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 <[email protected]> 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 <[email protected]> > 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 <[email protected]> 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 <[email protected]> >>> 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 >>> [email protected] >>> 303-523-3592 >>> _______________________________________________ >>> List sponsored by Redwood Alliance >>> >>> Pay optional member dues here: http://re-wrenches.org >>> >>> List Address: [email protected] >>> >>> 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/[email protected]/ >>> 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: [email protected] >>> >>> 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/[email protected]/ >>> 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: [email protected] >> >> 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/[email protected]/ >> 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: [email protected] > > 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/[email protected]/ > 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 >
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