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System System is Multiplus 5 kVA, BMV 702,Venus GX , grid connected. System is eing used as a UPS. I am busy moving over to lithiums ( making own LFP battery ) and would like to keep them float charged at 75- 80% and then cycle them to 100 % monthly for balancing . How do I do this?
Given the correct answers already provided, maybe I can tie your questions and their answers together.
Even with approved LFP batteries, it’s tricky to adjust charging to SOC or battery voltage because the discharge curve of LFP batteries is so flat that there isn’t much voltage range between 100% and 40%... about one volt on my 24 volt system. So settings based voltage alone are impossible to state for all but a few known, rigorously tested and approved batteries. Due to the wild variation in DIY battery packs, no manufacturer will venture a recommendation. Even if they did, I’d bet money it would be wrong.
In the end, even with a well known battery like the Battle Borns I have, I had to test, adjust, and repeat until I finally got things coming on and going off as needed.
As far as I’m aware, you would have to disable balancing, then manually change your bulk and absorption settings to ramp up your charge rate when desired.
I hope that is helpful.
DIY battery banks are not supported in Victron systems, so -while I'm sure that with careful experimentation you can probably force the behavior you would like- there is unfortunately not a tried-and-true method of doing this that we can point to, nor is there any guarantee of stable system behavior when using non-supported batteries.
At the end of the day, just make sure that you have an excellent BMS on the system with all the appropriate HV/LV and HT/LT cutouts or relays and then dial down your charge voltages until you get where you think you want to be... just be aware that you won't have manufacturer or dealer/distributor support in so doing, or in dealing with system instability that may result.
So you never answered my question - I understand that there could be issue s of warrantee but if the batteries were lead acid then how would you charge to a SOC value as opposed to a voltage ( ( using the BMV 702 via the Genius probably )
@merle.peter@gmail.com, on a Pb battery system you wouldn't charge to an SOC value, you would want to keep them full. That being said, since vbatt is directly correlative to SOC on a Pb bank, one could fairly accurately charge to a specific SOC by using voltage alone.
Unfortunately, that is not the case with LFP banks, as vbatt isn't an accurate predictor of SOC.
Once again, there's no tried-and-true method of doing what you're trying to do, since you're outside of the normal and designed use-case of the charger.
