Re: [Re-wrenches] Wacky FNDC behavior

[Jay]

I might be the only person on this blog that has good success with the fndc with both lead and lithium.  I’m curious if you all are yanking them out what are you using instead? Jay On Aug 26, 2026, at 7:29 AM, Blair May <tump3...@gmail.com> wrote: I too disable the FNDC, although I call it something else. You do seem to have a grasp on it, but Ill ask anyway. The batteries were fully charged prior to connecting? Again I have found, that device has allowed me to charge for many miles to disconnect it for excessive inaccurate readings.  On Aug 25, 2026, at 9:50 PM, Jason Szumlanski <ja...@floridasolardesigngroup.com> wrote: I have hesitantly removed the FNDC from a few systems for the same reason. The accuracy is poor, especially when batteries don't reach full charge frequently, which is an epidemic around here with undersized arrays and oversized lead acid batteries.  Accurate SOC is the holy grail of off-grid monitoring systems. My understanding is that EVs do this at least slightly better because they consider the rate vs capacity curve. My suspicion is the FNDC and other inverter and BMS systems ignore current rate versus capacity and are just dumb coulomb counters. But I don't know enough about how they work. Interested in learning, though! Jason Szumlanski Florida Solar Design Group On Tue, Aug 25, 2026, 8:55 PM William Miller <will...@millersolar.com> wrote: I’m with Jeremy in that I never use the FNDC. SOC calculation drift is a problem with any battery chemistry. Clients freak out about an inaccurate SOC reading and it causes headaches for all.  William Miller Miller Solar.com 805-438-5600 www.millersolar.com On Tue, Aug 25, 2026 at 5:49 PM All Solar <allsolarpenr...@gmail.com> wrote: FWIW, I usually unhook the FNDC, as I’ve found that it is more suited for a lead acid battery bank.  I do have one with  a large bank of 16 cell Lithiums and It often does report critically low SOC , so it’s not very useful in my opinion.  Jeremy Rodriguez All Solar Inc. Penrose CO 719-276-4954 Established 1998 On Tue, Aug 25, 2026 at 5:37 PM Jay <jay.pe...@gmail.com> wrote: Hi Zeke The lightning is the issue.  I would replace all the fndc wires, make sure the shunt is clean. And of course all the cat 5 cables.  Check every wire connection to every buss.  Good luck.  On Aug 25, 2026, at 5:14 PM, Zeke Yewdall <z...@darkforestsolar.com> wrote:  Trying to troubleshoot an outback FNDC that's giving completely wacky SOC readings.  Yet the current readings all appear to be reasonable. We just replaced the FNDC because of a lightning strike -- came in through the ethernet to the mate it seems (took out a router too), and took out the mate3s, hub10, and possibly the old FNDC.  So all of those are new.  Rest of the system is maybe 5 to 6 years old -- four radians, four FM100's, 1200AH of 16 cell LFP batteries (open loop).  Has been operating pretty well for the past few years other than some battery BMS failures.  About 6 weeks ago, it looks like the SOC started being completely wrong on the old FNDC as well... which makes me question something about wiring... a wire coming loose from the shunt or or ?? Whenever I do a spot check for amperage from it, it looks right.  But then it bounces from 100% SOC to 2% SOC in one hour... physically impossible.  And then, I drain 23kWh out of the batteries, and it stays at 100% SOC for the next 6 hours.  Recorded voltages also look good during spot checks, and for the recorded min-max on the graph (not sure if those are from the FNDC or the inverter though).  System is operating fine, as long as I do all controls based on voltage, not SOC. Of course the system is 2500 miles from me, so I'm also trying to troubleshoot remotely. I did ask outback tech support, and was surprised to get an answer back... which turned out to be completely useless.  They said the FNDC only works with lead acid batteries, and lithium batteries are used in closed loop.  Nevermind that I have well over a dozen radian systems with lithium batteries and FNDC's working correctly, and this exact system was working correctly that way for 5 years. Zeke -- Zeke Yewdall PV Engineer NABCEP #031508-89 z...@darkforestsolar.com 303-523-3592 _______________________________________________ List sponsored by Redwood Alliance Pay optional member dues here: http://re-wrenches.org List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org There are two list archives for searching. 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When one doesn't work, try the other: https://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/ http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: http://www.re-wrenches.org/etiquette.htm Check out or update participant bios: http://www.members.re-wrenches.org _______________________________________________ List sponsored by Redwood Alliance Pay optional member dues here: http://re-wrenches.org List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org There are two list archives for searching. When one doesn't work, try the other: https://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/ http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: http://www.re-wrenches.org/etiquette.htm Check out or update participant bios: http://www.members.re-wrenches.org Blair “TUMP” May Solarwinds Northernlights east "We must all learn to live together as brothers,  or we all will perish as fools." MLK _______________________________________________ List sponsored by Redwood Alliance Pay optional member dues here: http://re-wrenches.org List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org There are two list archives for searching. 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