Per the Subject, it appears that the Venus OS automatically enables DVCC when there is a BMS in the system, and the BMS upon reaching 100% SoC turns down the solar-produced charging current to zero. But that appears to happen before the cells have been given time to balance. How is balancing supposed to work?
I’ve been using a SmartSolar MPPT solar controller with a home-built 315AH 12v LiFePO4 batter that’s managed by an OverkillSolar BMS (rebranded JBD, I believe). The solar controller was programmed to run the battery up to 14.35v (3.6v / cell), when it can, and hold it there for an hour or two to let the BMS do its cell balancing thing. Then the system drops to 13.8v with the solar providing power to the load for as long as the Sun shines. There’s a DC power supply that kicks in as the battery drains, keeping things alive at night. Next day the cycle repeats. Often the battery never gets to 100%, but that’s intended. You don’t want a reservoir to be totally full very often.
The DC supply is adjusted so that the battery never drains completely, but also isn’t held so full as to not have a place for the solar energy to go. (I’d rather burn my own solar photons than electrons created by the local utility.) Once in a while I tweak things to force a 100% charge, to let the battery cells balance. That occurred yesterday. In the early afternoon I noticed that the battery had reached 100% SoC, but upon doing so the voltage dropped almost immediately to 13.5v, which is apparently what the BMS calls “fully charged”. The cells happen to be balanced pretty well (within 7-8 mv), but I still expected that the absorption phase should have lasted more than the few moments that it did, and even then, it only reached about 13.7v instead of the expected 14.35v.
I recently saw a video from the Off Grid Garage where they detailed their JBD BMS battery settings. Comparing my settings, I decided that I liked theirs better so did an update. The result of disconnecting the USB adapter from the BMS to switch over to Bluetooth (so I could make the changes) somehow reset what the BMS thought the battery’s Soc was. It was like 90+ percent at the time, and came back at 70%. {shrug} Decided it was time to run the battery back up to 100% in order to reset the SoC meter, and to watch the battery balance take place (the topic of this thread).
Bottom line, it wouldn’t balance.
I expect there’s a setting somewhere on what voltage the “DVCC algorithm” targets, but it strongly limited the battery voltage to less than what is needed for the balancing to start. I needed up to about 14.2v, and something was preventing anything more than 13.8v. I found that the setting for max voltage, but changing that didn’t seem to matter.
So I’ve turned off DVCC. I had initially read that DVCC needed to be on in order to view the battery data, but apparently that isn’t true. Turning off DVCC has let the SmartSolar controller manage the battery charging algorithm (bulk and absorbtion), with voltages that are compatible with the BMS. I can still see all the battery data, so I think I’m done.
No idea why or where the DVCC settings should be changed to let the BMS properly manage the inputs in a way consistent with its needs. At a high level, one would expect the BMS to be in the best position to do that, but oh well… If there is a setting that I missed somewhere, please let me know and I’ll give it another chance.
Balancing has nothing to do with DVCC, it is a function of the BMS.
DVCC’s sole purpose is to coordinate charging sources so that the voltage and current limits, requested by the BMS are honoured.
The battery manages the system.
It takes experience to program your own BMS, but if the battery is not balancing then you have configured it incorrectly, or, the system is not capable of getting to the voltage where balancing occurs.
This topic is a bit outside of the scope of the community, you would find more support on a site focused on your BMS or DIY batteries in general.
A stable system depends on a good battery, which is why too many DIY batteries inevitably end up on this forum with problems, typically which Victron can do little about.
Thanks, Nick. I understand in general what’s supposed to happen, and the battery DOES balance when given the opportunity to do so. But it needs more than 13.8v (3.45 per cell) in order to start, and when externally pushed over 14v it did in fact begin balancing. It wasn’t hugely out of balance, but when held at 14.2v there was about an hour of periodic balance action going on.
So the issue here is that there appears to be something in the DVCC system that was limiting (actively reducing) the solar from getting the battery to where balancing would start. If the BMS is doing that, I’d be curious why. Rather counter productive on its part, no? Is DVCC dependent solely on the BMS; if so, what specifically is the BMS telling DVCC to do? Perhaps I can walk that backward to some BMS parameter that needs to be changed. Or are there limits separate from the internal BMS configuration that can affect the DVCC behavior?
Right now I’m getting the behavior I (thought I) wanted from just the SmartSolar’s charging configuration, leaving DVCC out of it. What advantage would I have in flogging DVCC further to make it work the same?
The BMS should be sending 3 figures to Venus OS, CVL, CCL and DCL, charge voltage limit, charge current limit and discharge current limit. These can be seen on the correct page of Venus OS, they can also be plotted historically on VRM.
DVCC relays these first 2 figures to all connected charging devices and ensures all charging devices work to the same target voltage and limit the combined charging current if it could exceed the maximum.value. DVCC also has manually input values for these and DVCC works from the lower of the BMS or manual values.
Look at your manual settings in DVCC, if OK then check what the BMS is requesting, then work out why. Perhaps it stops absorption on tail current very quickly.
It’s always a good idea to use a meter and check voltage along the chain.
Bad connections and components can also cause voltage drop which could affect charging.
But, no, DVCC won’t lower voltage unless you set it to.
“The BMS should be sending 3 figures to Venus OS, CVL, CCL and DCL, charge voltage limit, charge current limit and discharge current limit.”
…
“DVCC also has manually input values for these and DVCC works from the lower of the BMS or manual values.”
Ha! Thank you; this is the key that I had missed. Apparently the Venus OS driver for the JBD / OverkillSolar BMS is taking the “cell 100% capacity voltage” x the number of cells, and using that as the limiting voltage level. My BMS is configured, I believe correctly, with a 3.350v 100% cell capacity level, and a 3.450v Balance start voltage. Taking the minimum explains why I never saw the voltage ever exceed 13.4v even when I changed the manual DVCC CVL entry to 14.2. That also explains why balancing can never occur in my system with DVCC enabled, since DVCC will never let a cell go above the 3.45v minimum needed to start balancing.
So to use DVCC either the BMS or the driver need a code change to use the “Cell Full Voltage” (3.55v cell / 14.2v net in my case) instead of the “Cell 100% capacity” value (why are there two separate statistics?), or I need to artificially change the 100% voltage to match the Cell Full, which makes me nervous. Again, I’m using the Off Grid Garage tutorials as my BMS configuration guide.
Given that my system consists only of the solar controller running the charging profile, and it’s algorithm seems to be working properly, I think (yes?) that leaving DVCC off is the right answer. Let me know if I’m missing something important here.
Thanks all for your help in getting to the root cause here.
