I like the Midnite 1000/2000A battery combiners for this purpose. The 250A DC breakers are not cheap, but they add a level of convenience and protection that I think it worthwhile.
Jason Szumlanski Principal Solar Designer | Florida Solar Design Group NABCEP Certified Solar Professional (PVIP) Florida State Certified Solar Contractor CVC56956 Florida Certified Electrical Contractor EC13013208 On Tue, Jul 15, 2025 at 1:36 PM William Miller via RE-wrenches < [email protected]> wrote: > Kirk: > > First of all, I apologize for the typo in the subject line. I fixed it. I > take my writing seriously but sometimes my devices fight me with > autocorrect. > > You make good point regarding a scenario with a battery combiner. Relying > on multiple BMSs for OCP would be a bad choice. > > Fortunately some cabinets come with breakers and those should be > sufficient. > > William Miller > Miller Solar.com > 805-438-5600 > www.millersolar.com > > > On Tue, Jul 15, 2025 at 9:50 AM Kirk Herander <[email protected]> wrote: > >> Hello William- >> >> I agree with your approach. >> >> I never historically have OCPD'ed lead acids at both ends of the battery >> cable run, but do now, esp. with high kwh LFP, where a DC combiner is in >> use. >> >> For instance, I had to design a 2000 ADC bus @ a nominal 55 vdc, for a >> 500 kwh Blue Ion LFP bank. To create enough space for cables and fuses, I >> started with -2- 2000A Midnite combiners, using one for positive and one >> for negative. >> >> Class T fuses are bolted directly to the DC bus (properly sized copper >> from copper.org charts), and the BI BMS breaker protects at each >> respective battery cabinet. >> >> Same thing on the DC to inverter cables, class T's bolted to the bus, to >> its respective outback Radian, where the standard 175A breaker resides. >> >> I would never again do a building power system of any type or size >> without both ends of a power cable (point of use & point of origin) being >> protected, A best practice IMO. >> >> >> >> On Tue, Jul 15, 2025 at 1:13 AM William Miller via RE-wrenches < >> [email protected]> wrote: >> >>> Friends: >>> >>> When I design a lead-acid battery system I decide if I need over-current >>> protection right at the battery or not. My logic is: if the leads are >>> short, don’t go through a wall and are well protected, I can rely on OCPD >>> in the BOS cabinets. If any of these criteria is a no I put appropriately >>> rated class T fuses right at the battery. >>> >>> I’m thinking that since BMS devices have over-current protection >>> baked-in, fusing at the battery would not be necessary for BMS equipped >>> batteries in any case. Do you agree with that premise? >>> >>> >>> William Miller >>> Miller Solar.com >>> 805-438-5600 >>> www.millersolar.com >>> _______________________________________________ >>> 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 >>> >>> >> >> -- >> >> >> *Kirk Herander / **[email protected] <[email protected]>* >> >> *Owner|Principal, VT Solar, LLC* >> >> *Celebrating our 34st Anniversary 1991-2025!!* >> >> dba Vermont Solar Engineering >> >> 802.559.1225 >> > _______________________________________________ > 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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