Hi guys. I have an 800AH battery bank connected to Multiplus compact 24/1600/40. Everything is working for over a year now. Theres 7m distance between batteries and multiplus so I used 35mm cable. I calculated volt drop over this distance would be around 0.3-0.4V and this is what I'm measuring however the multiplus is measuring around 0.7V lower than the battery terminal voltage. So for example, right now I have an 80% discharged bank which is inverting at 1200W (I've set it lower than max to increase battery life). My battery voltage is 24.7V, no issues there. The multiplus sees 24.0V. I've measured the voltage at multiplus and it is 24.3V, which ties up with my volt drop calculations. Again, no issues as the system works ok.
But, the only reason it does it that when I installed it, I noticed that the multiplus was shutting down early due to low voltage. I couldn't see any way to offset the voltage in the software. I'm using dynamic voltage adjustment and also temp sense. On my multiplus, there is no voltage sense terminals which would likely correct for this issue. Due to this, I had to "manipulate" the default voltage settings in the ESS assistant dynamic voltage page. I set all of them to 0.7V lower than the default values, so cut off at 24V becomes 23.3V etc. The only problem is...at lower current draw, the volt drop decreases so this could result in the battery potentially becoming dangerously discharged.
Now obviously, the ESS system uses SOC instead of terminal voltage to work out when its going to stop discharging and this has never been a problem since I changed the values....but it kinda seems like a work around and I've never really gotten to the bottom of why the multiplus is under-reading by the extra 0.3-0.4V in the first place. I think voltage sense cables would correct this but I have no where to connect them to and I don't really want to spend more money on a BMV which I imagine would also fix the issue.
Does anyone else have any ideas how to correct for this? I could increase my cable size to 50mm or 2 pairs of 35mm (don't fancy trying to fit 70mm in my DC breaker box) but this will only fix the volt drop in the cable and I would still have the problem with the remaining 0.3-0.4V. Is there a setting I've missed in the software maybe?
