Cable length(s) between battery and inverter
All Hoymiles inputs should be connected to the battery
Does this mean:
- "All DC+ cables" should have the same length? or
- "All DC- cables" should have the same length? or
- "All DC+ & DC- cables" should have the same length?
SW-Niko and spcqike were kind enough to write some explanations:
First of all, different length of cables = different losses.
It makes sense therefore, that all DC- cables have the same length and all DC+ cables have the same length.
However, there is a much more important reason:
If the DCminus inputs of the Hoymiles are connected internally, then the return current to the battery flows preferentially via the connection with the lower-impedance .
In other words, more current flows in one return line than in the other return line.
This could lead to an overload of the internal conductors or the external cables.
In the event of a fault (lower line high resistance or interrupted), the entire return current could flow via one line.
The external line can cope with this, but whether the internal connection can do the same, is questionable.
This is not the first time that Ground (DC-) is so important: This is the reason why galvanic isolation for the earth of the VE.Direkt cable is so important level shifter - optocoupler
A quick check on a HMS-2000 showed that all DC- lines are connected internally. No problem with galvanically isolated power sources (= solar panels), but if all inputs are connected to one source (= battery).... Yes!!!!
In Summary: Have same length DC- cables 👍
A more detailed explanation is here: https://github.com/helgeerbe/OpenDTU-OnBattery/wiki/HOYMILES
Most probably, differences in length of the DC+ cables is less of concern as the electronic limiter of the Hoymiles should offer enough protection
(to be translated) https://github.com/helgeerbe/OpenDTU-OnBattery/discussions/1084
From spcqike: der gleichmäßige und gute / feste Anschluss aller Kabel ist sehr wichtig. Da reicht manchmal schon eine nicht ganz feste Mutter, eine schlechte MC4-Stecker-Crimpung oder ein minderwertiger Zwischenstecker/Adapter :)
Zum technischen Hintergrund, was bei dir das Problem gewesen sein kann:
die DC(-) Ausgänge am Wechselrichter sind intern alle verbunden. (lediglich die DC(+) sind einzeln oder, wie beim HM-1500, paarweise verbunden). Hat nun eine DC(-) Leitung zur Batterie einen höheren Widerstand, sucht sich der Strom halt den Weg des Geringsten. der Strom von DC3(+) kann dann auch über DC1(-) zurück fließen. Bei Solarpaneelen ist das technisch unmöglich, bei Batterien hingegen quasi immer der Fall. Nun messen die Wechselrichter die Eingangsleistung im DC(-) Strang eines jeden Strings.
Dadurch passiert es, dass der Strom bspw. in DC3/4 rein kommt, durch vom MPPT umgesetzt wird aber über DC1/2 abfließt und dort als Leistung gezählt wird. Dadurch drosselt der WR die echte Leistung auf DC1/2, setzt dort weniger Strom um, er zählt ja aber noch den "Abfluss" von DC3/4 als imaginäre Produktion auf DC1/2.
See here: https://github.com/helgeerbe/OpenDTU-OnBattery/discussions/984
One member describes:
The string inputs to the inverter were completely assymettric (I was too busy one Sunday afternoon to put things together again that I mixed a set of cables):
Another member describes: Took me a while to understand what your issue was, but yeah, I can see it now. The inverter is set to a certain limit, but it does not produce as much.
I had the exact same learning curve. My setup was working fine, then some day I saw the inverter was not producing as much as the limit allowed. The power produced by channel was wildly different and fluctuating a lot. I also thought that I had tightened all screws properly. And I had, but on some only one out of two screws reached the cable. See this image:
The screw below is down a lot more than the screw above, and that's because the cable does not reach the second screw. I removed the blue collar from the ferrule, such that the cable went down further and the second screw could reach it. There were other connections and screws that loosened a little over time. After going over all of them, the issue was gone.
I thought this was the perfect opportunity to share this experience with a little more detail. I hope I can find and link to this discussion the next time I need it 😉
Regarding contact grease: I also thought about it, and learned that it is actually not conductive. One uses contact grease to lubricate fasteners and prevent corrosion, but you actually don't put it between contact surfaces that shall conduct electricity. At least that's what I researched a couple of months ago. I don't use any and rewired the whole setup into a new cabinet recently. After learning that connections must be very good, I took care to double check every single one and I had no issue firing up the new wiring.