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 >
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