[RE-wrenches] BMS as OCPD

[jay]

I always use breakers more because they are easy to reset and visible if tripped.  And if a class T  fuse blows, you don’t actually know it and then you have the situation of having to go out to replace a $100 fuse or have the customer replace a fuse with live ends which I stopped doing a long time ago. 

I wonder if midnite power is going to build a combiner box with breakers.  I’m sure the breakers used in their batteries are way less expensive than the big 250’s we have used.  I’d sure like to see one.  

A question on the Class T fuses.  Is it UL ok to mount the fuses not in a fuse holder?  Like to a buss bar?

jay

> On Jul 16, 2025, at 5:23 AM, Jason Szumlanski via RE-wrenches <re-wrenches at lists.re-wrenches.org> wrote:
> 
> 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 <mailto: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 <http://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 <http://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 <mailto: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 <http://www.millersolar.com/>
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>>> 
>>> 
>>> --
>>> 
>>> 
>>>  <>
>>> Kirk Herander /  <>kirkh at vermont.solar <mailto: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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