[RE-wrenches] BMS as OCPD

[Jason Szumlanski]

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 <
re-wrenches at lists.re-wrenches.org> 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 <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
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>>
>> --
>>
>>
>> *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
>>
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