 100Ah 24V Lifepo4 battery.
AC component at DC side detected when charging or discharging. Measured current with 2 pair of clamps similar values, also BMS of the batterie shows that current direction changes.
With no-load with DC current 0,55A measured AC current is around 2,6A.
Here oscilloscope picture for voltage AC component:
When charging battery with lab power supply (10A) no AC component detected.
Commonly referred to as “ripple”; see here: 2. Theory
Read the item linked to above. Ripple is made worse if the battery is too small and if the cables between the battery and Multiplus are too small. The cables need to be larger gauge than required for current capacity to keep resistance low.
Your 100Ah 24V battery is half the size of the minimum recommended 200Ah. How does your cable size compare to the recommended. See the manual installation section.
I will find a battery impedance meter, to check if everything is correct with battery. Regarding recommended 200-700Ah batteries i suggested that this is recommendation for lead-acid batteries with relatively high internal resistance.
Because i don’t see how high voltage drop can be present with current in range of 1A. Battery is now connected for with temporary 1m long 6mm2 Cu wires, so expected voltage drop in the wires should be 7,5mV/A.
But there is something I don’t see from theory link.
Can this effect be present when charging? Because it exist even with very small charging current with multi and don’t exist when charging with lab power supply?
Can this actually go alternative to current on DC side - periodically crossing the zero, as on the oscillogram?
It can be present when converting ac to dc during the charging process as the rectified current will be variable and it relies on the DC capacitors to smooth this and the higher the battery and wiring resistance, the higher the ripple voltage.
All of this needs a lot of current, or very bad resistance…
System ripple monitor don’t see significant ripple:
0,3V appears when i tried charge with around 20A, but all other numbers with no external load.
could also be that the inverter is doing power factor correction, where it will take power from, and then push power back into the battery in order to get the mains input current in phase with the mains input voltage.
If i read this right, then thats also a problem, 6mm2 is way to thin
For no-load? <10mV voltage drop?
I don’t have crimple tool for my 50mm2 cable now so only option to double 6mm2 cable, but this will not work. Quite precise measurements show, that there is no voltage drop on the wires. Measured both, “+to -” on battery vs “+ to -” on inverter. Also between + of battery and + of inverter, voltage drop below 0,1V with inverter self consumption.
So I think, its not related to the wires there is a problem with the battery or the inverter.
As I wrote, it’s was not a plan… But 50 supposed to work with 100A. we now talking about 0,5A DC and around 2,6A AC. If it not work with 6mm2 and 0,5A, no chance it will work with 50mm2 and 100A.
Measurements:
DC current 0,6A
AC current 2,6A
Battery voltage with nothing connected: 26,672
Battery DC voltage inverter connected: 26,669xV
Inverter terminals DC voltage:26,645xV
x - is changing
“-” wire AC voltage drop between battery and inverter “-” terminals 45mV
“-” wire DC voltage drop between and inverter “-” terminals 10mV
So the short version of the post; you haven’t installed the system correctly and are wondering why it is behaving what you consider to be strangely?
Any other option how to check it ?
But what should be accepted value of AC current for no-load conditions?
Reconnected everything with 50mm2 wires, as expected - no effect. Voltage drop reduced but currents are the same.
AC current exactly the same, voltage drop on the cable is very low, i think its near to sensitivity of my measurement devices.
I tried to measure battery internal impedance with Fnirsi HRM-10, but it cannot measure it, values change between 0 and 2mOhm. But I think can be effect of BMS, can it ?
Yes, you are right. It’s even lower.
But the questions is still the currents. My initial question was is it normal when working with inverter?
This is the question, because there definitely shall be some in capacitor at scheme and this can not be good for battery?!
Something wrong with settings?
Is there some way to get official comment from Victron?
Because i got following answer from dealer “That is definetly not normal, I can’t provide a solution there because I never heard of that problem”.
Its at 102 hz so is at the switching frequency of the mosfet or igbts in the inverter.
Is the scope in AC coupling mode so it can filter out the DC?
