[RE-wrenches] BMS as OCPD

[William Miller]

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 <kirkh at vermont.solar> 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 <
> re-wrenches at lists.re-wrenches.org> 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: RE-wrenches at 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
>>
>>
>
> --
>
>
> *Kirk Herander / **kirkh at vermont.solar <kirkh at vermont.solar>*
>
> *Owner|Principal, VT Solar, LLC*
>
> *Celebrating our 34st Anniversary 1991-2025!!*
>
> dba Vermont Solar Engineering
>
> 802.559.1225
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20250715/3e02735b/attachment.htm>